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Introduce automounter dentry ops

From:  David Howells <>
Subject:  [PATCH 00/17] Introduce automounter dentry ops
Date:  Thu, 30 Sep 2010 19:14:55 +0100
Message-ID:  <>
Archive-link:  Article

The attached patches introduce a couple of new dentry operations for use by
automounters and make AFS, NFS, CIFS and autofs4 use them.  This means that
these filesystems no longer have to abuse lookup(), follow_link() and
d_revalidate() to achieve the desired effect.

There are two dentry operations provided:

 (1) struct vfsmount *(*d_automount)(struct path *path);

     This is used by follow_automount() in fs/namei.c to ask the filesystem
     that owns the dentry at the current path point to mount something on

     It is called if either the inode belonging to the given dentry is flagged
     S_AUTOMOUNT or the dentry is flagged DMANAGED_AUTOMOUNT, and if the dentry
     has nothing mounted on it in the current namespace when someone attempts
     to use that dentry.

     No locks will be held when this is called.

     d_op->d_automount() may return one of:

     (a) The vfsmount mounted upon that dentry, in which case pathwalk will
     	 move to the root dentry of that vfsmount.

     (b) NULL if something was already mounted there, in which case pathwalk
     	 will loop around and recheck the mountings.

     (c) -EISDIR, in which case pathwalk will stop at this point and attempt to
     	 use that dentry as the object of interest.  If the current dentry is
     	 not terminal within the path, -EREMOTE will be returned.

     (d) An error value, to be returned immediately.

     Automount transits are counted as symlinks to prevent circular references
     from being a problem.  If one is detected, -ELOOP will be returned.

     If stat() is given AT_NO_AUTOMOUNT then d_op->d_automount() will not be
     invoked on a terminal dentry; instead that dentry will be returned by

     follow_automount() also does not invoke d_op->d_automount() if the caller
     gave AT_SYMLINK_NOFOLLOW to stat(), but rather returns the base dentry.

 (2) int (*d_manage)(struct path *path, bool mounting_here);

     This is called by managed_dentry() or follow_down() in fs/namei.c to
     indicate to a filesystem that pathwalk is about to step off of the current
     path point and walk to another point in the path.

     This is called if DMANAGED_TRANSIT is set on a dentry.

     This can then be used by autofs to stop non-daemon processes from walking
     until it has finished constructing or expiring the tree behind the dentry.
     It could also be used to prevent undesirables from mounting on this

     @mounting_here is true if called from follow_down() from mount, in which
     case namespace_sem is held exclusively by the caller of follow_down().
     Otherwise, no locks are held.

     d_op->d_manage() may return one of:

     (a) 0 to continue the pathwalk as normal.

     (b) -EISDIR to prevent managed_dentry() from crossing to a mounted
     	 filesystem or calling d_op->d_automount(), in which case the dentry
     	 will be treated as an ordinary directory.

     (c) An error to abort the pathwalk completely.

To make this work for autofs, d_mounted in struct dentry has become d_managed.
This used to be a count of the number of mounts on this dentry.  The bottom 28
bits are still that (DMANAGED_MOUNTPOINT).  The upper four bits contain a
couple of flags, DMANAGED_TRANSIT and DMANAGED_AUTOMOUNT.  This allows
managed_dentry() to test all three conditions with minimumal overhead if none
of them are true.

For other filesystems, setting S_AUTOMOUNT is sufficient.  This is noted by
__d_instantiate() which will set DMANAGED_AUTOMOUNT automatically if it is
seen.  Checking S_AUTOMOUNT doesn't work for autofs, however, since the dentry
might not have an inode, hence why a dentry flag also.

S_AUTOMOUNT and d_automount() are introduced in patch 1; d_manage(), d_managed
and DMANAGED_* are introduced in patch 7.


David Howells (8):
      Make follow_down() handle d_manage()
      Make dentry::d_mounted into a more general field for special function dirs
      Add an AT_NO_AUTOMOUNT flag to suppress terminal automount
      Remove the automount through follow_link() kludge code from pathwalk
      CIFS: Use d_automount() rather than abusing follow_link()
      NFS: Use d_automount() rather than abusing follow_link()
      AFS: Use d_automount() rather than abusing follow_link()
      Add a dentry op to handle automounting rather than abusing follow_link()

Ian Kent (9):
      autofs4 - bump version
      autofs4 - add v4 pseudo direct mount support
      autofs4 - fix wait validation
      autofs4: cleanup autofs4_free_ino()
      autofs4: cleanup dentry operations
      autofs4: cleanup inode operations
      autofs4: removed unused code
      autofs4: add d_manage() dentry operation
      autofs4: add d_automount() dentry operation

 Documentation/filesystems/Locking |    2 
 Documentation/filesystems/vfs.txt |   22 +
 fs/afs/dir.c                      |    1 
 fs/afs/inode.c                    |    3 
 fs/afs/internal.h                 |    1 
 fs/afs/mntpt.c                    |   47 +--
 fs/autofs/dirhash.c               |    5 
 fs/autofs4/autofs_i.h             |  100 ++++--
 fs/autofs4/dev-ioctl.c            |    2 
 fs/autofs4/expire.c               |   42 ++
 fs/autofs4/inode.c                |   28 --
 fs/autofs4/root.c                 |  668 ++++++++++++++++---------------------
 fs/autofs4/waitq.c                |   17 +
 fs/cifs/cifs_dfs_ref.c            |  134 ++++---
 fs/cifs/cifsfs.h                  |    6 
 fs/cifs/dir.c                     |    2 
 fs/cifs/inode.c                   |    8 
 fs/dcache.c                       |    7 
 fs/namei.c                        |  243 +++++++++++--
 fs/namespace.c                    |   20 +
 fs/nfs/dir.c                      |    4 
 fs/nfs/inode.c                    |    4 
 fs/nfs/internal.h                 |    1 
 fs/nfs/namespace.c                |   87 ++---
 fs/nfsd/vfs.c                     |    5 
 fs/stat.c                         |    4 
 include/linux/auto_fs4.h          |    2 
 include/linux/dcache.h            |   19 +
 include/linux/fcntl.h             |    1 
 include/linux/fs.h                |    2 
 include/linux/namei.h             |    5 
 include/linux/nfs_fs.h            |    1 
 32 files changed, 836 insertions(+), 657 deletions(-)

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