NFS: Fix the fsid revalidation in nfs_update_inode()

When we detect that we've crossed a mountpoint on the remote server, we
must take care not to use that inode to revalidate the fsid on our
current superblock. To do so, we label the inode as a remote mountpoint,
and check for that in nfs_update_inode().
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/inode.c

@@ -299,6 +299,7 @@ nfs_fhget(struct super_block *sb, struct nfs_fh *fh, struct nfs_fattr *fattr)
 				else
 					inode->i_op = &nfs_mountpoint_inode_operations;
 				inode->i_fop = NULL;
+				set_bit(NFS_INO_MOUNTPOINT, &nfsi->flags);
 			}
 		} else if (S_ISLNK(inode->i_mode))
 			inode->i_op = &nfs_symlink_inode_operations;
@@ -1003,8 +1004,9 @@ static int nfs_update_inode(struct inode *inode, struct nfs_fattr *fattr)
 
 	server = NFS_SERVER(inode);
 	/* Update the fsid? */
-	if (S_ISDIR(inode->i_mode)
-			&& !nfs_fsid_equal(&server->fsid, &fattr->fsid))
+	if (S_ISDIR(inode->i_mode) &&
+			!nfs_fsid_equal(&server->fsid, &fattr->fsid) &&
+			!test_bit(NFS_INO_MOUNTPOINT, &nfsi->flags))
 		server->fsid = fattr->fsid;
 
 	/*

include/linux/nfs_fs.h

@@ -195,6 +195,7 @@ struct nfs_inode {
 #define NFS_INO_ADVISE_RDPLUS	(1)		/* advise readdirplus */
 #define NFS_INO_STALE		(2)		/* possible stale inode */
 #define NFS_INO_ACL_LRU_SET	(3)		/* Inode is on the LRU list */
+#define NFS_INO_MOUNTPOINT	(4)		/* inode is remote mountpoint */
 
 static inline struct nfs_inode *NFS_I(const struct inode *inode)
 {