[PATCH 1/3] NFS: Don't invalidate a submounted dentry in nfs_prime_dcache()

[Trond Myklebust <trond.myklebust@primarydata.com>]

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>
---
 fs/nfs/dir.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 9b0c55cb2a2e..0da617a61c0b 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -479,6 +479,10 @@ 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);
-- 
2.1.0