xfs: drain the buffer LRU on mount

Log recovery of v4 filesystems does not use buffer verifiers because
log recovery historically can result in transient buffer corruption
when target buffers might be ahead of the log after a crash. v5
filesystems work around this problem with metadata LSN ordering.

While this log recovery verifier behavior is necessary on v4 supers,
it can result in leaving buffers around in the LRU without verifiers
attached for a significant amount of time. This leads to use of
unverified buffers while the filesystem is in active use, long after
recovery has completed.

To address this problem, drain all buffers from the LRU as a final
step of the log mount sequence. Note that this is done
unconditionally to provide a consistently clean cache footprint,
regardless of superblock version or log state. As a side effect,
this ensures that all cache resident, unverified buffers are
reclaimed after log recovery and therefore must be recreated with
verifiers on subsequent use.

Reported-by: Darrick Wong <darrick.wong@oracle.com>
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_log.c

@@ -744,6 +744,7 @@ xfs_log_mount_finish(
 {
 	int	error = 0;
 	bool	readonly = (mp->m_flags & XFS_MOUNT_RDONLY);
+	bool	recovered = mp->m_log->l_flags & XLOG_RECOVERY_NEEDED;
 
 	if (mp->m_flags & XFS_MOUNT_NORECOVERY) {
 		ASSERT(mp->m_flags & XFS_MOUNT_RDONLY);
@@ -780,6 +781,21 @@ xfs_log_mount_finish(
 	mp->m_super->s_flags &= ~MS_ACTIVE;
 	evict_inodes(mp->m_super);
 
+	/*
+	 * Drain the buffer LRU after log recovery. This is required for v4
+	 * filesystems to avoid leaving around buffers with NULL verifier ops,
+	 * but we do it unconditionally to make sure we're always in a clean
+	 * cache state after mount.
+	 *
+	 * Don't push in the error case because the AIL may have pending intents
+	 * that aren't removed until recovery is cancelled.
+	 */
+	if (!error && recovered) {
+		xfs_log_force(mp, XFS_LOG_SYNC);
+		xfs_ail_push_all_sync(mp->m_ail);
+	}
+	xfs_wait_buftarg(mp->m_ddev_targp);
+
 	if (readonly)
 		mp->m_flags |= XFS_MOUNT_RDONLY;