[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>

3 files changed, 10 insertions, 8 deletions

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 1045da582b9..3abf08f1b14 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -142,15 +142,16 @@ see also dquot_operations section.
 
 --------------------------- file_system_type ---------------------------
 prototypes:
-        struct super_block *(*get_sb) (struct file_system_type *, int,
-                        const char *, void *);
+        struct int (*get_sb) (struct file_system_type *, int,
+                        const char *, void *, struct vfsmount *);
         void (*kill_sb) (struct super_block *);
 locking rules:
                 may block       BKL
 get_sb          yes             yes
 kill_sb         yes             yes
 
-->get_sb() returns error or a locked superblock (exclusive on ->s_umount).
+->get_sb() returns error or 0 with locked superblock attached to the vfsmount
+(exclusive on ->s_umount).
 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
 unlocks and drops the reference.
 
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 2f388460cbe..5531694059a 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -50,10 +50,11 @@ Turn your foo_read_super() into a function that would return 0 in case of
 success and negative number in case of error (-EINVAL unless you have more
 informative error value to report).  Call it foo_fill_super().  Now declare
 
-struct super_block foo_get_sb(struct file_system_type *fs_type,
-        int flags, const char *dev_name, void *data)
+int foo_get_sb(struct file_system_type *fs_type,
+        int flags, const char *dev_name, void *data, struct vfsmount *mnt)
 {
-        return get_sb_bdev(fs_type, flags, dev_name, data, ext2_fill_super);
+        return get_sb_bdev(fs_type, flags, dev_name, data, foo_fill_super,
+                           mnt);
 }
 
 (or similar with s/bdev/nodev/ or s/bdev/single/, depending on the kind of
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 3a2e5520c1e..dd7d0dcedc8 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -113,8 +113,8 @@ members are defined:
 struct file_system_type {
         const char *name;
         int fs_flags;
-        struct super_block *(*get_sb) (struct file_system_type *, int,
-                                       const char *, void *);
+        struct int (*get_sb) (struct file_system_type *, int,
+                              const char *, void *, struct vfsmount *);
         void (*kill_sb) (struct super_block *);
         struct module *owner;
         struct file_system_type * next;