[PATCH] VFS: Permit filesystem to override root dentry on mount

Extend the get_sb() filesystem operation to take an extra argument that
permits the VFS to pass in the target vfsmount that defines the mountpoint.

The filesystem is then required to manually set the superblock and root dentry
pointers.  For most filesystems, this should be done with simple_set_mnt()
which will set the superblock pointer and then set the root dentry to the
superblock's s_root (as per the old default behaviour).

The get_sb() op now returns an integer as there's now no need to return the
superblock pointer.

This patch permits a superblock to be implicitly shared amongst several mount
points, such as can be done with NFS to avoid potential inode aliasing.  In
such a case, simple_set_mnt() would not be called, and instead the mnt_root
and mnt_sb would be set directly.

The patch also makes the following changes:

 (*) the get_sb_*() convenience functions in the core kernel now take a vfsmount
     pointer argument and return an integer, so most filesystems have to change
     very little.

 (*) If one of the convenience function is not used, then get_sb() should
     normally call simple_set_mnt() to instantiate the vfsmount. This will
     always return 0, and so can be tail-called from get_sb().

 (*) generic_shutdown_super() now calls shrink_dcache_sb() to clean up the
     dcache upon superblock destruction rather than shrink_dcache_anon().

     This is required because the superblock may now have multiple trees that
     aren't actually bound to s_root, but that still need to be cleaned up. The
     currently called functions assume that the whole tree is rooted at s_root,
     and that anonymous dentries are not the roots of trees which results in
     dentries being left unculled.

     However, with the way NFS superblock sharing are currently set to be
     implemented, these assumptions are violated: the root of the filesystem is
     simply a dummy dentry and inode (the real inode for '/' may well be
     inaccessible), and all the vfsmounts are rooted on anonymous[*] dentries
     with child trees.

     [*] Anonymous until discovered from another tree.

 (*) The documentation has been adjusted, including the additional bit of
     changing ext2_* into foo_* in the documentation.

[akpm@osdl.org: convert ipath_fs, do other stuff]
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Nathan Scott <nathans@sgi.com>
Cc: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

7 files changed, 27 insertions, 24 deletions

diff --git a/drivers/infiniband/core/uverbs_main.c b/drivers/infiniband/core/uverbs_main.c
index 5ec2d49e9bb..e57d3c50f75 100644
--- a/drivers/infiniband/core/uverbs_main.c
+++ b/drivers/infiniband/core/uverbs_main.c
@@ -821,11 +821,12 @@ static void ib_uverbs_remove_one(struct ib_device *device)
 	kref_put(&uverbs_dev->ref, ib_uverbs_release_dev);
 }
 
-static struct super_block *uverbs_event_get_sb(struct file_system_type *fs_type, int flags,
-					       const char *dev_name, void *data)
+static int uverbs_event_get_sb(struct file_system_type *fs_type, int flags,
+			       const char *dev_name, void *data,
+			       struct vfsmount *mnt)
 {
 	return get_sb_pseudo(fs_type, "infinibandevent:", NULL,
-			     INFINIBANDEVENTFS_MAGIC);
+			     INFINIBANDEVENTFS_MAGIC, mnt);
 }
 
 static struct file_system_type uverbs_event_fs = {
diff --git a/drivers/infiniband/hw/ipath/ipath_fs.c b/drivers/infiniband/hw/ipath/ipath_fs.c
index e274120567e..63de3046aff 100644
--- a/drivers/infiniband/hw/ipath/ipath_fs.c
+++ b/drivers/infiniband/hw/ipath/ipath_fs.c
@@ -542,13 +542,14 @@ bail:
 	return ret;
 }
 
-static struct super_block *ipathfs_get_sb(struct file_system_type *fs_type,
-				        int flags, const char *dev_name,
-					void *data)
+static int ipathfs_get_sb(struct file_system_type *fs_type, int flags,
+			const char *dev_name, void *data, struct vfsmount *mnt)
 {
-	ipath_super = get_sb_single(fs_type, flags, data,
-				    ipathfs_fill_super);
-	return ipath_super;
+	int ret = get_sb_single(fs_type, flags, data,
+				    ipathfs_fill_super, mnt);
+	if (ret >= 0)
+		ipath_super = mnt->mnt_sb;
+	return ret;
 }
 
 static void ipathfs_kill_super(struct super_block *s)
diff --git a/drivers/isdn/capi/capifs.c b/drivers/isdn/capi/capifs.c
index 0a37aded4b5..9ea6bd0ddc3 100644
--- a/drivers/isdn/capi/capifs.c
+++ b/drivers/isdn/capi/capifs.c
@@ -121,10 +121,10 @@ fail:
 	return -ENOMEM;
 }
 
-static struct super_block *capifs_get_sb(struct file_system_type *fs_type,
-	int flags, const char *dev_name, void *data)
+static int capifs_get_sb(struct file_system_type *fs_type,
+	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
 {
-	return get_sb_single(fs_type, flags, data, capifs_fill_super);
+	return get_sb_single(fs_type, flags, data, capifs_fill_super, mnt);
 }
 
 static struct file_system_type capifs_fs_type = {
diff --git a/drivers/misc/ibmasm/ibmasmfs.c b/drivers/misc/ibmasm/ibmasmfs.c
index 26a230b6ff8..4a35caff5d0 100644
--- a/drivers/misc/ibmasm/ibmasmfs.c
+++ b/drivers/misc/ibmasm/ibmasmfs.c
@@ -90,10 +90,11 @@ static void ibmasmfs_create_files (struct super_block *sb, struct dentry *root);
 static int ibmasmfs_fill_super (struct super_block *sb, void *data, int silent);
 
 
-static struct super_block *ibmasmfs_get_super(struct file_system_type *fst,
-			int flags, const char *name, void *data)
+static int ibmasmfs_get_super(struct file_system_type *fst,
+			int flags, const char *name, void *data,
+			struct vfsmount *mnt)
 {
-	return get_sb_single(fst, flags, data, ibmasmfs_fill_super);
+	return get_sb_single(fst, flags, data, ibmasmfs_fill_super, mnt);
 }
 
 static struct super_operations ibmasmfs_s_ops = {
diff --git a/drivers/oprofile/oprofilefs.c b/drivers/oprofile/oprofilefs.c
index b62da9b0cbf..71c2da277d6 100644
--- a/drivers/oprofile/oprofilefs.c
+++ b/drivers/oprofile/oprofilefs.c
@@ -272,10 +272,10 @@ static int oprofilefs_fill_super(struct super_block * sb, void * data, int silen
 }
 
 
-static struct super_block *oprofilefs_get_sb(struct file_system_type *fs_type,
-	int flags, const char *dev_name, void *data)
+static int oprofilefs_get_sb(struct file_system_type *fs_type,
+	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
 {
-	return get_sb_single(fs_type, flags, data, oprofilefs_fill_super);
+	return get_sb_single(fs_type, flags, data, oprofilefs_fill_super, mnt);
 }
 
 
diff --git a/drivers/usb/core/inode.c b/drivers/usb/core/inode.c
index 3cf945cc5b9..95f5ad923b0 100644
--- a/drivers/usb/core/inode.c
+++ b/drivers/usb/core/inode.c
@@ -543,10 +543,10 @@ static void fs_remove_file (struct dentry *dentry)
 
 /* --------------------------------------------------------------------- */
 
-static struct super_block *usb_get_sb(struct file_system_type *fs_type,
-	int flags, const char *dev_name, void *data)
+static int usb_get_sb(struct file_system_type *fs_type,
+	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
 {
-	return get_sb_single(fs_type, flags, data, usbfs_fill_super);
+	return get_sb_single(fs_type, flags, data, usbfs_fill_super, mnt);
 }
 
 static struct file_system_type usb_fs_type = {
diff --git a/drivers/usb/gadget/inode.c b/drivers/usb/gadget/inode.c
index aef0722b8f1..3bdc5e3ba23 100644
--- a/drivers/usb/gadget/inode.c
+++ b/drivers/usb/gadget/inode.c
@@ -2070,11 +2070,11 @@ enomem0:
 }
 
 /* "mount -t gadgetfs path /dev/gadget" ends up here */
-static struct super_block *
+static int
 gadgetfs_get_sb (struct file_system_type *t, int flags,
-		const char *path, void *opts)
+		const char *path, void *opts, struct vfsmount *mnt)
 {
-	return get_sb_single (t, flags, opts, gadgetfs_fill_super);
+	return get_sb_single (t, flags, opts, gadgetfs_fill_super, mnt);
 }
 
 static void