xfs: ratelimit unmount time per-buffer I/O error alert

At unmount time, XFS emits an alert for every in-core buffer that
might have undergone a write error. In practice this behavior is
probably reasonable given that the filesystem is likely short lived
once I/O errors begin to occur consistently. Under certain test or
otherwise expected error conditions, this can spam the logs and slow
down the unmount.

Now that we have a ratelimit mechanism specifically for buffer
alerts, reuse it for the per-buffer alerts in xfs_wait_buftarg().
Also lift the final repair message out of the loop so it always
prints and assert that the metadata error handling code has shut
down the fs.
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_buf.c

@@ -1657,7 +1657,8 @@ xfs_wait_buftarg(
 	struct xfs_buftarg	*btp)
 {
 	LIST_HEAD(dispose);
-	int loop = 0;
+	int			loop = 0;
+	bool			write_fail = false;
 
 	/*
 	 * First wait on the buftarg I/O count for all in-flight buffers to be
@@ -1685,17 +1686,29 @@ xfs_wait_buftarg(
 			bp = list_first_entry(&dispose, struct xfs_buf, b_lru);
 			list_del_init(&bp->b_lru);
 			if (bp->b_flags & XBF_WRITE_FAIL) {
-				xfs_alert(btp->bt_mount,
+				write_fail = true;
+				xfs_buf_alert_ratelimited(bp,
+					"XFS: Corruption Alert",
 "Corruption Alert: Buffer at daddr 0x%llx had permanent write failures!",
 					(long long)bp->b_bn);
-				xfs_alert(btp->bt_mount,
-"Please run xfs_repair to determine the extent of the problem.");
 			}
 			xfs_buf_rele(bp);
 		}
 		if (loop++ != 0)
 			delay(100);
 	}
+
+	/*
+	 * If one or more failed buffers were freed, that means dirty metadata
+	 * was thrown away. This should only ever happen after I/O completion
+	 * handling has elevated I/O error(s) to permanent failures and shuts
+	 * down the fs.
+	 */
+	if (write_fail) {
+		ASSERT(XFS_FORCED_SHUTDOWN(btp->bt_mount));
+		xfs_alert(btp->bt_mount,
+	      "Please run xfs_repair to determine the extent of the problem.");
+	}
 }
 
 static enum lru_status