NFS: Don't invalidate a submounted dentry in nfs_prime_dcache()

If we're traversing a directory which contains a submounted filesystem,
or one that has a referral, the NFS server that is processing the READDIR
request will often return information for the underlying (mounted-on)
directory. It may, or may not, also return filehandle information.

If this happens, and the lookup in nfs_prime_dcache() returns the
dentry for the submounted directory, the filehandle comparison will
fail, and we call d_invalidate(). Post-commit 8ed936b5671bf
("vfs: Lazily remove mounts on unlinked files and directories."), this
means the entire subtree is unmounted.

The following minimal patch addresses this problem by punting on
the invalidation if there is a submount.

Kudos to Neil Brown <neilb@suse.de> for having tracked down this
issue (see link).

Reported-by: Nix <nix@esperi.org.uk>
Link: http://lkml.kernel.org/r/87iofju9ht.fsf@spindle.srvr.nix
Cc: stable@vger.kernel.org # 3.18+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 9b0c55cb2a2e..4ad7fff9ccaf 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -469,6 +469,8 @@ void nfs_prime_dcache(struct dentry *parent, struct nfs_entry *entry)
         struct inode *inode;
         int status;
 
+        if (!(entry->fattr->valid & NFS_ATTR_FATTR_FSID))
+                return;
         if (filename.name[0] == '.') {
                 if (filename.len == 1)
                         return;
@@ -479,6 +481,10 @@ void nfs_prime_dcache(struct dentry *parent, struct nfs_entry *entry)
 
         dentry = d_lookup(parent, &filename);
         if (dentry != NULL) {
+                /* Is there a mountpoint here? If so, just exit */
+                if (!nfs_fsid_equal(&NFS_SB(dentry->d_sb)->fsid,
+                                        &entry->fattr->fsid))
+                        goto out;
                 if (nfs_same_file(dentry, entry)) {
                         nfs_set_verifier(dentry, nfs_save_change_attribute(dir));
                         status = nfs_refresh_inode(dentry->d_inode, entry->fattr);