More fun with unmounting ESTALE directories.

NeilBrown
2013-02-12T00:38:28Z

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I've been exploring difficulties with unmounting stale directories and
discovered another bug.

If I:

SERVER: mkdir /foo/bar #and make sure it is exported
CLIENT: mount -o vers=4 server:/foo/bar /mnt
SERVER: rm -r /foo
CLIENT: > /mnt/baz # gets an error of course
CLIENT: ls -l /mnt # error again
CLIENT: umount /mnt

The result of that last command is:

/mnt was not found in /proc/mounts
/mnt was not found in /proc/mounts

Strange?

cat /proc/mounts

.....
10.0.2.2://foo/bar /mnt\040(deleted) nfs4 rw,relatime,vers=4,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,clientaddr=10.0.2.15,minorversion=0,local_lock=none,addr=10.0.2.2 0 0
....

Notice the "\040(deleted)".

NFS has unhashed that directory because it is obviously bad, and d_path()
notices and adds " (deleted)".

Now I might be able to argue that NFS shouldn't be unhashing a directory that
is a mountpoint - it certainly seems strange behaviour.

But I think I can more strongly argue that /proc/mounts shouldn't be showing
the mounted directory, but instead the directory that it is mounted on.
Obviously these both have the same name so it shouldn't matter ... except
that here is a case where it does.

I "fixed" it with

--- a/fs/proc_namespace.c
+++ b/fs/proc_namespace.c
@@ -93,7 +93,7 @@ static int show_vfsmnt(struct seq_file *m, struct vfsmount *mnt)
{
struct mount *r = real_mount(mnt);
int err = 0;
- struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };
+ struct path mnt_path = { .dentry = r->mnt_mountpoint, .mnt = &(r->mnt_parent)->mnt };
struct super_block *sb = mnt_path.dentry->d_sb;

if (sb->s_op->show_devname) {

though I suspect that isn't safe and needs some locking.

Probably both should be fixed: NFS should not invalidate any mounted
directory, and show_vfsmnt() should report the mointpoint, not the mounted
directory.

I can't figure out any way to get NFS to not invalidate the mounted directory.
I think it happens in nfs_lookup_revalidate() when it calls d_drop(), but I
don't know how to tell if a given dentry is a mnt_root for any mountpoint.

Suggestions? Thoughts?

Thanks,
NeilBrown

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