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authorLinus Torvalds <torvalds@linux-foundation.org>2009-04-03 10:07:43 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2009-04-03 10:07:43 -0700
commit3cc50ac0dbda5100684e570247782330155d35e0 (patch)
treef4b8f22d1725ebe65d2fe658d292dabacd7ed564 /Documentation
parentd9b9be024a6628a01d8730d1fd0b5f25658a2794 (diff)
parentb797cac7487dee6bfddeb161631c1bbc54fa3cdb (diff)
Merge git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-2.6-fscache
* git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-2.6-fscache: (41 commits) NFS: Add mount options to enable local caching on NFS NFS: Display local caching state NFS: Store pages from an NFS inode into a local cache NFS: Read pages from FS-Cache into an NFS inode NFS: nfs_readpage_async() needs to be accessible as a fallback for local caching NFS: Add read context retention for FS-Cache to call back with NFS: FS-Cache page management NFS: Add some new I/O counters for FS-Cache doing things for NFS NFS: Invalidate FsCache page flags when cache removed NFS: Use local disk inode cache NFS: Define and create inode-level cache objects NFS: Define and create superblock-level objects NFS: Define and create server-level objects NFS: Register NFS for caching and retrieve the top-level index NFS: Permit local filesystem caching to be enabled for NFS NFS: Add FS-Cache option bit and debug bit NFS: Add comment banners to some NFS functions FS-Cache: Make kAFS use FS-Cache CacheFiles: A cache that backs onto a mounted filesystem CacheFiles: Export things for CacheFiles ...
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/filesystems/caching/backend-api.txt658
-rw-r--r--Documentation/filesystems/caching/cachefiles.txt501
-rw-r--r--Documentation/filesystems/caching/fscache.txt333
-rw-r--r--Documentation/filesystems/caching/netfs-api.txt778
-rw-r--r--Documentation/filesystems/caching/object.txt313
-rw-r--r--Documentation/filesystems/caching/operations.txt213
-rw-r--r--Documentation/slow-work.txt174
7 files changed, 2970 insertions, 0 deletions
diff --git a/Documentation/filesystems/caching/backend-api.txt b/Documentation/filesystems/caching/backend-api.txt
new file mode 100644
index 00000000000..382d52cdaf2
--- /dev/null
+++ b/Documentation/filesystems/caching/backend-api.txt
@@ -0,0 +1,658 @@
+ ==========================
+ FS-CACHE CACHE BACKEND API
+ ==========================
+
+The FS-Cache system provides an API by which actual caches can be supplied to
+FS-Cache for it to then serve out to network filesystems and other interested
+parties.
+
+This API is declared in <linux/fscache-cache.h>.
+
+
+====================================
+INITIALISING AND REGISTERING A CACHE
+====================================
+
+To start off, a cache definition must be initialised and registered for each
+cache the backend wants to make available. For instance, CacheFS does this in
+the fill_super() operation on mounting.
+
+The cache definition (struct fscache_cache) should be initialised by calling:
+
+ void fscache_init_cache(struct fscache_cache *cache,
+ struct fscache_cache_ops *ops,
+ const char *idfmt,
+ ...);
+
+Where:
+
+ (*) "cache" is a pointer to the cache definition;
+
+ (*) "ops" is a pointer to the table of operations that the backend supports on
+ this cache; and
+
+ (*) "idfmt" is a format and printf-style arguments for constructing a label
+ for the cache.
+
+
+The cache should then be registered with FS-Cache by passing a pointer to the
+previously initialised cache definition to:
+
+ int fscache_add_cache(struct fscache_cache *cache,
+ struct fscache_object *fsdef,
+ const char *tagname);
+
+Two extra arguments should also be supplied:
+
+ (*) "fsdef" which should point to the object representation for the FS-Cache
+ master index in this cache. Netfs primary index entries will be created
+ here. FS-Cache keeps the caller's reference to the index object if
+ successful and will release it upon withdrawal of the cache.
+
+ (*) "tagname" which, if given, should be a text string naming this cache. If
+ this is NULL, the identifier will be used instead. For CacheFS, the
+ identifier is set to name the underlying block device and the tag can be
+ supplied by mount.
+
+This function may return -ENOMEM if it ran out of memory or -EEXIST if the tag
+is already in use. 0 will be returned on success.
+
+
+=====================
+UNREGISTERING A CACHE
+=====================
+
+A cache can be withdrawn from the system by calling this function with a
+pointer to the cache definition:
+
+ void fscache_withdraw_cache(struct fscache_cache *cache);
+
+In CacheFS's case, this is called by put_super().
+
+
+========
+SECURITY
+========
+
+The cache methods are executed one of two contexts:
+
+ (1) that of the userspace process that issued the netfs operation that caused
+ the cache method to be invoked, or
+
+ (2) that of one of the processes in the FS-Cache thread pool.
+
+In either case, this may not be an appropriate context in which to access the
+cache.
+
+The calling process's fsuid, fsgid and SELinux security identities may need to
+be masqueraded for the duration of the cache driver's access to the cache.
+This is left to the cache to handle; FS-Cache makes no effort in this regard.
+
+
+===================================
+CONTROL AND STATISTICS PRESENTATION
+===================================
+
+The cache may present data to the outside world through FS-Cache's interfaces
+in sysfs and procfs - the former for control and the latter for statistics.
+
+A sysfs directory called /sys/fs/fscache/<cachetag>/ is created if CONFIG_SYSFS
+is enabled. This is accessible through the kobject struct fscache_cache::kobj
+and is for use by the cache as it sees fit.
+
+
+========================
+RELEVANT DATA STRUCTURES
+========================
+
+ (*) Index/Data file FS-Cache representation cookie:
+
+ struct fscache_cookie {
+ struct fscache_object_def *def;
+ struct fscache_netfs *netfs;
+ void *netfs_data;
+ ...
+ };
+
+ The fields that might be of use to the backend describe the object
+ definition, the netfs definition and the netfs's data for this cookie.
+ The object definition contain functions supplied by the netfs for loading
+ and matching index entries; these are required to provide some of the
+ cache operations.
+
+
+ (*) In-cache object representation:
+
+ struct fscache_object {
+ int debug_id;
+ enum {
+ FSCACHE_OBJECT_RECYCLING,
+ ...
+ } state;
+ spinlock_t lock
+ struct fscache_cache *cache;
+ struct fscache_cookie *cookie;
+ ...
+ };
+
+ Structures of this type should be allocated by the cache backend and
+ passed to FS-Cache when requested by the appropriate cache operation. In
+ the case of CacheFS, they're embedded in CacheFS's internal object
+ structures.
+
+ The debug_id is a simple integer that can be used in debugging messages
+ that refer to a particular object. In such a case it should be printed
+ using "OBJ%x" to be consistent with FS-Cache.
+
+ Each object contains a pointer to the cookie that represents the object it
+ is backing. An object should retired when put_object() is called if it is
+ in state FSCACHE_OBJECT_RECYCLING. The fscache_object struct should be
+ initialised by calling fscache_object_init(object).
+
+
+ (*) FS-Cache operation record:
+
+ struct fscache_operation {
+ atomic_t usage;
+ struct fscache_object *object;
+ unsigned long flags;
+ #define FSCACHE_OP_EXCLUSIVE
+ void (*processor)(struct fscache_operation *op);
+ void (*release)(struct fscache_operation *op);
+ ...
+ };
+
+ FS-Cache has a pool of threads that it uses to give CPU time to the
+ various asynchronous operations that need to be done as part of driving
+ the cache. These are represented by the above structure. The processor
+ method is called to give the op CPU time, and the release method to get
+ rid of it when its usage count reaches 0.
+
+ An operation can be made exclusive upon an object by setting the
+ appropriate flag before enqueuing it with fscache_enqueue_operation(). If
+ an operation needs more processing time, it should be enqueued again.
+
+
+ (*) FS-Cache retrieval operation record:
+
+ struct fscache_retrieval {
+ struct fscache_operation op;
+ struct address_space *mapping;
+ struct list_head *to_do;
+ ...
+ };
+
+ A structure of this type is allocated by FS-Cache to record retrieval and
+ allocation requests made by the netfs. This struct is then passed to the
+ backend to do the operation. The backend may get extra refs to it by
+ calling fscache_get_retrieval() and refs may be discarded by calling
+ fscache_put_retrieval().
+
+ A retrieval operation can be used by the backend to do retrieval work. To
+ do this, the retrieval->op.processor method pointer should be set
+ appropriately by the backend and fscache_enqueue_retrieval() called to
+ submit it to the thread pool. CacheFiles, for example, uses this to queue
+ page examination when it detects PG_lock being cleared.
+
+ The to_do field is an empty list available for the cache backend to use as
+ it sees fit.
+
+
+ (*) FS-Cache storage operation record:
+
+ struct fscache_storage {
+ struct fscache_operation op;
+ pgoff_t store_limit;
+ ...
+ };
+
+ A structure of this type is allocated by FS-Cache to record outstanding
+ writes to be made. FS-Cache itself enqueues this operation and invokes
+ the write_page() method on the object at appropriate times to effect
+ storage.
+
+
+================
+CACHE OPERATIONS
+================
+
+The cache backend provides FS-Cache with a table of operations that can be
+performed on the denizens of the cache. These are held in a structure of type:
+
+ struct fscache_cache_ops
+
+ (*) Name of cache provider [mandatory]:
+
+ const char *name
+
+ This isn't strictly an operation, but should be pointed at a string naming
+ the backend.
+
+
+ (*) Allocate a new object [mandatory]:
+
+ struct fscache_object *(*alloc_object)(struct fscache_cache *cache,
+ struct fscache_cookie *cookie)
+
+ This method is used to allocate a cache object representation to back a
+ cookie in a particular cache. fscache_object_init() should be called on
+ the object to initialise it prior to returning.
+
+ This function may also be used to parse the index key to be used for
+ multiple lookup calls to turn it into a more convenient form. FS-Cache
+ will call the lookup_complete() method to allow the cache to release the
+ form once lookup is complete or aborted.
+
+
+ (*) Look up and create object [mandatory]:
+
+ void (*lookup_object)(struct fscache_object *object)
+
+ This method is used to look up an object, given that the object is already
+ allocated and attached to the cookie. This should instantiate that object
+ in the cache if it can.
+
+ The method should call fscache_object_lookup_negative() as soon as
+ possible if it determines the object doesn't exist in the cache. If the
+ object is found to exist and the netfs indicates that it is valid then
+ fscache_obtained_object() should be called once the object is in a
+ position to have data stored in it. Similarly, fscache_obtained_object()
+ should also be called once a non-present object has been created.
+
+ If a lookup error occurs, fscache_object_lookup_error() should be called
+ to abort the lookup of that object.
+
+
+ (*) Release lookup data [mandatory]:
+
+ void (*lookup_complete)(struct fscache_object *object)
+
+ This method is called to ask the cache to release any resources it was
+ using to perform a lookup.
+
+
+ (*) Increment object refcount [mandatory]:
+
+ struct fscache_object *(*grab_object)(struct fscache_object *object)
+
+ This method is called to increment the reference count on an object. It
+ may fail (for instance if the cache is being withdrawn) by returning NULL.
+ It should return the object pointer if successful.
+
+
+ (*) Lock/Unlock object [mandatory]:
+
+ void (*lock_object)(struct fscache_object *object)
+ void (*unlock_object)(struct fscache_object *object)
+
+ These methods are used to exclusively lock an object. It must be possible
+ to schedule with the lock held, so a spinlock isn't sufficient.
+
+
+ (*) Pin/Unpin object [optional]:
+
+ int (*pin_object)(struct fscache_object *object)
+ void (*unpin_object)(struct fscache_object *object)
+
+ These methods are used to pin an object into the cache. Once pinned an
+ object cannot be reclaimed to make space. Return -ENOSPC if there's not
+ enough space in the cache to permit this.
+
+
+ (*) Update object [mandatory]:
+
+ int (*update_object)(struct fscache_object *object)
+
+ This is called to update the index entry for the specified object. The
+ new information should be in object->cookie->netfs_data. This can be
+ obtained by calling object->cookie->def->get_aux()/get_attr().
+
+
+ (*) Discard object [mandatory]:
+
+ void (*drop_object)(struct fscache_object *object)
+
+ This method is called to indicate that an object has been unbound from its
+ cookie, and that the cache should release the object's resources and
+ retire it if it's in state FSCACHE_OBJECT_RECYCLING.
+
+ This method should not attempt to release any references held by the
+ caller. The caller will invoke the put_object() method as appropriate.
+
+
+ (*) Release object reference [mandatory]:
+
+ void (*put_object)(struct fscache_object *object)
+
+ This method is used to discard a reference to an object. The object may
+ be freed when all the references to it are released.
+
+
+ (*) Synchronise a cache [mandatory]:
+
+ void (*sync)(struct fscache_cache *cache)
+
+ This is called to ask the backend to synchronise a cache with its backing
+ device.
+
+
+ (*) Dissociate a cache [mandatory]:
+
+ void (*dissociate_pages)(struct fscache_cache *cache)
+
+ This is called to ask a cache to perform any page dissociations as part of
+ cache withdrawal.
+
+
+ (*) Notification that the attributes on a netfs file changed [mandatory]:
+
+ int (*attr_changed)(struct fscache_object *object);
+
+ This is called to indicate to the cache that certain attributes on a netfs
+ file have changed (for example the maximum size a file may reach). The
+ cache can read these from the netfs by calling the cookie's get_attr()
+ method.
+
+ The cache may use the file size information to reserve space on the cache.
+ It should also call fscache_set_store_limit() to indicate to FS-Cache the
+ highest byte it's willing to store for an object.
+
+ This method may return -ve if an error occurred or the cache object cannot
+ be expanded. In such a case, the object will be withdrawn from service.
+
+ This operation is run asynchronously from FS-Cache's thread pool, and
+ storage and retrieval operations from the netfs are excluded during the
+ execution of this operation.
+
+
+ (*) Reserve cache space for an object's data [optional]:
+
+ int (*reserve_space)(struct fscache_object *object, loff_t size);
+
+ This is called to request that cache space be reserved to hold the data
+ for an object and the metadata used to track it. Zero size should be
+ taken as request to cancel a reservation.
+
+ This should return 0 if successful, -ENOSPC if there isn't enough space
+ available, or -ENOMEM or -EIO on other errors.
+
+ The reservation may exceed the current size of the object, thus permitting
+ future expansion. If the amount of space consumed by an object would
+ exceed the reservation, it's permitted to refuse requests to allocate
+ pages, but not required. An object may be pruned down to its reservation
+ size if larger than that already.
+
+
+ (*) Request page be read from cache [mandatory]:
+
+ int (*read_or_alloc_page)(struct fscache_retrieval *op,
+ struct page *page,
+ gfp_t gfp)
+
+ This is called to attempt to read a netfs page from the cache, or to
+ reserve a backing block if not. FS-Cache will have done as much checking
+ as it can before calling, but most of the work belongs to the backend.
+
+ If there's no page in the cache, then -ENODATA should be returned if the
+ backend managed to reserve a backing block; -ENOBUFS or -ENOMEM if it
+ didn't.
+
+ If there is suitable data in the cache, then a read operation should be
+ queued and 0 returned. When the read finishes, fscache_end_io() should be
+ called.
+
+ The fscache_mark_pages_cached() should be called for the page if any cache
+ metadata is retained. This will indicate to the netfs that the page needs
+ explicit uncaching. This operation takes a pagevec, thus allowing several
+ pages to be marked at once.
+
+ The retrieval record pointed to by op should be retained for each page
+ queued and released when I/O on the page has been formally ended.
+ fscache_get/put_retrieval() are available for this purpose.
+
+ The retrieval record may be used to get CPU time via the FS-Cache thread
+ pool. If this is desired, the op->op.processor should be set to point to
+ the appropriate processing routine, and fscache_enqueue_retrieval() should
+ be called at an appropriate point to request CPU time. For instance, the
+ retrieval routine could be enqueued upon the completion of a disk read.
+ The to_do field in the retrieval record is provided to aid in this.
+
+ If an I/O error occurs, fscache_io_error() should be called and -ENOBUFS
+ returned if possible or fscache_end_io() called with a suitable error
+ code..
+
+
+ (*) Request pages be read from cache [mandatory]:
+
+ int (*read_or_alloc_pages)(struct fscache_retrieval *op,
+ struct list_head *pages,
+ unsigned *nr_pages,
+ gfp_t gfp)
+
+ This is like the read_or_alloc_page() method, except it is handed a list
+ of pages instead of one page. Any pages on which a read operation is
+ started must be added to the page cache for the specified mapping and also
+ to the LRU. Such pages must also be removed from the pages list and
+ *nr_pages decremented per page.
+
+ If there was an error such as -ENOMEM, then that should be returned; else
+ if one or more pages couldn't be read or allocated, then -ENOBUFS should
+ be returned; else if one or more pages couldn't be read, then -ENODATA
+ should be returned. If all the pages are dispatched then 0 should be
+ returned.
+
+
+ (*) Request page be allocated in the cache [mandatory]:
+
+ int (*allocate_page)(struct fscache_retrieval *op,
+ struct page *page,
+ gfp_t gfp)
+
+ This is like the read_or_alloc_page() method, except that it shouldn't
+ read from the cache, even if there's data there that could be retrieved.
+ It should, however, set up any internal metadata required such that
+ the write_page() method can write to the cache.
+
+ If there's no backing block available, then -ENOBUFS should be returned
+ (or -ENOMEM if there were other problems). If a block is successfully
+ allocated, then the netfs page should be marked and 0 returned.
+
+
+ (*) Request pages be allocated in the cache [mandatory]:
+
+ int (*allocate_pages)(struct fscache_retrieval *op,
+ struct list_head *pages,
+ unsigned *nr_pages,
+ gfp_t gfp)
+
+ This is an multiple page version of the allocate_page() method. pages and
+ nr_pages should be treated as for the read_or_alloc_pages() method.
+
+
+ (*) Request page be written to cache [mandatory]:
+
+ int (*write_page)(struct fscache_storage *op,
+ struct page *page);
+
+ This is called to write from a page on which there was a previously
+ successful read_or_alloc_page() call or similar. FS-Cache filters out
+ pages that don't have mappings.
+
+ This method is called asynchronously from the FS-Cache thread pool. It is
+ not required to actually store anything, provided -ENODATA is then
+ returned to the next read of this page.
+
+ If an error occurred, then a negative error code should be returned,
+ otherwise zero should be returned. FS-Cache will take appropriate action
+ in response to an error, such as withdrawing this object.
+
+ If this method returns success then FS-Cache will inform the netfs
+ appropriately.
+
+
+ (*) Discard retained per-page metadata [mandatory]:
+
+ void (*uncache_page)(struct fscache_object *object, struct page *page)
+
+ This is called when a netfs page is being evicted from the pagecache. The
+ cache backend should tear down any internal representation or tracking it
+ maintains for this page.
+
+
+==================
+FS-CACHE UTILITIES
+==================
+
+FS-Cache provides some utilities that a cache backend may make use of:
+
+ (*) Note occurrence of an I/O error in a cache:
+
+ void fscache_io_error(struct fscache_cache *cache)
+
+ This tells FS-Cache that an I/O error occurred in the cache. After this
+ has been called, only resource dissociation operations (object and page
+ release) will be passed from the netfs to the cache backend for the
+ specified cache.
+
+ This does not actually withdraw the cache. That must be done separately.
+
+
+ (*) Invoke the retrieval I/O completion function:
+
+ void fscache_end_io(struct fscache_retrieval *op, struct page *page,
+ int error);
+
+ This is called to note the end of an attempt to retrieve a page. The
+ error value should be 0 if successful and an error otherwise.
+
+
+ (*) Set highest store limit:
+
+ void fscache_set_store_limit(struct fscache_object *object,
+ loff_t i_size);
+
+ This sets the limit FS-Cache imposes on the highest byte it's willing to
+ try and store for a netfs. Any page over this limit is automatically
+ rejected by fscache_read_alloc_page() and co with -ENOBUFS.
+
+
+ (*) Mark pages as being cached:
+
+ void fscache_mark_pages_cached(struct fscache_retrieval *op,
+ struct pagevec *pagevec);
+
+ This marks a set of pages as being cached. After this has been called,
+ the netfs must call fscache_uncache_page() to unmark the pages.
+
+
+ (*) Perform coherency check on an object:
+
+ enum fscache_checkaux fscache_check_aux(struct fscache_object *object,
+ const void *data,
+ uint16_t datalen);
+
+ This asks the netfs to perform a coherency check on an object that has
+ just been looked up. The cookie attached to the object will determine the
+ netfs to use. data and datalen should specify where the auxiliary data
+ retrieved from the cache can be found.
+
+ One of three values will be returned:
+
+ (*) FSCACHE_CHECKAUX_OKAY
+
+ The coherency data indicates the object is valid as is.
+
+ (*) FSCACHE_CHECKAUX_NEEDS_UPDATE
+
+ The coherency data needs updating, but otherwise the object is
+ valid.
+
+ (*) FSCACHE_CHECKAUX_OBSOLETE
+
+ The coherency data indicates that the object is obsolete and should
+ be discarded.
+
+
+ (*) Initialise a freshly allocated object:
+
+ void fscache_object_init(struct fscache_object *object);
+
+ This initialises all the fields in an object representation.
+
+
+ (*) Indicate the destruction of an object:
+
+ void fscache_object_destroyed(struct fscache_cache *cache);
+
+ This must be called to inform FS-Cache that an object that belonged to a
+ cache has been destroyed and deallocated. This will allow continuation
+ of the cache withdrawal process when it is stopped pending destruction of
+ all the objects.
+
+
+ (*) Indicate negative lookup on an object:
+
+ void fscache_object_lookup_negative(struct fscache_object *object);
+
+ This is called to indicate to FS-Cache that a lookup process for an object
+ found a negative result.
+
+ This changes the state of an object to permit reads pending on lookup
+ completion to go off and start fetching data from the netfs server as it's
+ known at this point that there can't be any data in the cache.
+
+ This may be called multiple times on an object. Only the first call is
+ significant - all subsequent calls are ignored.
+
+
+ (*) Indicate an object has been obtained:
+
+ void fscache_obtained_object(struct fscache_object *object);
+
+ This is called to indicate to FS-Cache that a lookup process for an object
+ produced a positive result, or that an object was created. This should
+ only be called once for any particular object.
+
+ This changes the state of an object to indicate:
+
+ (1) if no call to fscache_object_lookup_negative() has been made on
+ this object, that there may be data available, and that reads can
+ now go and look for it; and
+
+ (2) that writes may now proceed against this object.
+
+
+ (*) Indicate that object lookup failed:
+
+ void fscache_object_lookup_error(struct fscache_object *object);
+
+ This marks an object as having encountered a fatal error (usually EIO)
+ and causes it to move into a state whereby it will be withdrawn as soon
+ as possible.
+
+
+ (*) Get and release references on a retrieval record:
+
+ void fscache_get_retrieval(struct fscache_retrieval *op);
+ void fscache_put_retrieval(struct fscache_retrieval *op);
+
+ These two functions are used to retain a retrieval record whilst doing
+ asynchronous data retrieval and block allocation.
+
+
+ (*) Enqueue a retrieval record for processing.
+
+ void fscache_enqueue_retrieval(struct fscache_retrieval *op);
+
+ This enqueues a retrieval record for processing by the FS-Cache thread
+ pool. One of the threads in the pool will invoke the retrieval record's
+ op->op.processor callback function. This function may be called from
+ within the callback function.
+
+
+ (*) List of object state names:
+
+ const char *fscache_object_states[];
+
+ For debugging purposes, this may be used to turn the state that an object
+ is in into a text string for display purposes.
diff --git a/Documentation/filesystems/caching/cachefiles.txt b/Documentation/filesystems/caching/cachefiles.txt
new file mode 100644
index 00000000000..c78a49b7bba
--- /dev/null
+++ b/Documentation/filesystems/caching/cachefiles.txt
@@ -0,0 +1,501 @@
+ ===============================================
+ CacheFiles: CACHE ON ALREADY MOUNTED FILESYSTEM
+ ===============================================
+
+Contents:
+
+ (*) Overview.
+
+ (*) Requirements.
+
+ (*) Configuration.
+
+ (*) Starting the cache.
+
+ (*) Things to avoid.
+
+ (*) Cache culling.
+
+ (*) Cache structure.
+
+ (*) Security model and SELinux.
+
+ (*) A note on security.
+
+ (*) Statistical information.
+
+ (*) Debugging.
+
+
+========
+OVERVIEW
+========
+
+CacheFiles is a caching backend that's meant to use as a cache a directory on
+an already mounted filesystem of a local type (such as Ext3).
+
+CacheFiles uses a userspace daemon to do some of the cache management - such as
+reaping stale nodes and culling. This is called cachefilesd and lives in
+/sbin.
+
+The filesystem and data integrity of the cache are only as good as those of the
+filesystem providing the backing services. Note that CacheFiles does not
+attempt to journal anything since the journalling interfaces of the various
+filesystems are very specific in nature.
+
+CacheFiles creates a misc character device - "/dev/cachefiles" - that is used
+to communication with the daemon. Only one thing may have this open at once,
+and whilst it is open, a cache is at least partially in existence. The daemon
+opens this and sends commands down it to control the cache.
+
+CacheFiles is currently limited to a single cache.
+
+CacheFiles attempts to maintain at least a certain percentage of free space on
+the filesystem, shrinking the cache by culling the objects it contains to make
+space if necessary - see the "Cache Culling" section. This means it can be
+placed on the same medium as a live set of data, and will expand to make use of
+spare space and automatically contract when the set of data requires more
+space.
+
+
+============
+REQUIREMENTS
+============
+
+The use of CacheFiles and its daemon requires the following features to be
+available in the system and in the cache filesystem:
+
+ - dnotify.
+
+ - extended attributes (xattrs).
+
+ - openat() and friends.
+
+ - bmap() support on files in the filesystem (FIBMAP ioctl).
+
+ - The use of bmap() to detect a partial page at the end of the file.
+
+It is strongly recommended that the "dir_index" option is enabled on Ext3
+filesystems being used as a cache.
+
+
+=============
+CONFIGURATION
+=============
+
+The cache is configured by a script in /etc/cachefilesd.conf. These commands
+set up cache ready for use. The following script commands are available:
+
+ (*) brun <N>%
+ (*) bcull <N>%
+ (*) bstop <N>%
+ (*) frun <N>%
+ (*) fcull <N>%
+ (*) fstop <N>%
+
+ Configure the culling limits. Optional. See the section on culling
+ The defaults are 7% (run), 5% (cull) and 1% (stop) respectively.
+
+ The commands beginning with a 'b' are file space (block) limits, those
+ beginning with an 'f' are file count limits.
+
+ (*) dir <path>
+
+ Specify the directory containing the root of the cache. Mandatory.
+
+ (*) tag <name>
+
+ Specify a tag to FS-Cache to use in distinguishing multiple caches.
+ Optional. The default is "CacheFiles".
+
+ (*) debug <mask>
+
+ Specify a numeric bitmask to control debugging in the kernel module.
+ Optional. The default is zero (all off). The following values can be
+ OR'd into the mask to collect various information:
+
+ 1 Turn on trace of function entry (_enter() macros)
+ 2 Turn on trace of function exit (_leave() macros)
+ 4 Turn on trace of internal debug points (_debug())
+
+ This mask can also be set through sysfs, eg:
+
+ echo 5 >/sys/modules/cachefiles/parameters/debug
+
+
+==================
+STARTING THE CACHE
+==================
+
+The cache is started by running the daemon. The daemon opens the cache device,
+configures the cache and tells it to begin caching. At that point the cache
+binds to fscache and the cache becomes live.
+
+The daemon is run as follows:
+
+ /sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>]
+
+The flags are:
+
+ (*) -d
+
+ Increase the debugging level. This can be specified multiple times and
+ is cumulative with itself.
+
+ (*) -s
+
+ Send messages to stderr instead of syslog.
+
+ (*) -n
+
+ Don't daemonise and go into background.
+
+ (*) -f <configfile>
+
+ Use an alternative configuration file rather than the default one.
+
+
+===============
+THINGS TO AVOID
+===============
+
+Do not mount other things within the cache as this will cause problems. The
+kernel module contains its own very cut-down path walking facility that ignores
+mountpoints, but the daemon can't avoid them.
+
+Do not create, rename or unlink files and directories in the cache whilst the
+cache is active, as this may cause the state to become uncertain.
+
+Renaming files in the cache might make objects appear to be other objects (the
+filename is part of the lookup key).
+
+Do not change or remove the extended attributes attached to cache files by the
+cache as this will cause the cache state management to get confused.
+
+Do not create files or directories in the cache, lest the cache get confused or
+serve incorrect data.
+
+Do not chmod files in the cache. The module creates things with minimal
+permissions to prevent random users being able to access them directly.
+
+
+=============
+CACHE CULLING
+=============
+
+The cache may need culling occasionally to make space. This involves
+discarding objects from the cache that have been used less recently than
+anything else. Culling is based on the access time of data objects. Empty
+directories are culled if not in use.
+
+Cache culling is done on the basis of the percentage of blocks and the
+percentage of files available in the underlying filesystem. There are six
+"limits":
+
+ (*) brun
+ (*) frun
+
+ If the amount of free space and the number of available files in the cache
+ rises above both these limits, then culling is turned off.
+
+ (*) bcull
+ (*) fcull
+
+ If the amount of available space or the number of available files in the
+ cache falls below either of these limits, then culling is started.
+
+ (*) bstop
+ (*) fstop
+
+ If the amount of available space or the number of available files in the
+ cache falls below either of these limits, then no further allocation of
+ disk space or files is permitted until culling has raised things above
+ these limits again.
+
+These must be configured thusly:
+
+ 0 <= bstop < bcull < brun < 100
+ 0 <= fstop < fcull < frun < 100
+
+Note that these are percentages of available space and available files, and do
+_not_ appear as 100 minus the percentage displayed by the "df" program.
+
+The userspace daemon scans the cache to build up a table of cullable objects.
+These are then culled in least recently used order. A new scan of the cache is
+started as soon as space is made in the table. Objects will be skipped if
+their atimes have changed or if the kernel module says it is still using them.
+
+
+===============
+CACHE STRUCTURE
+===============
+
+The CacheFiles module will create two directories in the directory it was
+given:
+
+ (*) cache/
+
+ (*) graveyard/
+
+The active cache objects all reside in the first directory. The CacheFiles
+kernel module moves any retired or culled objects that it can't simply unlink
+to the graveyard from which the daemon will actually delete them.
+
+The daemon uses dnotify to monitor the graveyard directory, and will delete
+anything that appears therein.
+
+
+The module represents index objects as directories with the filename "I..." or
+"J...". Note that the "cache/" directory is itself a special index.
+
+Data objects are represented as files if they have no children, or directories
+if they do. Their filenames all begin "D..." or "E...". If represented as a
+directory, data objects will have a file in the directory called "data" that
+actually holds the data.
+
+Special objects are similar to data objects, except their filenames begin
+"S..." or "T...".
+
+
+If an object has children, then it will be represented as a directory.
+Immediately in the representative directory are a collection of directories
+named for hash values of the child object keys with an '@' prepended. Into
+this directory, if possible, will be placed the representations of the child
+objects:
+
+ INDEX INDEX INDEX DATA FILES
+ ========= ========== ================================= ================
+ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400
+ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry
+ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry
+ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry
+
+
+If the key is so long that it exceeds NAME_MAX with the decorations added on to
+it, then it will be cut into pieces, the first few of which will be used to
+make a nest of directories, and the last one of which will be the objects
+inside the last directory. The names of the intermediate directories will have
+'+' prepended:
+
+ J1223/@23/+xy...z/+kl...m/Epqr
+
+
+Note that keys are raw data, and not only may they exceed NAME_MAX in size,
+they may also contain things like '/' and NUL characters, and so they may not
+be suitable for turning directly into a filename.
+
+To handle this, CacheFiles will use a suitably printable filename directly and
+"base-64" encode ones that aren't directly suitable. The two versions of
+object filenames indicate the encoding:
+
+ OBJECT TYPE PRINTABLE ENCODED
+ =============== =============== ===============
+ Index "I..." "J..."
+ Data "D..." "E..."
+ Special "S..." "T..."
+
+Intermediate directories are always "@" or "+" as appropriate.
+
+
+Each object in the cache has an extended attribute label that holds the object
+type ID (required to distinguish special objects) and the auxiliary data from
+the netfs. The latter is used to detect stale objects in the cache and update
+or retire them.
+
+
+Note that CacheFiles will erase from the cache any file it doesn't recognise or
+any file of an incorrect type (such as a FIFO file or a device file).
+
+
+==========================
+SECURITY MODEL AND SELINUX
+==========================
+
+CacheFiles is implemented to deal properly with the LSM security features of
+the Linux kernel and the SELinux facility.
+
+One of the problems that CacheFiles faces is that it is generally acting on
+behalf of a process, and running in that process's context, and that includes a
+security context that is not appropriate for accessing the cache - either
+because the files in the cache are inaccessible to that process, or because if
+the process creates a file in the cache, that file may be inaccessible to other
+processes.
+
+The way CacheFiles works is to temporarily change the security context (fsuid,
+fsgid and actor security label) that the process acts as - without changing the
+security context of the process when it the target of an operation performed by
+some other process (so signalling and suchlike still work correctly).
+
+
+When the CacheFiles module is asked to bind to its cache, it:
+
+ (1) Finds the security label attached to the root cache directory and uses
+ that as the security label with which it will create files. By default,
+ this is:
+
+ cachefiles_var_t
+
+ (2) Finds the security label of the process which issued the bind request
+ (presumed to be the cachefilesd daemon), which by default will be:
+
+ cachefilesd_t
+
+ and asks LSM to supply a security ID as which it should act given the
+ daemon's label. By default, this will be:
+
+ cachefiles_kernel_t
+
+ SELinux transitions the daemon's security ID to the module's security ID
+ based on a rule of this form in the policy.
+
+ type_transition <daemon's-ID> kernel_t : process <module's-ID>;
+
+ For instance:
+
+ type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t;
+
+
+The module's security ID gives it permission to create, move and remove files
+and directories in the cache, to find and access directories and files in the
+cache, to set and access extended attributes on cache objects, and to read and
+write files in the cache.
+
+The daemon's security ID gives it only a very restricted set of permissions: it
+may scan directories, stat files and erase files and directories. It may
+not read or write files in the cache, and so it is precluded from accessing the
+data cached therein; nor is it permitted to create new files in the cache.
+
+
+There are policy source files available in:
+
+ http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2
+
+and later versions. In that tarball, see the files:
+
+ cachefilesd.te
+ cachefilesd.fc
+ cachefilesd.if
+
+They are built and installed directly by the RPM.
+
+If a non-RPM based system is being used, then copy the above files to their own
+directory and run:
+
+ make -f /usr/share/selinux/devel/Makefile
+ semodule -i cachefilesd.pp
+
+You will need checkpolicy and selinux-policy-devel installed prior to the
+build.
+
+
+By default, the cache is located in /var/fscache, but if it is desirable that
+it should be elsewhere, than either the above policy files must be altered, or
+an auxiliary policy must be installed to label the alternate location of the
+cache.
+
+For instructions on how to add an auxiliary policy to enable the cache to be
+located elsewhere when SELinux is in enforcing mode, please see:
+
+ /usr/share/doc/cachefilesd-*/move-cache.txt
+
+When the cachefilesd rpm is installed; alternatively, the document can be found
+in the sources.
+
+
+==================
+A NOTE ON SECURITY
+==================
+
+CacheFiles makes use of the split security in the task_struct. It allocates
+its own task_security structure, and redirects current->act_as to point to it
+when it acts on behalf of another process, in that process's context.
+
+The reason it does this is that it calls vfs_mkdir() and suchlike rather than
+bypassing security and calling inode ops directly. Therefore the VFS and LSM
+may deny the CacheFiles access to the cache data because under some
+circumstances the caching code is running in the security context of whatever
+process issued the original syscall on the netfs.
+
+Furthermore, should CacheFiles create a file or directory, the security
+parameters with that object is created (UID, GID, security label) would be
+derived from that process that issued the system call, thus potentially
+preventing other processes from accessing the cache - including CacheFiles's
+cache management daemon (cachefilesd).
+
+What is required is to temporarily override the security of the process that
+issued the system call. We can't, however, just do an in-place change of the
+security data as that affects the process as an object, not just as a subject.
+This means it may lose signals or ptrace events for example, and affects what
+the process looks like in /proc.
+
+So CacheFiles makes use of a logical split in the security between the
+objective security (task->sec) and the subjective security (task->act_as). The
+objective security holds the intrinsic security properties of a process and is
+never overridden. This is what appears in /proc, and is what is used when a
+process is the target of an operation by some other process (SIGKILL for
+example).
+
+The subjective security holds the active security properties of a process, and
+may be overridden. This is not seen externally, and is used whan a process
+acts upon another object, for example SIGKILLing another process or opening a
+file.
+
+LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request
+for CacheFiles to run in a context of a specific security label, or to create
+files and directories with another security label.
+
+
+=======================
+STATISTICAL INFORMATION
+=======================
+
+If FS-Cache is compiled with the following option enabled:
+
+ CONFIG_CACHEFILES_HISTOGRAM=y
+
+then it will gather certain statistics and display them through a proc file.
+
+ (*) /proc/fs/cachefiles/histogram
+
+ cat /proc/fs/cachefiles/histogram
+ JIFS SECS LOOKUPS MKDIRS CREATES
+ ===== ===== ========= ========= =========
+
+ This shows the breakdown of the number of times each amount of time
+ between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The
+ columns are as follows:
+
+ COLUMN TIME MEASUREMENT
+ ======= =======================================================
+ LOOKUPS Length of time to perform a lookup on the backing fs
+ MKDIRS Length of time to perform a mkdir on the backing fs
+ CREATES Length of time to perform a create on the backing fs
+
+ Each row shows the number of events that took a particular range of times.
+ Each step is 1 jiffy in size. The JIFS column indicates the particular
+ jiffy range covered, and the SECS field the equivalent number of seconds.
+
+
+=========
+DEBUGGING
+=========
+
+If CONFIG_CACHEFILES_DEBUG is enabled, the CacheFiles facility can have runtime
+debugging enabled by adjusting the value in:
+
+ /sys/module/cachefiles/parameters/debug
+
+This is a bitmask of debugging streams to enable:
+
+ BIT VALUE STREAM POINT
+ ======= ======= =============================== =======================
+ 0 1 General Function entry trace
+ 1 2 Function exit trace
+ 2 4 General
+
+The appropriate set of values should be OR'd together and the result written to
+the control file. For example:
+
+ echo $((1|4|8)) >/sys/module/cachefiles/parameters/debug
+
+will turn on all function entry debugging.
diff --git a/Documentation/filesystems/caching/fscache.txt b/Documentation/filesystems/caching/fscache.txt
new file mode 100644
index 00000000000..9e94b9491d8
--- /dev/null
+++ b/Documentation/filesystems/caching/fscache.txt
@@ -0,0 +1,333 @@
+ ==========================
+ General Filesystem Caching
+ ==========================
+
+========
+OVERVIEW
+========
+
+This facility is a general purpose cache for network filesystems, though it
+could be used for caching other things such as ISO9660 filesystems too.
+
+FS-Cache mediates between cache backends (such as CacheFS) and network
+filesystems:
+
+ +---------+
+ | | +--------------+
+ | NFS |--+ | |
+ | | | +-->| CacheFS |
+ +---------+ | +----------+ | | /dev/hda5 |
+ | | | | +--------------+
+ +---------+ +-->| | |
+ | | | |--+
+ | AFS |----->| FS-Cache |
+ | | | |--+
+ +---------+ +-->| | |
+ | | | | +--------------+
+ +---------+ | +----------+ | | |
+ | | | +-->| CacheFiles |
+ | ISOFS |--+ | /var/cache |
+ | | +--------------+
+ +---------+
+
+Or to look at it another way, FS-Cache is a module that provides a caching
+facility to a network filesystem such that the cache is transparent to the
+user:
+
+ +---------+
+ | |
+ | Server |
+ | |
+ +---------+
+ | NETWORK
+ ~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ |
+ | +----------+
+ V | |
+ +---------+ | |
+ | | | |
+ | NFS |----->| FS-Cache |
+ | | | |--+
+ +---------+ | | | +--------------+ +--------------+
+ | | | | | | | |
+ V +----------+ +-->| CacheFiles |-->| Ext3 |
+ +---------+ | /var/cache | | /dev/sda6 |
+ | | +--------------+ +--------------+
+ | VFS | ^ ^
+ | | | |
+ +---------+ +--------------+ |
+ | KERNEL SPACE | |
+ ~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|~~~~~~|~~~~
+ | USER SPACE | |
+ V | |
+ +---------+ +--------------+
+ | | | |
+ | Process | | cachefilesd |
+ | | | |
+ +---------+ +--------------+
+
+
+FS-Cache does not follow the idea of completely loading every netfs file
+opened in its entirety into a cache before permitting it to be accessed and
+then serving the pages out of that cache rather than the netfs inode because:
+
+ (1) It must be practical to operate without a cache.
+
+ (2) The size of any accessible file must not be limited to the size of the
+ cache.
+
+ (3) The combined size of all opened files (this includes mapped libraries)
+ must not be limited to the size of the cache.
+
+ (4) The user should not be forced to download an entire file just to do a
+ one-off access of a small portion of it (such as might be done with the
+ "file" program).
+
+It instead serves the cache out in PAGE_SIZE chunks as and when requested by
+the netfs('s) using it.
+
+
+FS-Cache provides the following facilities:
+
+ (1) More than one cache can be used at once. Caches can be selected
+ explicitly by use of tags.
+
+ (2) Caches can be added / removed at any time.
+
+ (3) The netfs is provided with an interface that allows either party to
+ withdraw caching facilities from a file (required for (2)).
+
+ (4) The interface to the netfs returns as few errors as possible, preferring
+ rather to let the netfs remain oblivious.
+
+ (5) Cookies are used to represent indices, files and other objects to the
+ netfs. The simplest cookie is just a NULL pointer - indicating nothing
+ cached there.
+
+ (6) The netfs is allowed to propose - dynamically - any index hierarchy it
+ desires, though it must be aware that the index search function is
+ recursive, stack space is limited, and indices can only be children of
+ indices.
+
+ (7) Data I/O is done direct to and from the netfs's pages. The netfs
+ indicates that page A is at index B of the data-file represented by cookie
+ C, and that it should be read or written. The cache backend may or may
+ not start I/O on that page, but if it does, a netfs callback will be
+ invoked to indicate completion. The I/O may be either synchronous or
+ asynchronous.
+
+ (8) Cookies can be "retired" upon release. At this point FS-Cache will mark
+ them as obsolete and the index hierarchy rooted at that point will get
+ recycled.
+
+ (9) The netfs provides a "match" function for index searches. In addition to
+ saying whether a match was made or not, this can also specify that an
+ entry should be updated or deleted.
+
+(10) As much as possible is done asynchronously.
+
+
+FS-Cache maintains a virtual indexing tree in which all indices, files, objects
+and pages are kept. Bits of this tree may actually reside in one or more
+caches.
+
+ FSDEF
+ |
+ +------------------------------------+
+ | |
+ NFS AFS
+ | |
+ +--------------------------+ +-----------+
+ | | | |
+ homedir mirror afs.org redhat.com
+ | | |
+ +------------+ +---------------+ +----------+
+ | | | | | |
+ 00001 00002 00007 00125 vol00001 vol00002
+ | | | | |
+ +---+---+ +-----+ +---+ +------+------+ +-----+----+
+ | | | | | | | | | | | | |
+PG0 PG1 PG2 PG0 XATTR PG0 PG1 DIRENT DIRENT DIRENT R/W R/O Bak
+ | |
+ PG0 +-------+
+ | |
+ 00001 00003
+ |
+ +---+---+
+ | | |
+ PG0 PG1 PG2
+
+In the example above, you can see two netfs's being backed: NFS and AFS. These
+have different index hierarchies:
+
+ (*) The NFS primary index contains per-server indices. Each server index is
+ indexed by NFS file handles to get data file objects. Each data file
+ objects can have an array of pages, but may also have further child
+ objects, such as extended attributes and directory entries. Extended
+ attribute objects themselves have page-array contents.
+
+ (*) The AFS primary index contains per-cell indices. Each cell index contains
+ per-logical-volume indices. Each of volume index contains up to three
+ indices for the read-write, read-only and backup mirrors of those volumes.
+ Each of these contains vnode data file objects, each of which contains an
+ array of pages.
+
+The very top index is the FS-Cache master index in which individual netfs's
+have entries.
+
+Any index object may reside in more than one cache, provided it only has index
+children. Any index with non-index object children will be assumed to only
+reside in one cache.
+
+
+The netfs API to FS-Cache can be found in:
+
+ Documentation/filesystems/caching/netfs-api.txt
+
+The cache backend API to FS-Cache can be found in:
+
+ Documentation/filesystems/caching/backend-api.txt
+
+A description of the internal representations and object state machine can be
+found in:
+
+ Documentation/filesystems/caching/object.txt
+
+
+=======================
+STATISTICAL INFORMATION
+=======================
+
+If FS-Cache is compiled with the following options enabled:
+
+ CONFIG_FSCACHE_STATS=y
+ CONFIG_FSCACHE_HISTOGRAM=y
+
+then it will gather certain statistics and display them through a number of
+proc files.
+
+ (*) /proc/fs/fscache/stats
+
+ This shows counts of a number of events that can happen in FS-Cache:
+
+ CLASS EVENT MEANING
+ ======= ======= =======================================================
+ Cookies idx=N Number of index cookies allocated
+ dat=N Number of data storage cookies allocated
+ spc=N Number of special cookies allocated
+ Objects alc=N Number of objects allocated
+ nal=N Number of object allocation failures
+ avl=N Number of objects that reached the available state
+ ded=N Number of objects that reached the dead state
+ ChkAux non=N Number of objects that didn't have a coherency check
+ ok=N Number of objects that passed a coherency check
+ upd=N Number of objects that needed a coherency data update
+ obs=N Number of objects that were declared obsolete
+ Pages mrk=N Number of pages marked as being cached
+ unc=N Number of uncache page requests seen
+ Acquire n=N Number of acquire cookie requests seen
+ nul=N Number of acq reqs given a NULL parent
+ noc=N Number of acq reqs rejected due to no cache available
+ ok=N Number of acq reqs succeeded
+ nbf=N Number of acq reqs rejected due to error
+ oom=N Number of acq reqs failed on ENOMEM
+ Lookups n=N Number of lookup calls made on cache backends
+ neg=N Number of negative lookups made
+ pos=N Number of positive lookups made
+ crt=N Number of objects created by lookup
+ Updates n=N Number of update cookie requests seen
+ nul=N Number of upd reqs given a NULL parent
+ run=N Number of upd reqs granted CPU time
+ Relinqs n=N Number of relinquish cookie requests seen
+ nul=N Number of rlq reqs given a NULL parent
+ wcr=N Number of rlq reqs waited on completion of creation
+ AttrChg n=N Number of attribute changed requests seen
+ ok=N Number of attr changed requests queued
+ nbf=N Number of attr changed rejected -ENOBUFS
+ oom=N Number of attr changed failed -ENOMEM
+ run=N Number of attr changed ops given CPU time
+ Allocs n=N Number of allocation requests seen
+ ok=N Number of successful alloc reqs
+ wt=N Number of alloc reqs that waited on lookup completion
+ nbf=N Number of alloc reqs rejected -ENOBUFS
+ ops=N Number of alloc reqs submitted
+ owt=N Number of alloc reqs waited for CPU time
+ Retrvls n=N Number of retrieval (read) requests seen
+ ok=N Number of successful retr reqs
+ wt=N Number of retr reqs that waited on lookup completion
+ nod=N Number of retr reqs returned -ENODATA
+ nbf=N Number of retr reqs rejected -ENOBUFS
+ int=N Number of retr reqs aborted -ERESTARTSYS
+ oom=N Number of retr reqs failed -ENOMEM
+ ops=N Number of retr reqs submitted
+ owt=N Number of retr reqs waited for CPU time
+ Stores n=N Number of storage (write) requests seen
+ ok=N Number of successful store reqs
+ agn=N Number of store reqs on a page already pending storage
+ nbf=N Number of store reqs rejected -ENOBUFS
+ oom=N Number of store reqs failed -ENOMEM
+ ops=N Number of store reqs submitted
+ run=N Number of store reqs granted CPU time
+ Ops pend=N Number of times async ops added to pending queues
+ run=N Number of times async ops given CPU time
+ enq=N Number of times async ops queued for processing
+ dfr=N Number of async ops queued for deferred release
+ rel=N Number of async ops released
+ gc=N Number of deferred-release async ops garbage collected
+
+
+ (*) /proc/fs/fscache/histogram
+
+ cat /proc/fs/fscache/histogram
+ JIFS SECS OBJ INST OP RUNS OBJ RUNS RETRV DLY RETRIEVLS
+ ===== ===== ========= ========= ========= ========= =========
+
+ This shows the breakdown of the number of times each amount of time
+ between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The
+ columns are as follows:
+
+ COLUMN TIME MEASUREMENT
+ ======= =======================================================
+ OBJ INST Length of time to instantiate an object
+ OP RUNS Length of time a call to process an operation took
+ OBJ RUNS Length of time a call to process an object event took
+ RETRV DLY Time between an requesting a read and lookup completing
+ RETRIEVLS Time between beginning and end of a retrieval
+
+ Each row shows the number of events that took a particular range of times.
+ Each step is 1 jiffy in size. The JIFS column indicates the particular
+ jiffy range covered, and the SECS field the equivalent number of seconds.
+
+
+=========
+DEBUGGING
+=========
+
+If CONFIG_FSCACHE_DEBUG is enabled, the FS-Cache facility can have runtime
+debugging enabled by adjusting the value in:
+
+ /sys/module/fscache/parameters/debug
+
+This is a bitmask of debugging streams to enable:
+
+ BIT VALUE STREAM POINT
+ ======= ======= =============================== =======================
+ 0 1 Cache management Function entry trace
+ 1 2 Function exit trace
+ 2 4 General
+ 3 8 Cookie management Function entry trace
+ 4 16 Function exit trace
+ 5 32 General
+ 6 64 Page handling Function entry trace
+ 7 128 Function exit trace
+ 8 256 General
+ 9 512 Operation management Function entry trace
+ 10 1024 Function exit trace
+ 11 2048 General
+
+The appropriate set of values should be OR'd together and the result written to
+the control file. For example:
+
+ echo $((1|8|64)) >/sys/module/fscache/parameters/debug
+
+will turn on all function entry debugging.
diff --git a/Documentation/filesystems/caching/netfs-api.txt b/Documentation/filesystems/caching/netfs-api.txt
new file mode 100644
index 00000000000..4db125b3a5c
--- /dev/null
+++ b/Documentation/filesystems/caching/netfs-api.txt
@@ -0,0 +1,778 @@
+ ===============================
+ FS-CACHE NETWORK FILESYSTEM API
+ ===============================
+
+There's an API by which a network filesystem can make use of the FS-Cache
+facilities. This is based around a number of principles:
+
+ (1) Caches can store a number of different object types. There are two main
+ object types: indices and files. The first is a special type used by
+ FS-Cache to make finding objects faster and to make retiring of groups of
+ objects easier.
+
+ (2) Every index, file or other object is represented by a cookie. This cookie
+ may or may not have anything associated with it, but the netfs doesn't
+ need to care.
+
+ (3) Barring the top-level index (one entry per cached netfs), the index
+ hierarchy for each netfs is structured according the whim of the netfs.
+
+This API is declared in <linux/fscache.h>.
+
+This document contains the following sections:
+
+ (1) Network filesystem definition
+ (2) Index definition
+ (3) Object definition
+ (4) Network filesystem (un)registration
+ (5) Cache tag lookup
+ (6) Index registration
+ (7) Data file registration
+ (8) Miscellaneous object registration
+ (9) Setting the data file size
+ (10) Page alloc/read/write
+ (11) Page uncaching
+ (12) Index and data file update
+ (13) Miscellaneous cookie operations
+ (14) Cookie unregistration
+ (15) Index and data file invalidation
+ (16) FS-Cache specific page flags.
+
+
+=============================
+NETWORK FILESYSTEM DEFINITION
+=============================
+
+FS-Cache needs a description of the network filesystem. This is specified
+using a record of the following structure:
+
+ struct fscache_netfs {
+ uint32_t version;
+ const char *name;
+ struct fscache_cookie *primary_index;
+ ...
+ };
+
+This first two fields should be filled in before registration, and the third
+will be filled in by the registration function; any other fields should just be
+ignored and are for internal use only.
+
+The fields are:
+
+ (1) The name of the netfs (used as the key in the toplevel index).
+
+ (2) The version of the netfs (if the name matches but the version doesn't, the
+ entire in-cache hierarchy for this netfs will be scrapped and begun
+ afresh).
+
+ (3) The cookie representing the primary index will be allocated according to
+ another parameter passed into the registration function.
+
+For example, kAFS (linux/fs/afs/) uses the following definitions to describe
+itself:
+
+ struct fscache_netfs afs_cache_netfs = {
+ .version = 0,
+ .name = "afs",
+ };
+
+
+================
+INDEX DEFINITION
+================
+
+Indices are used for two purposes:
+
+ (1) To aid the finding of a file based on a series of keys (such as AFS's
+ "cell", "volume ID", "vnode ID").
+
+ (2) To make it easier to discard a subset of all the files cached based around
+ a particular key - for instance to mirror the removal of an AFS volume.
+
+However, since it's unlikely that any two netfs's are going to want to define
+their index hierarchies in quite the same way, FS-Cache tries to impose as few
+restraints as possible on how an index is structured and where it is placed in
+the tree. The netfs can even mix indices and data files at the same level, but
+it's not recommended.
+
+Each index entry consists of a key of indeterminate length plus some auxilliary
+data, also of indeterminate length.
+
+There are some limits on indices:
+
+ (1) Any index containing non-index objects should be restricted to a single
+ cache. Any such objects created within an index will be created in the
+ first cache only. The cache in which an index is created can be
+ controlled by cache tags (see below).
+
+ (2) The entry data must be atomically journallable, so it is limited to about
+ 400 bytes at present. At least 400 bytes will be available.
+
+ (3) The depth of the index tree should be judged with care as the search
+ function is recursive. Too many layers will run the kernel out of stack.
+
+
+=================
+OBJECT DEFINITION
+=================
+
+To define an object, a structure of the following type should be filled out:
+
+ struct fscache_cookie_def
+ {
+ uint8_t name[16];
+ uint8_t type;
+
+ struct fscache_cache_tag *(*select_cache)(
+ const void *parent_netfs_data,
+ const void *cookie_netfs_data);
+
+ uint16_t (*get_key)(const void *cookie_netfs_data,
+ void *buffer,
+ uint16_t bufmax);
+
+ void (*get_attr)(const void *cookie_netfs_data,
+ uint64_t *size);
+
+ uint16_t (*get_aux)(const void *cookie_netfs_data,
+ void *buffer,
+ uint16_t bufmax);
+
+ enum fscache_checkaux (*check_aux)(void *cookie_netfs_data,
+ const void *data,
+ uint16_t datalen);
+
+ void (*get_context)(void *cookie_netfs_data, void *context);
+
+ void (*put_context)(void *cookie_netfs_data, void *context);
+
+ void (*mark_pages_cached)(void *cookie_netfs_data,
+ struct address_space *mapping,
+ struct pagevec *cached_pvec);
+
+ void (*now_uncached)(void *cookie_netfs_data);
+ };
+
+This has the following fields:
+
+ (1) The type of the object [mandatory].
+
+ This is one of the following values:
+
+ (*) FSCACHE_COOKIE_TYPE_INDEX
+
+ This defines an index, which is a special FS-Cache type.
+
+ (*) FSCACHE_COOKIE_TYPE_DATAFILE
+
+ This defines an ordinary data file.
+
+ (*) Any other value between 2 and 255
+
+ This defines an extraordinary object such as an XATTR.
+
+ (2) The name of the object type (NUL terminated unless all 16 chars are used)
+ [optional].
+
+ (3) A function to select the cache in which to store an index [optional].
+
+ This function is invoked when an index needs to be instantiated in a cache
+ during the instantiation of a non-index object. Only the immediate index
+ parent for the non-index object will be queried. Any indices above that
+ in the hierarchy may be stored in multiple caches. This function does not
+ need to be supplied for any non-index object or any index that will only
+ have index children.
+
+ If this function is not supplied or if it returns NULL then the first
+ cache in the parent's list will be chosed, or failing that, the first
+ cache in the master list.
+
+ (4) A function to retrieve an object's key from the netfs [mandatory].
+
+ This function will be called with the netfs data that was passed to the
+ cookie acquisition function and the maximum length of key data that it may
+ provide. It should write the required key data into the given buffer and
+ return the quantity it wrote.
+
+ (5) A function to retrieve attribute data from the netfs [optional].
+
+ This function will be called with the netfs data that was passed to the
+ cookie acquisition function. It should return the size of the file if
+ this is a data file. The size may be used to govern how much cache must
+ be reserved for this file in the cache.
+
+ If the function is absent, a file size of 0 is assumed.
+
+ (6) A function to retrieve auxilliary data from the netfs [optional].
+
+ This function will be called with the netfs data that was passed to the
+ cookie acquisition function and the maximum length of auxilliary data that
+ it may provide. It should write the auxilliary data into the given buffer
+ and return the quantity it wrote.
+
+ If this function is absent, the auxilliary data length will be set to 0.
+
+ The length of the auxilliary data buffer may be dependent on the key
+ length. A netfs mustn't rely on being able to provide more than 400 bytes
+ for both.
+
+ (7) A function to check the auxilliary data [optional].
+
+ This function will be called to check that a match found in the cache for
+ this object is valid. For instance with AFS it could check the auxilliary
+ data against the data version number returned by the server to determine
+ whether the index entry in a cache is still valid.
+
+ If this function is absent, it will be assumed that matching objects in a
+ cache are always valid.
+
+ If present, the function should return one of the following values:
+
+ (*) FSCACHE_CHECKAUX_OKAY - the entry is okay as is
+ (*) FSCACHE_CHECKAUX_NEEDS_UPDATE - the entry requires update
+ (*) FSCACHE_CHECKAUX_OBSOLETE - the entry should be deleted
+
+ This function can also be used to extract data from the auxilliary data in
+ the cache and copy it into the netfs's structures.
+
+ (8) A pair of functions to manage contexts for the completion callback
+ [optional].
+
+ The cache read/write functions are passed a context which is then passed
+ to the I/O completion callback function. To ensure this context remains
+ valid until after the I/O completion is called, two functions may be
+ provided: one to get an extra reference on the context, and one to drop a
+ reference to it.
+
+ If the context is not used or is a type of object that won't go out of
+ scope, then these functions are not required. These functions are not
+ required for indices as indices may not contain data. These functions may
+ be called in interrupt context and so may not sleep.
+
+ (9) A function to mark a page as retaining cache metadata [optional].
+
+ This is called by the cache to indicate that it is retaining in-memory
+ information for this page and that the netfs should uncache the page when
+ it has finished. This does not indicate whether there's data on the disk
+ or not. Note that several pages at once may be presented for marking.
+
+ The PG_fscache bit is set on the pages before this function would be
+ called, so the function need not be provided if this is sufficient.
+
+ This function is not required for indices as they're not permitted data.
+
+(10) A function to unmark all the pages retaining cache metadata [mandatory].
+
+ This is called by FS-Cache to indicate that a backing store is being
+ unbound from a cookie and that all the marks on the pages should be
+ cleared to prevent confusion. Note that the cache will have torn down all
+ its tracking information so that the pages don't need to be explicitly
+ uncached.
+
+ This function is not required for indices as they're not permitted data.
+
+
+===================================
+NETWORK FILESYSTEM (UN)REGISTRATION
+===================================
+
+The first step is to declare the network filesystem to the cache. This also
+involves specifying the layout of the primary index (for AFS, this would be the
+"cell" level).
+
+The registration function is:
+
+ int fscache_register_netfs(struct fscache_netfs *netfs);
+
+It just takes a pointer to the netfs definition. It returns 0 or an error as
+appropriate.
+
+For kAFS, registration is done as follows:
+
+ ret = fscache_register_netfs(&afs_cache_netfs);
+
+The last step is, of course, unregistration:
+
+ void fscache_unregister_netfs(struct fscache_netfs *netfs);
+
+
+================
+CACHE TAG LOOKUP
+================
+
+FS-Cache permits the use of more than one cache. To permit particular index
+subtrees to be bound to particular caches, the second step is to look up cache
+representation tags. This step is optional; it can be left entirely up to
+FS-Cache as to which cache should be used. The problem with doing that is that
+FS-Cache will always pick the first cache that was registered.
+
+To get the representation for a named tag:
+
+ struct fscache_cache_tag *fscache_lookup_cache_tag(const char *name);
+
+This takes a text string as the name and returns a representation of a tag. It
+will never return an error. It may return a dummy tag, however, if it runs out
+of memory; this will inhibit caching with this tag.
+
+Any representation so obtained must be released by passing it to this function:
+
+ void fscache_release_cache_tag(struct fscache_cache_tag *tag);
+
+The tag will be retrieved by FS-Cache when it calls the object definition
+operation select_cache().
+
+
+==================
+INDEX REGISTRATION
+==================
+
+The third step is to inform FS-Cache about part of an index hierarchy that can
+be used to locate files. This is done by requesting a cookie for each index in
+the path to the file:
+
+ struct fscache_cookie *
+ fscache_acquire_cookie(struct fscache_cookie *parent,
+ const struct fscache_object_def *def,
+ void *netfs_data);
+
+This function creates an index entry in the index represented by parent,
+filling in the index entry by calling the operations pointed to by def.
+
+Note that this function never returns an error - all errors are handled
+internally. It may, however, return NULL to indicate no cookie. It is quite
+acceptable to pass this token back to this function as the parent to another
+acquisition (or even to the relinquish cookie, read page and write page
+functions - see below).
+
+Note also that no indices are actually created in a cache until a non-index
+object needs to be created somewhere down the hierarchy. Furthermore, an index
+may be created in several different caches independently at different times.
+This is all handled transparently, and the netfs doesn't see any of it.
+
+For example, with AFS, a cell would be added to the primary index. This index
+entry would have a dependent inode containing a volume location index for the
+volume mappings within this cell:
+
+ cell->cache =
+ fscache_acquire_cookie(afs_cache_netfs.primary_index,
+ &afs_cell_cache_index_def,
+ cell);
+
+Then when a volume location was accessed, it would be entered into the cell's
+index and an inode would be allocated that acts as a volume type and hash chain
+combination:
+
+ vlocation->cache =
+ fscache_acquire_cookie(cell->cache,
+ &afs_vlocation_cache_index_def,
+ vlocation);
+
+And then a particular flavour of volume (R/O for example) could be added to
+that index, creating another index for vnodes (AFS inode equivalents):
+
+ volume->cache =
+ fscache_acquire_cookie(vlocation->cache,
+ &afs_volume_cache_index_def,
+ volume);
+
+
+======================
+DATA FILE REGISTRATION
+======================
+
+The fourth step is to request a data file be created in the cache. This is
+identical to index cookie acquisition. The only difference is that the type in
+the object definition should be something other than index type.
+
+ vnode->cache =
+ fscache_acquire_cookie(volume->cache,
+ &afs_vnode_cache_object_def,
+ vnode);
+
+
+=================================
+MISCELLANEOUS OBJECT REGISTRATION
+=================================
+
+An optional step is to request an object of miscellaneous type be created in
+the cache. This is almost identical to index cookie acquisition. The only
+difference is that the type in the object definition should be something other
+than index type. Whilst the parent object could be an index, it's more likely
+it would be some other type of object such as a data file.
+
+ xattr->cache =
+ fscache_acquire_cookie(vnode->cache,
+ &afs_xattr_cache_object_def,
+ xattr);
+
+Miscellaneous objects might be used to store extended attributes or directory
+entries for example.
+
+
+==========================
+SETTING THE DATA FILE SIZE
+==========================
+
+The fifth step is to set the physical attributes of the file, such as its size.
+This doesn't automatically reserve any space in the cache, but permits the
+cache to adjust its metadata for data tracking appropriately:
+
+ int fscache_attr_changed(struct fscache_cookie *cookie);
+
+The cache will return -ENOBUFS if there is no backing cache or if there is no
+space to allocate any extra metadata required in the cache. The attributes
+will be accessed with the get_attr() cookie definition operation.
+
+Note that attempts to read or write data pages in the cache over this size may
+be rebuffed with -ENOBUFS.
+
+This operation schedules an attribute adjustment to happen asynchronously at
+some point in the future, and as such, it may happen after the function returns
+to the caller. The attribute adjustment excludes read and write operations.
+
+
+=====================
+PAGE READ/ALLOC/WRITE
+=====================
+
+And the sixth step is to store and retrieve pages in the cache. There are
+three functions that are used to do this.
+
+Note:
+
+ (1) A page should not be re-read or re-allocated without uncaching it first.
+
+ (2) A read or allocated page must be uncached when the netfs page is released
+ from the pagecache.
+
+ (3) A page should only be written to the cache if previous read or allocated.
+
+This permits the cache to maintain its page tracking in proper order.
+
+
+PAGE READ
+---------
+
+Firstly, the netfs should ask FS-Cache to examine the caches and read the
+contents cached for a particular page of a particular file if present, or else
+allocate space to store the contents if not:
+
+ typedef
+ void (*fscache_rw_complete_t)(struct page *page,
+ void *context,
+ int error);
+
+ int fscache_read_or_alloc_page(struct fscache_cookie *cookie,
+ struct page *page,
+ fscache_rw_complete_t end_io_func,
+ void *context,
+ gfp_t gfp);
+
+The cookie argument must specify a cookie for an object that isn't an index,
+the page specified will have the data loaded into it (and is also used to
+specify the page number), and the gfp argument is used to control how any
+memory allocations made are satisfied.
+
+If the cookie indicates the inode is not cached:
+
+ (1) The function will return -ENOBUFS.
+
+Else if there's a copy of the page resident in the cache:
+
+ (1) The mark_pages_cached() cookie operation will be called on that page.
+
+ (2) The function will submit a request to read the data from the cache's
+ backing device directly into the page specified.
+
+ (3) The function will return 0.
+
+ (4) When the read is complete, end_io_func() will be invoked with:
+
+ (*) The netfs data supplied when the cookie was created.
+
+ (*) The page descriptor.
+
+ (*) The context argument passed to the above function. This will be
+ maintained with the get_context/put_context functions mentioned above.
+
+ (*) An argument that's 0 on success or negative for an error code.
+
+ If an error occurs, it should be assumed that the page contains no usable
+ data.
+
+ end_io_func() will be called in process context if the read is results in
+ an error, but it might be called in interrupt context if the read is
+ successful.
+
+Otherwise, if there's not a copy available in cache, but the cache may be able
+to store the page:
+
+ (1) The mark_pages_cached() cookie operation will be called on that page.
+
+ (2) A block may be reserved in the cache and attached to the object at the
+ appropriate place.
+
+ (3) The function will return -ENODATA.
+
+This function may also return -ENOMEM or -EINTR, in which case it won't have
+read any data from the cache.
+
+
+PAGE ALLOCATE
+-------------
+
+Alternatively, if there's not expected to be any data in the cache for a page
+because the file has been extended, a block can simply be allocated instead:
+
+ int fscache_alloc_page(struct fscache_cookie *cookie,
+ struct page *page,
+ gfp_t gfp);
+
+This is similar to the fscache_read_or_alloc_page() function, except that it
+never reads from the cache. It will return 0 if a block has been allocated,
+rather than -ENODATA as the other would. One or the other must be performed
+before writing to the cache.
+
+The mark_pages_cached() cookie operation will be called on the page if
+successful.
+
+
+PAGE WRITE
+----------
+
+Secondly, if the netfs changes the contents of the page (either due to an
+initial download or if a user performs a write), then the page should be
+written back to the cache:
+
+ int fscache_write_page(struct fscache_cookie *cookie,
+ struct page *page,
+ gfp_t gfp);
+
+The cookie argument must specify a data file cookie, the page specified should
+contain the data to be written (and is also used to specify the page number),
+and the gfp argument is used to control how any memory allocations made are
+satisfied.
+
+The page must have first been read or allocated successfully and must not have
+been uncached before writing is performed.
+
+If the cookie indicates the inode is not cached then:
+
+ (1) The function will return -ENOBUFS.
+
+Else if space can be allocated in the cache to hold this page:
+
+ (1) PG_fscache_write will be set on the page.
+
+ (2) The function will submit a request to write the data to cache's backing
+ device directly from the page specified.
+
+ (3) The function will return 0.
+
+ (4) When the write is complete PG_fscache_write is cleared on the page and
+ anyone waiting for that bit will be woken up.
+
+Else if there's no space available in the cache, -ENOBUFS will be returned. It
+is also possible for the PG_fscache_write bit to be cleared when no write took
+place if unforeseen circumstances arose (such as a disk error).
+
+Writing takes place asynchronously.
+
+
+MULTIPLE PAGE READ
+------------------
+
+A facility is provided to read several pages at once, as requested by the
+readpages() address space operation:
+
+ int fscache_read_or_alloc_pages(struct fscache_cookie *cookie,
+ struct address_space *mapping,
+ struct list_head *pages,
+ int *nr_pages,
+ fscache_rw_complete_t end_io_func,
+ void *context,
+ gfp_t gfp);
+
+This works in a similar way to fscache_read_or_alloc_page(), except:
+
+ (1) Any page it can retrieve data for is removed from pages and nr_pages and
+ dispatched for reading to the disk. Reads of adjacent pages on disk may
+ be merged for greater efficiency.
+
+ (2) The mark_pages_cached() cookie operation will be called on several pages
+ at once if they're being read or allocated.
+
+ (3) If there was an general error, then that error will be returned.
+
+ Else if some pages couldn't be allocated or read, then -ENOBUFS will be
+ returned.
+
+ Else if some pages couldn't be read but were allocated, then -ENODATA will
+ be returned.
+
+ Otherwise, if all pages had reads dispatched, then 0 will be returned, the
+ list will be empty and *nr_pages will be 0.
+
+ (4) end_io_func will be called once for each page being read as the reads
+ complete. It will be called in process context if error != 0, but it may
+ be called in interrupt context if there is no error.
+
+Note that a return of -ENODATA, -ENOBUFS or any other error does not preclude
+some of the pages being read and some being allocated. Those pages will have
+been marked appropriately and will need uncaching.
+
+
+==============
+PAGE UNCACHING
+==============
+
+To uncache a page, this function should be called:
+
+ void fscache_uncache_page(struct fscache_cookie *cookie,
+ struct page *page);
+
+This function permits the cache to release any in-memory representation it
+might be holding for this netfs page. This function must be called once for
+each page on which the read or write page functions above have been called to
+make sure the cache's in-memory tracking information gets torn down.
+
+Note that pages can't be explicitly deleted from the a data file. The whole
+data file must be retired (see the relinquish cookie function below).
+
+Furthermore, note that this does not cancel the asynchronous read or write
+operation started by the read/alloc and write functions, so the page
+invalidation and release functions must use:
+
+ bool fscache_check_page_write(struct fscache_cookie *cookie,
+ struct page *page);
+
+to see if a page is being written to the cache, and:
+
+ void fscache_wait_on_page_write(struct fscache_cookie *cookie,
+ struct page *page);
+
+to wait for it to finish if it is.
+
+
+==========================
+INDEX AND DATA FILE UPDATE
+==========================
+
+To request an update of the index data for an index or other object, the
+following function should be called:
+
+ void fscache_update_cookie(struct fscache_cookie *cookie);
+
+This function will refer back to the netfs_data pointer stored in the cookie by
+the acquisition function to obtain the data to write into each revised index
+entry. The update method in the parent index definition will be called to
+transfer the data.
+
+Note that partial updates may happen automatically at other times, such as when
+data blocks are added to a data file object.
+
+
+===============================
+MISCELLANEOUS COOKIE OPERATIONS
+===============================
+
+There are a number of operations that can be used to control cookies:
+
+ (*) Cookie pinning:
+
+ int fscache_pin_cookie(struct fscache_cookie *cookie);
+ void fscache_unpin_cookie(struct fscache_cookie *cookie);
+
+ These operations permit data cookies to be pinned into the cache and to
+ have the pinning removed. They are not permitted on index cookies.
+
+ The pinning function will return 0 if successful, -ENOBUFS in the cookie
+ isn't backed by a cache, -EOPNOTSUPP if the cache doesn't support pinning,
+ -ENOSPC if there isn't enough space to honour the operation, -ENOMEM or
+ -EIO if there's any other problem.
+
+ (*) Data space reservation:
+
+ int fscache_reserve_space(struct fscache_cookie *cookie, loff_t size);
+
+ This permits a netfs to request cache space be reserved to store up to the
+ given amount of a file. It is permitted to ask for more than the current
+ size of the file to allow for future file expansion.
+
+ If size is given as zero then the reservation will be cancelled.
+
+ The function will return 0 if successful, -ENOBUFS in the cookie isn't
+ backed by a cache, -EOPNOTSUPP if the cache doesn't support reservations,
+ -ENOSPC if there isn't enough space to honour the operation, -ENOMEM or
+ -EIO if there's any other problem.
+
+ Note that this doesn't pin an object in a cache; it can still be culled to
+ make space if it's not in use.
+
+
+=====================
+COOKIE UNREGISTRATION
+=====================
+
+To get rid of a cookie, this function should be called.
+
+ void fscache_relinquish_cookie(struct fscache_cookie *cookie,
+ int retire);
+
+If retire is non-zero, then the object will be marked for recycling, and all
+copies of it will be removed from all active caches in which it is present.
+Not only that but all child objects will also be retired.
+
+If retire is zero, then the object may be available again when next the
+acquisition function is called. Retirement here will overrule the pinning on a
+cookie.
+
+One very important note - relinquish must NOT be called for a cookie unless all
+the cookies for "child" indices, objects and pages have been relinquished
+first.
+
+
+================================
+INDEX AND DATA FILE INVALIDATION
+================================
+
+There is no direct way to invalidate an index subtree or a data file. To do
+this, the caller should relinquish and retire the cookie they have, and then
+acquire a new one.
+
+
+===========================
+FS-CACHE SPECIFIC PAGE FLAG
+===========================
+
+FS-Cache makes use of a page flag, PG_private_2, for its own purpose. This is
+given the alternative name PG_fscache.
+
+PG_fscache is used to indicate that the page is known by the cache, and that
+the cache must be informed if the page is going to go away. It's an indication
+to the netfs that the cache has an interest in this page, where an interest may
+be a pointer to it, resources allocated or reserved for it, or I/O in progress
+upon it.
+
+The netfs can use this information in methods such as releasepage() to
+determine whether it needs to uncache a page or update it.
+
+Furthermore, if this bit is set, releasepage() and invalidatepage() operations
+will be called on a page to get rid of it, even if PG_private is not set. This
+allows caching to attempted on a page before read_cache_pages() to be called
+after fscache_read_or_alloc_pages() as the former will try and release pages it
+was given under certain circumstances.
+
+This bit does not overlap with such as PG_private. This means that FS-Cache
+can be used with a filesystem that uses the block buffering code.
+
+There are a number of operations defined on this flag:
+
+ int PageFsCache(struct page *page);
+ void SetPageFsCache(struct page *page)
+ void ClearPageFsCache(struct page *page)
+ int TestSetPageFsCache(struct page *page)
+ int TestClearPageFsCache(struct page *page)
+
+These functions are bit test, bit set, bit clear, bit test and set and bit
+test and clear operations on PG_fscache.
diff --git a/Documentation/filesystems/caching/object.txt b/Documentation/filesystems/caching/object.txt
new file mode 100644
index 00000000000..e8b0a35d8fe
--- /dev/null
+++ b/Documentation/filesystems/caching/object.txt
@@ -0,0 +1,313 @@
+ ====================================================
+ IN-KERNEL CACHE OBJECT REPRESENTATION AND MANAGEMENT
+ ====================================================
+
+By: David Howells <dhowells@redhat.com>
+
+Contents:
+
+ (*) Representation
+
+ (*) Object management state machine.
+
+ - Provision of cpu time.
+ - Locking simplification.
+
+ (*) The set of states.
+
+ (*) The set of events.
+
+
+==============
+REPRESENTATION
+==============
+
+FS-Cache maintains an in-kernel representation of each object that a netfs is
+currently interested in. Such objects are represented by the fscache_cookie
+struct and are referred to as cookies.
+
+FS-Cache also maintains a separate in-kernel representation of the objects that
+a cache backend is currently actively caching. Such objects are represented by
+the fscache_object struct. The cache backends allocate these upon request, and
+are expected to embed them in their own representations. These are referred to
+as objects.
+
+There is a 1:N relationship between cookies and objects. A cookie may be
+represented by multiple objects - an index may exist in more than one cache -
+or even by no objects (it may not be cached).
+
+Furthermore, both cookies and objects are hierarchical. The two hierarchies
+correspond, but the cookies tree is a superset of the union of the object trees
+of multiple caches:
+
+ NETFS INDEX TREE : CACHE 1 : CACHE 2
+ : :
+ : +-----------+ :
+ +----------->| IObject | :
+ +-----------+ | : +-----------+ :
+ | ICookie |-------+ : | :
+ +-----------+ | : | : +-----------+
+ | +------------------------------>| IObject |
+ | : | : +-----------+
+ | : V : |
+ | : +-----------+ : |
+ V +----------->| IObject | : |
+ +-----------+ | : +-----------+ : |
+ | ICookie |-------+ : | : V
+ +-----------+ | : | : +-----------+
+ | +------------------------------>| IObject |
+ +-----+-----+ : | : +-----------+
+ | | : | : |
+ V | : V : |
+ +-----------+ | : +-----------+ : |
+ | ICookie |------------------------->| IObject | : |
+ +-----------+ | : +-----------+ : |
+ | V : | : V
+ | +-----------+ : | : +-----------+
+ | | ICookie |-------------------------------->| IObject |
+ | +-----------+ : | : +-----------+
+ V | : V : |
+ +-----------+ | : +-----------+ : |
+ | DCookie |------------------------->| DObject | : |
+ +-----------+ | : +-----------+ : |
+ | : : |
+ +-------+-------+ : : |
+ | | : : |
+ V V : : V
+ +-----------+ +-----------+ : : +-----------+
+ | DCookie | | DCookie |------------------------>| DObject |
+ +-----------+ +-----------+ : : +-----------+
+ : :
+
+In the above illustration, ICookie and IObject represent indices and DCookie
+and DObject represent data storage objects. Indices may have representation in
+multiple caches, but currently, non-index objects may not. Objects of any type
+may also be entirely unrepresented.
+
+As far as the netfs API goes, the netfs is only actually permitted to see
+pointers to the cookies. The cookies themselves and any objects attached to
+those cookies are hidden from it.
+
+
+===============================
+OBJECT MANAGEMENT STATE MACHINE
+===============================
+
+Within FS-Cache, each active object is managed by its own individual state
+machine. The state for an object is kept in the fscache_object struct, in
+object->state. A cookie may point to a set of objects that are in different
+states.
+
+Each state has an action associated with it that is invoked when the machine
+wakes up in that state. There are four logical sets of states:
+
+ (1) Preparation: states that wait for the parent objects to become ready. The
+ representations are hierarchical, and it is expected that an object must
+ be created or accessed with respect to its parent object.
+
+ (2) Initialisation: states that perform lookups in the cache and validate
+ what's found and that create on disk any missing metadata.
+
+ (3) Normal running: states that allow netfs operations on objects to proceed
+ and that update the state of objects.
+
+ (4) Termination: states that detach objects from their netfs cookies, that
+ delete objects from disk, that handle disk and system errors and that free
+ up in-memory resources.
+
+
+In most cases, transitioning between states is in response to signalled events.
+When a state has finished processing, it will usually set the mask of events in
+which it is interested (object->event_mask) and relinquish the worker thread.
+Then when an event is raised (by calling fscache_raise_event()), if the event
+is not masked, the object will be queued for processing (by calling
+fscache_enqueue_object()).
+
+
+PROVISION OF CPU TIME
+---------------------
+
+The work to be done by the various states is given CPU time by the threads of
+the slow work facility (see Documentation/slow-work.txt). This is used in
+preference to the workqueue facility because:
+
+ (1) Threads may be completely occupied for very long periods of time by a
+ particular work item. These state actions may be doing sequences of
+ synchronous, journalled disk accesses (lookup, mkdir, create, setxattr,
+ getxattr, truncate, unlink, rmdir, rename).
+
+ (2) Threads may do little actual work, but may rather spend a lot of time
+ sleeping on I/O. This means that single-threaded and 1-per-CPU-threaded
+ workqueues don't necessarily have the right numbers of threads.
+
+
+LOCKING SIMPLIFICATION
+----------------------
+
+Because only one worker thread may be operating on any particular object's
+state machine at once, this simplifies the locking, particularly with respect
+to disconnecting the netfs's representation of a cache object (fscache_cookie)
+from the cache backend's representation (fscache_object) - which may be
+requested from either end.
+
+
+=================
+THE SET OF STATES
+=================
+
+The object state machine has a set of states that it can be in. There are
+preparation states in which the object sets itself up and waits for its parent
+object to transit to a state that allows access to its children:
+
+ (1) State FSCACHE_OBJECT_INIT.
+
+ Initialise the object and wait for the parent object to become active. In
+ the cache, it is expected that it will not be possible to look an object
+ up from the parent object, until that parent object itself has been looked
+ up.
+
+There are initialisation states in which the object sets itself up and accesses
+disk for the object metadata:
+
+ (2) State FSCACHE_OBJECT_LOOKING_UP.
+
+ Look up the object on disk, using the parent as a starting point.
+ FS-Cache expects the cache backend to probe the cache to see whether this
+ object is represented there, and if it is, to see if it's valid (coherency
+ management).
+
+ The cache should call fscache_object_lookup_negative() to indicate lookup
+ failure for whatever reason, and should call fscache_obtained_object() to
+ indicate success.
+
+ At the completion of lookup, FS-Cache will let the netfs go ahead with
+ read operations, no matter whether the file is yet cached. If not yet
+ cached, read operations will be immediately rejected with ENODATA until
+ the first known page is uncached - as to that point there can be no data
+ to be read out of the cache for that file that isn't currently also held
+ in the pagecache.
+
+ (3) State FSCACHE_OBJECT_CREATING.
+
+ Create an object on disk, using the parent as a starting point. This
+ happens if the lookup failed to find the object, or if the object's
+ coherency data indicated what's on disk is out of date. In this state,
+ FS-Cache expects the cache to create
+
+ The cache should call fscache_obtained_object() if creation completes
+ successfully, fscache_object_lookup_negative() otherwise.
+
+ At the completion of creation, FS-Cache will start processing write
+ operations the netfs has queued for an object. If creation failed, the
+ write ops will be transparently discarded, and nothing recorded in the
+ cache.
+
+There are some normal running states in which the object spends its time
+servicing netfs requests:
+
+ (4) State FSCACHE_OBJECT_AVAILABLE.
+
+ A transient state in which pending operations are started, child objects
+ are permitted to advance from FSCACHE_OBJECT_INIT state, and temporary
+ lookup data is freed.
+
+ (5) State FSCACHE_OBJECT_ACTIVE.
+
+ The normal running state. In this state, requests the netfs makes will be
+ passed on to the cache.
+
+ (6) State FSCACHE_OBJECT_UPDATING.
+
+ The state machine comes here to update the object in the cache from the
+ netfs's records. This involves updating the auxiliary data that is used
+ to maintain coherency.
+
+And there are terminal states in which an object cleans itself up, deallocates
+memory and potentially deletes stuff from disk:
+
+ (7) State FSCACHE_OBJECT_LC_DYING.
+
+ The object comes here if it is dying because of a lookup or creation
+ error. This would be due to a disk error or system error of some sort.
+ Temporary data is cleaned up, and the parent is released.
+
+ (8) State FSCACHE_OBJECT_DYING.
+
+ The object comes here if it is dying due to an error, because its parent
+ cookie has been relinquished by the netfs or because the cache is being
+ withdrawn.
+
+ Any child objects waiting on this one are given CPU time so that they too
+ can destroy themselves. This object waits for all its children to go away
+ before advancing to the next state.
+
+ (9) State FSCACHE_OBJECT_ABORT_INIT.
+
+ The object comes to this state if it was waiting on its parent in
+ FSCACHE_OBJECT_INIT, but its parent died. The object will destroy itself
+ so that the parent may proceed from the FSCACHE_OBJECT_DYING state.
+
+(10) State FSCACHE_OBJECT_RELEASING.
+(11) State FSCACHE_OBJECT_RECYCLING.
+
+ The object comes to one of these two states when dying once it is rid of
+ all its children, if it is dying because the netfs relinquished its
+ cookie. In the first state, the cached data is expected to persist, and
+ in the second it will be deleted.
+
+(12) State FSCACHE_OBJECT_WITHDRAWING.
+
+ The object transits to this state if the cache decides it wants to
+ withdraw the object from service, perhaps to make space, but also due to
+ error or just because the whole cache is being withdrawn.
+
+(13) State FSCACHE_OBJECT_DEAD.
+
+ The object transits to this state when the in-memory object record is
+ ready to be deleted. The object processor shouldn't ever see an object in
+ this state.
+
+
+THE SET OF EVENTS
+-----------------
+
+There are a number of events that can be raised to an object state machine:
+
+ (*) FSCACHE_OBJECT_EV_UPDATE
+
+ The netfs requested that an object be updated. The state machine will ask
+ the cache backend to update the object, and the cache backend will ask the
+ netfs for details of the change through its cookie definition ops.
+
+ (*) FSCACHE_OBJECT_EV_CLEARED
+
+ This is signalled in two circumstances:
+
+ (a) when an object's last child object is dropped and
+
+ (b) when the last operation outstanding on an object is completed.
+
+ This is used to proceed from the dying state.
+
+ (*) FSCACHE_OBJECT_EV_ERROR
+
+ This is signalled when an I/O error occurs during the processing of some
+ object.
+
+ (*) FSCACHE_OBJECT_EV_RELEASE
+ (*) FSCACHE_OBJECT_EV_RETIRE
+
+ These are signalled when the netfs relinquishes a cookie it was using.
+ The event selected depends on whether the netfs asks for the backing
+ object to be retired (deleted) or retained.
+
+ (*) FSCACHE_OBJECT_EV_WITHDRAW
+
+ This is signalled when the cache backend wants to withdraw an object.
+ This means that the object will have to be detached from the netfs's
+ cookie.
+
+Because the withdrawing releasing/retiring events are all handled by the object
+state machine, it doesn't matter if there's a collision with both ends trying
+to sever the connection at the same time. The state machine can just pick
+which one it wants to honour, and that effects the other.
diff --git a/Documentation/filesystems/caching/operations.txt b/Documentation/filesystems/caching/operations.txt
new file mode 100644
index 00000000000..b6b070c57cb
--- /dev/null
+++ b/Documentation/filesystems/caching/operations.txt
@@ -0,0 +1,213 @@
+ ================================
+ ASYNCHRONOUS OPERATIONS HANDLING
+ ================================
+
+By: David Howells <dhowells@redhat.com>
+
+Contents:
+
+ (*) Overview.
+
+ (*) Operation record initialisation.
+
+ (*) Parameters.
+
+ (*) Procedure.
+
+ (*) Asynchronous callback.
+
+
+========
+OVERVIEW
+========
+
+FS-Cache has an asynchronous operations handling facility that it uses for its
+data storage and retrieval routines. Its operations are represented by
+fscache_operation structs, though these are usually embedded into some other
+structure.
+
+This facility is available to and expected to be be used by the cache backends,
+and FS-Cache will create operations and pass them off to the appropriate cache
+backend for completion.
+
+To make use of this facility, <linux/fscache-cache.h> should be #included.
+
+
+===============================
+OPERATION RECORD INITIALISATION
+===============================
+
+An operation is recorded in an fscache_operation struct:
+
+ struct fscache_operation {
+ union {
+ struct work_struct fast_work;
+ struct slow_work slow_work;
+ };
+ unsigned long flags;
+ fscache_operation_processor_t processor;
+ ...
+ };
+
+Someone wanting to issue an operation should allocate something with this
+struct embedded in it. They should initialise it by calling:
+
+ void fscache_operation_init(struct fscache_operation *op,
+ fscache_operation_release_t release);
+
+with the operation to be initialised and the release function to use.
+
+The op->flags parameter should be set to indicate the CPU time provision and
+the exclusivity (see the Parameters section).
+
+The op->fast_work, op->slow_work and op->processor flags should be set as
+appropriate for the CPU time provision (see the Parameters section).
+
+FSCACHE_OP_WAITING may be set in op->flags prior to each submission of the
+operation and waited for afterwards.
+
+
+==========
+PARAMETERS
+==========
+
+There are a number of parameters that can be set in the operation record's flag
+parameter. There are three options for the provision of CPU time in these
+operations:
+
+ (1) The operation may be done synchronously (FSCACHE_OP_MYTHREAD). A thread
+ may decide it wants to handle an operation itself without deferring it to
+ another thread.
+
+ This is, for example, used in read operations for calling readpages() on
+ the backing filesystem in CacheFiles. Although readpages() does an
+ asynchronous data fetch, the determination of whether pages exist is done
+ synchronously - and the netfs does not proceed until this has been
+ determined.
+
+ If this option is to be used, FSCACHE_OP_WAITING must be set in op->flags
+ before submitting the operation, and the operating thread must wait for it
+ to be cleared before proceeding:
+
+ wait_on_bit(&op->flags, FSCACHE_OP_WAITING,
+ fscache_wait_bit, TASK_UNINTERRUPTIBLE);
+
+
+ (2) The operation may be fast asynchronous (FSCACHE_OP_FAST), in which case it
+ will be given to keventd to process. Such an operation is not permitted
+ to sleep on I/O.
+
+ This is, for example, used by CacheFiles to copy data from a backing fs
+ page to a netfs page after the backing fs has read the page in.
+
+ If this option is used, op->fast_work and op->processor must be
+ initialised before submitting the operation:
+
+ INIT_WORK(&op->fast_work, do_some_work);
+
+
+ (3) The operation may be slow asynchronous (FSCACHE_OP_SLOW), in which case it
+ will be given to the slow work facility to process. Such an operation is
+ permitted to sleep on I/O.
+
+ This is, for example, used by FS-Cache to handle background writes of
+ pages that have just been fetched from a remote server.
+
+ If this option is used, op->slow_work and op->processor must be
+ initialised before submitting the operation:
+
+ fscache_operation_init_slow(op, processor)
+
+
+Furthermore, operations may be one of two types:
+
+ (1) Exclusive (FSCACHE_OP_EXCLUSIVE). Operations of this type may not run in
+ conjunction with any other operation on the object being operated upon.
+
+ An example of this is the attribute change operation, in which the file
+ being written to may need truncation.
+
+ (2) Shareable. Operations of this type may be running simultaneously. It's
+ up to the operation implementation to prevent interference between other
+ operations running at the same time.
+
+
+=========
+PROCEDURE
+=========
+
+Operations are used through the following procedure:
+
+ (1) The submitting thread must allocate the operation and initialise it
+ itself. Normally this would be part of a more specific structure with the
+ generic op embedded within.
+
+ (2) The submitting thread must then submit the operation for processing using
+ one of the following two functions:
+
+ int fscache_submit_op(struct fscache_object *object,
+ struct fscache_operation *op);
+
+ int fscache_submit_exclusive_op(struct fscache_object *object,
+ struct fscache_operation *op);
+
+ The first function should be used to submit non-exclusive ops and the
+ second to submit exclusive ones. The caller must still set the
+ FSCACHE_OP_EXCLUSIVE flag.
+
+ If successful, both functions will assign the operation to the specified
+ object and return 0. -ENOBUFS will be returned if the object specified is
+ permanently unavailable.
+
+ The operation manager will defer operations on an object that is still
+ undergoing lookup or creation. The operation will also be deferred if an
+ operation of conflicting exclusivity is in progress on the object.
+
+ If the operation is asynchronous, the manager will retain a reference to
+ it, so the caller should put their reference to it by passing it to:
+
+ void fscache_put_operation(struct fscache_operation *op);
+
+ (3) If the submitting thread wants to do the work itself, and has marked the
+ operation with FSCACHE_OP_MYTHREAD, then it should monitor
+ FSCACHE_OP_WAITING as described above and check the state of the object if
+ necessary (the object might have died whilst the thread was waiting).
+
+ When it has finished doing its processing, it should call
+ fscache_put_operation() on it.
+
+ (4) The operation holds an effective lock upon the object, preventing other
+ exclusive ops conflicting until it is released. The operation can be
+ enqueued for further immediate asynchronous processing by adjusting the
+ CPU time provisioning option if necessary, eg:
+
+ op->flags &= ~FSCACHE_OP_TYPE;
+ op->flags |= ~FSCACHE_OP_FAST;
+
+ and calling:
+
+ void fscache_enqueue_operation(struct fscache_operation *op)
+
+ This can be used to allow other things to have use of the worker thread
+ pools.
+
+
+=====================
+ASYNCHRONOUS CALLBACK
+=====================
+
+When used in asynchronous mode, the worker thread pool will invoke the
+processor method with a pointer to the operation. This should then get at the
+container struct by using container_of():
+
+ static void fscache_write_op(struct fscache_operation *_op)
+ {
+ struct fscache_storage *op =
+ container_of(_op, struct fscache_storage, op);
+ ...
+ }
+
+The caller holds a reference on the operation, and will invoke
+fscache_put_operation() when the processor function returns. The processor
+function is at liberty to call fscache_enqueue_operation() or to take extra
+references.
diff --git a/Documentation/slow-work.txt b/Documentation/slow-work.txt
new file mode 100644
index 00000000000..ebc50f808ea
--- /dev/null
+++ b/Documentation/slow-work.txt
@@ -0,0 +1,174 @@
+ ====================================
+ SLOW WORK ITEM EXECUTION THREAD POOL
+ ====================================
+
+By: David Howells <dhowells@redhat.com>
+
+The slow work item execution thread pool is a pool of threads for performing
+things that take a relatively long time, such as making mkdir calls.
+Typically, when processing something, these items will spend a lot of time
+blocking a thread on I/O, thus making that thread unavailable for doing other
+work.
+
+The standard workqueue model is unsuitable for this class of work item as that
+limits the owner to a single thread or a single thread per CPU. For some
+tasks, however, more threads - or fewer - are required.
+
+There is just one pool per system. It contains no threads unless something
+wants to use it - and that something must register its interest first. When
+the pool is active, the number of threads it contains is dynamic, varying
+between a maximum and minimum setting, depending on the load.
+
+
+====================
+CLASSES OF WORK ITEM
+====================
+
+This pool support two classes of work items:
+
+ (*) Slow work items.
+
+ (*) Very slow work items.
+
+The former are expected to finish much quicker than the latter.
+
+An operation of the very slow class may do a batch combination of several
+lookups, mkdirs, and a create for instance.
+
+An operation of the ordinarily slow class may, for example, write stuff or
+expand files, provided the time taken to do so isn't too long.
+
+Operations of both types may sleep during execution, thus tying up the thread
+loaned to it.
+
+
+THREAD-TO-CLASS ALLOCATION
+--------------------------
+
+Not all the threads in the pool are available to work on very slow work items.
+The number will be between one and one fewer than the number of active threads.
+This is configurable (see the "Pool Configuration" section).
+
+All the threads are available to work on ordinarily slow work items, but a
+percentage of the threads will prefer to work on very slow work items.
+
+The configuration ensures that at least one thread will be available to work on
+very slow work items, and at least one thread will be available that won't work
+on very slow work items at all.
+
+
+=====================
+USING SLOW WORK ITEMS
+=====================
+
+Firstly, a module or subsystem wanting to make use of slow work items must
+register its interest:
+
+ int ret = slow_work_register_user();
+
+This will return 0 if successful, or a -ve error upon failure.
+
+
+Slow work items may then be set up by:
+
+ (1) Declaring a slow_work struct type variable:
+
+ #include <linux/slow-work.h>
+
+ struct slow_work myitem;
+
+ (2) Declaring the operations to be used for this item:
+
+ struct slow_work_ops myitem_ops = {
+ .get_ref = myitem_get_ref,
+ .put_ref = myitem_put_ref,
+ .execute = myitem_execute,
+ };
+
+ [*] For a description of the ops, see section "Item Operations".
+
+ (3) Initialising the item:
+
+ slow_work_init(&myitem, &myitem_ops);
+
+ or:
+
+ vslow_work_init(&myitem, &myitem_ops);
+
+ depending on its class.
+
+A suitably set up work item can then be enqueued for processing:
+
+ int ret = slow_work_enqueue(&myitem);
+
+This will return a -ve error if the thread pool is unable to gain a reference
+on the item, 0 otherwise.
+
+
+The items are reference counted, so there ought to be no need for a flush
+operation. When all a module's slow work items have been processed, and the
+module has no further interest in the facility, it should unregister its
+interest:
+
+ slow_work_unregister_user();
+
+
+===============
+ITEM OPERATIONS
+===============
+
+Each work item requires a table of operations of type struct slow_work_ops.
+All members are required:
+
+ (*) Get a reference on an item:
+
+ int (*get_ref)(struct slow_work *work);
+
+ This allows the thread pool to attempt to pin an item by getting a
+ reference on it. This function should return 0 if the reference was
+ granted, or a -ve error otherwise. If an error is returned,
+ slow_work_enqueue() will fail.
+
+ The reference is held whilst the item is queued and whilst it is being
+ executed. The item may then be requeued with the same reference held, or
+ the reference will be released.
+
+ (*) Release a reference on an item:
+
+ void (*put_ref)(struct slow_work *work);
+
+ This allows the thread pool to unpin an item by releasing the reference on
+ it. The thread pool will not touch the item again once this has been
+ called.
+
+ (*) Execute an item:
+
+ void (*execute)(struct slow_work *work);
+
+ This should perform the work required of the item. It may sleep, it may
+ perform disk I/O and it may wait for locks.
+
+
+==================
+POOL CONFIGURATION
+==================
+
+The slow-work thread pool has a number of configurables:
+
+ (*) /proc/sys/kernel/slow-work/min-threads
+
+ The minimum number of threads that should be in the pool whilst it is in
+ use. This may be anywhere between 2 and max-threads.
+
+ (*) /proc/sys/kernel/slow-work/max-threads
+
+ The maximum number of threads that should in the pool. This may be
+ anywhere between min-threads and 255 or NR_CPUS * 2, whichever is greater.
+
+ (*) /proc/sys/kernel/slow-work/vslow-percentage
+
+ The percentage of active threads in the pool that may be used to execute
+ very slow work items. This may be between 1 and 99. The resultant number
+ is bounded to between 1 and one fewer than the number of active threads.
+ This ensures there is always at least one thread that can process very
+ slow work items, and always at least one thread that won't.