diff options
| -rw-r--r-- | include/linux/mm.h | 68 | ||||
| -rw-r--r-- | include/linux/page-flags.h | 35 | ||||
| -rw-r--r-- | mm/filemap.c | 8 |
3 files changed, 64 insertions, 47 deletions
diff --git a/include/linux/mm.h b/include/linux/mm.h index 2db4229a006..f2018775b99 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -219,7 +219,8 @@ struct inode; * Each physical page in the system has a struct page associated with * it to keep track of whatever it is we are using the page for at the * moment. Note that we have no way to track which tasks are using - * a page. + * a page, though if it is a pagecache page, rmap structures can tell us + * who is mapping it. */ struct page { unsigned long flags; /* Atomic flags, some possibly @@ -299,8 +300,7 @@ struct page { */ /* - * Drop a ref, return true if the logical refcount fell to zero (the page has - * no users) + * Drop a ref, return true if the refcount fell to zero (the page has no users) */ static inline int put_page_testzero(struct page *page) { @@ -356,43 +356,55 @@ void split_page(struct page *page, unsigned int order); * For the non-reserved pages, page_count(page) denotes a reference count. * page_count() == 0 means the page is free. page->lru is then used for * freelist management in the buddy allocator. - * page_count() == 1 means the page is used for exactly one purpose - * (e.g. a private data page of one process). + * page_count() > 0 means the page has been allocated. * - * A page may be used for kmalloc() or anyone else who does a - * __get_free_page(). In this case the page_count() is at least 1, and - * all other fields are unused but should be 0 or NULL. The - * management of this page is the responsibility of the one who uses - * it. + * Pages are allocated by the slab allocator in order to provide memory + * to kmalloc and kmem_cache_alloc. In this case, the management of the + * page, and the fields in 'struct page' are the responsibility of mm/slab.c + * unless a particular usage is carefully commented. (the responsibility of + * freeing the kmalloc memory is the caller's, of course). * - * The other pages (we may call them "process pages") are completely + * A page may be used by anyone else who does a __get_free_page(). + * In this case, page_count still tracks the references, and should only + * be used through the normal accessor functions. The top bits of page->flags + * and page->virtual store page management information, but all other fields + * are unused and could be used privately, carefully. The management of this + * page is the responsibility of the one who allocated it, and those who have + * subsequently been given references to it. + * + * The other pages (we may call them "pagecache pages") are completely * managed by the Linux memory manager: I/O, buffers, swapping etc. * The following discussion applies only to them. * - * A page may belong to an inode's memory mapping. In this case, - * page->mapping is the pointer to the inode, and page->index is the - * file offset of the page, in units of PAGE_CACHE_SIZE. + * A pagecache page contains an opaque `private' member, which belongs to the + * page's address_space. Usually, this is the address of a circular list of + * the page's disk buffers. PG_private must be set to tell the VM to call + * into the filesystem to release these pages. * - * A page contains an opaque `private' member, which belongs to the - * page's address_space. Usually, this is the address of a circular - * list of the page's disk buffers. + * A page may belong to an inode's memory mapping. In this case, page->mapping + * is the pointer to the inode, and page->index is the file offset of the page, + * in units of PAGE_CACHE_SIZE. * - * For pages belonging to inodes, the page_count() is the number of - * attaches, plus 1 if `private' contains something, plus one for - * the page cache itself. + * If pagecache pages are not associated with an inode, they are said to be + * anonymous pages. These may become associated with the swapcache, and in that + * case PG_swapcache is set, and page->private is an offset into the swapcache. * - * Instead of keeping dirty/clean pages in per address-space lists, we instead - * now tag pages as dirty/under writeback in the radix tree. + * In either case (swapcache or inode backed), the pagecache itself holds one + * reference to the page. Setting PG_private should also increment the + * refcount. The each user mapping also has a reference to the page. * - * There is also a per-mapping radix tree mapping index to the page - * in memory if present. The tree is rooted at mapping->root. + * The pagecache pages are stored in a per-mapping radix tree, which is + * rooted at mapping->page_tree, and indexed by offset. + * Where 2.4 and early 2.6 kernels kept dirty/clean pages in per-address_space + * lists, we instead now tag pages as dirty/writeback in the radix tree. * - * All process pages can do I/O: + * All pagecache pages may be subject to I/O: * - inode pages may need to be read from disk, * - inode pages which have been modified and are MAP_SHARED may need - * to be written to disk, - * - private pages which have been modified may need to be swapped out - * to swap space and (later) to be read back into memory. + * to be written back to the inode on disk, + * - anonymous pages (including MAP_PRIVATE file mappings) which have been + * modified may need to be swapped out to swap space and (later) to be read + * back into memory. */ /* diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 5748642e9f3..9d7921dd50f 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -13,24 +13,25 @@ * PG_reserved is set for special pages, which can never be swapped out. Some * of them might not even exist (eg empty_bad_page)... * - * The PG_private bitflag is set if page->private contains a valid value. + * The PG_private bitflag is set on pagecache pages if they contain filesystem + * specific data (which is normally at page->private). It can be used by + * private allocations for its own usage. * - * During disk I/O, PG_locked is used. This bit is set before I/O and - * reset when I/O completes. page_waitqueue(page) is a wait queue of all tasks - * waiting for the I/O on this page to complete. + * During initiation of disk I/O, PG_locked is set. This bit is set before I/O + * and cleared when writeback _starts_ or when read _completes_. PG_writeback + * is set before writeback starts and cleared when it finishes. + * + * PG_locked also pins a page in pagecache, and blocks truncation of the file + * while it is held. + * + * page_waitqueue(page) is a wait queue of all tasks waiting for the page + * to become unlocked. * * PG_uptodate tells whether the page's contents is valid. When a read * completes, the page becomes uptodate, unless a disk I/O error happened. * - * For choosing which pages to swap out, inode pages carry a PG_referenced bit, - * which is set any time the system accesses that page through the (mapping, - * index) hash table. This referenced bit, together with the referenced bit - * in the page tables, is used to manipulate page->age and move the page across - * the active, inactive_dirty and inactive_clean lists. - * - * Note that the referenced bit, the page->lru list_head and the active, - * inactive_dirty and inactive_clean lists are protected by the - * zone->lru_lock, and *NOT* by the usual PG_locked bit! + * PG_referenced, PG_reclaim are used for page reclaim for anonymous and + * file-backed pagecache (see mm/vmscan.c). * * PG_error is set to indicate that an I/O error occurred on this page. * @@ -42,6 +43,10 @@ * space, they need to be kmapped separately for doing IO on the pages. The * struct page (these bits with information) are always mapped into kernel * address space... + * + * PG_buddy is set to indicate that the page is free and in the buddy system + * (see mm/page_alloc.c). + * */ /* @@ -74,7 +79,7 @@ #define PG_checked 8 /* kill me in 2.5.<early>. */ #define PG_arch_1 9 #define PG_reserved 10 -#define PG_private 11 /* Has something at ->private */ +#define PG_private 11 /* If pagecache, has fs-private data */ #define PG_writeback 12 /* Page is under writeback */ #define PG_nosave 13 /* Used for system suspend/resume */ @@ -83,7 +88,7 @@ #define PG_mappedtodisk 16 /* Has blocks allocated on-disk */ #define PG_reclaim 17 /* To be reclaimed asap */ -#define PG_nosave_free 18 /* Free, should not be written */ +#define PG_nosave_free 18 /* Used for system suspend/resume */ #define PG_buddy 19 /* Page is free, on buddy lists */ diff --git a/mm/filemap.c b/mm/filemap.c index d5af1cab426..afcdc72b5e9 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -599,8 +599,8 @@ void fastcall __lock_page_nosync(struct page *page) * @mapping: the address_space to search * @offset: the page index * - * A rather lightweight function, finding and getting a reference to a - * hashed page atomically. + * Is there a pagecache struct page at the given (mapping, offset) tuple? + * If yes, increment its refcount and return it; if no, return NULL. */ struct page * find_get_page(struct address_space *mapping, unsigned long offset) { @@ -987,7 +987,7 @@ page_not_up_to_date: /* Get exclusive access to the page ... */ lock_page(page); - /* Did it get unhashed before we got the lock? */ + /* Did it get truncated before we got the lock? */ if (!page->mapping) { unlock_page(page); page_cache_release(page); @@ -1627,7 +1627,7 @@ no_cached_page: page_not_uptodate: lock_page(page); - /* Did it get unhashed while we waited for it? */ + /* Did it get truncated while we waited for it? */ if (!page->mapping) { unlock_page(page); goto err; |