xfs: Don't flush stale inodes

Because inodes remain in cache much longer than inode buffers do
under memory pressure, we can get the situation where we have
stale, dirty inodes being reclaimed but the backing storage has
been freed.  Hence we should never, ever flush XFS_ISTALE inodes
to disk as there is no guarantee that the backing buffer is in
cache and still marked stale when the flush occurs.
Signed-off-by: NDave Chinner <david@fromorbit.com>
Signed-off-by: NAlex Elder <aelder@sgi.com>

fs/xfs/xfs_inode.c

@@ -2841,10 +2841,14 @@ xfs_iflush(
 	mp = ip->i_mount;
 
 	/*
-	 * If the inode isn't dirty, then just release the inode
-	 * flush lock and do nothing.
+	 * If the inode isn't dirty, then just release the inode flush lock and
+	 * do nothing. Treat stale inodes the same; we cannot rely on the
+	 * backing buffer remaining stale in cache for the remaining life of
+	 * the stale inode and so xfs_itobp() below may give us a buffer that
+	 * no longer contains inodes below. Doing this stale check here also
+	 * avoids forcing the log on pinned, stale inodes.
 	 */
-	if (xfs_inode_clean(ip)) {
+	if (xfs_inode_clean(ip) || xfs_iflags_test(ip, XFS_ISTALE)) {
 		xfs_ifunlock(ip);
 		return 0;
 	}