• David Howells's avatar
    [PATCH] VFS: Permit filesystem to override root dentry on mount · 454e2398
    David Howells authored
    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: default avatarDavid Howells <dhowells@redhat.com>
    Acked-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    Cc: Nathan Scott <nathans@sgi.com>
    Cc: Roland Dreier <rolandd@cisco.com>
    Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>