[PATCH] NFS: Revalidate once when holding a delegation

[Benjamin Coddington <bcodding@redhat.com>]

If we skip lookup revalidataion while holding a delegation, we might miss
that the file has changed directories on the server.  This can happen if
the file is moved in between the client caching a dentry and obtaining a
delegation.  That can be reproduced on a single system with this bash:

mkdir -p /exports/dir{1,2}
exportfs -o rw localhost:/exports
mount -t nfs -ov4.1,noac localhost:/exports /mnt/localhost

touch /exports/dir1/fubar

cat /mnt/localhost/dir1/fubar
mv /mnt/localhost/dir{1,2}/fubar

mv /exports/dir{2,1}/fubar

cat /mnt/localhost/dir1/fubar
mv /mnt/localhost/dir{1,2}/fubar

In this example, the final `mv` will stat source and destination and find
they are the same dentry.

To fix this without giving up the increased lookup performance that holding
a delegation provides, let's revalidate the dentry only once after
obtaining a delegation by placing a magic value in the dentry's verifier.

Suggested-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
---
 fs/nfs/dir.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index e180033e35cf..81a5a28d0015 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1073,6 +1073,7 @@ int nfs_neg_need_reval(struct inode *dir, struct dentry *dentry,
 	return !nfs_check_verifier(dir, dentry, flags & LOOKUP_RCU);
 }
 
+#define NFS_DELEGATION_VERF 0xfeeddeaf
 static int
 nfs_lookup_revalidate_done(struct inode *dir, struct dentry *dentry,
 			   struct inode *inode, int error)
@@ -1133,6 +1134,7 @@ nfs_lookup_revalidate_dentry(struct inode *dir, struct dentry *dentry,
 	struct nfs_fh *fhandle;
 	struct nfs_fattr *fattr;
 	struct nfs4_label *label;
+	unsigned long verifier;
 	int ret;
 
 	ret = -ENOMEM;
@@ -1154,6 +1156,11 @@ nfs_lookup_revalidate_dentry(struct inode *dir, struct dentry *dentry,
 	if (nfs_refresh_inode(inode, fattr) < 0)
 		goto out;
 
+	if (NFS_PROTO(dir)->have_delegation(inode, FMODE_READ))
+		verifier = NFS_DELEGATION_VERF;
+	else
+		verifier = nfs_save_change_attribute(dir);
+
 	nfs_setsecurity(inode, fattr, label);
 	nfs_set_verifier(dentry, nfs_save_change_attribute(dir));
 
@@ -1167,6 +1174,15 @@ nfs_lookup_revalidate_dentry(struct inode *dir, struct dentry *dentry,
 	return nfs_lookup_revalidate_done(dir, dentry, inode, ret);
 }
 
+static int nfs_delegation_matches_dentry(struct inode *dir,
+			struct dentry *dentry, struct inode *inode)
+{
+	if (NFS_PROTO(dir)->have_delegation(inode, FMODE_READ) &&
+		dentry->d_time == NFS_DELEGATION_VERF)
+		return 1;
+	return 0;
+}
+
 /*
  * This is called every time the dcache has a lookup hit,
  * and we should check whether we can really trust that
@@ -1197,7 +1213,7 @@ nfs_do_lookup_revalidate(struct inode *dir, struct dentry *dentry,
 		goto out_bad;
 	}
 
-	if (NFS_PROTO(dir)->have_delegation(inode, FMODE_READ))
+	if (nfs_delegation_matches_dentry(dir, dentry, inode))
 		return nfs_lookup_revalidate_delegated(dir, dentry, inode);
 
 	/* Force a full look up iff the parent directory has changed */
@@ -1635,7 +1651,7 @@ nfs4_do_lookup_revalidate(struct inode *dir, struct dentry *dentry,
 	if (inode == NULL)
 		goto full_reval;
 
-	if (NFS_PROTO(dir)->have_delegation(inode, FMODE_READ))
+	if (nfs_delegation_matches_dentry(dir, dentry, inode))
 		return nfs_lookup_revalidate_delegated(dir, dentry, inode);
 
 	/* NFS only supports OPEN on regular files */
-- 
2.20.1