xfs: optimise CRC updates

Nick Piggin reported that the CRC overhead in an fsync heavy
workload was higher than expected on a Power8 machine. Part of this
was to do with the fact that the power8 CRC implementation is not
efficient for CRC lengths of less than 512 bytes, and so the way we
split the CRCs over the CRC field means a lot of the CRCs are
reduced to being less than than optimal size.

To optimise this, change the CRC update mechanism to zero the CRC
field first, and then compute the CRC in one pass over the buffer
and write the result back into the buffer. We can do this safely
because anything writing a CRC has exclusive access to the buffer
the CRC is being calculated over.

We leave the CRC verify code the same - it still splits the CRC
calculation - because we do not want read-only operations modifying
the underlying buffer. This is because read-only operations may not
have an exclusive access to the buffer guaranteed, and so temporary
modifications could leak out to to other processes accessing the
buffer concurrently.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

1 files changed, 7 insertions, 5 deletions

diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 9b3d7c76915d..56b7a2f6aaf2 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -5113,19 +5113,21 @@ xlog_recover_process(
 	struct list_head	*buffer_list)
 {
 	int			error;
+	__le32			old_crc = rhead->h_crc;
 	__le32			crc;
 
+
 	crc = xlog_cksum(log, rhead, dp, be32_to_cpu(rhead->h_len));
 
 	/*
 	 * Nothing else to do if this is a CRC verification pass. Just return
 	 * if this a record with a non-zero crc. Unfortunately, mkfs always
-	 * sets h_crc to 0 so we must consider this valid even on v5 supers.
+	 * sets old_crc to 0 so we must consider this valid even on v5 supers.
 	 * Otherwise, return EFSBADCRC on failure so the callers up the stack
 	 * know precisely what failed.
 	 */
 	if (pass == XLOG_RECOVER_CRCPASS) {
-		if (rhead->h_crc && crc != rhead->h_crc)
+		if (old_crc && crc != old_crc)
 			return -EFSBADCRC;
 		return 0;
 	}
@@ -5136,11 +5138,11 @@ xlog_recover_process(
 	 * zero CRC check prevents warnings from being emitted when upgrading
 	 * the kernel from one that does not add CRCs by default.
 	 */
-	if (crc != rhead->h_crc) {
-		if (rhead->h_crc || xfs_sb_version_hascrc(&log->l_mp->m_sb)) {
+	if (crc != old_crc) {
+		if (old_crc || xfs_sb_version_hascrc(&log->l_mp->m_sb)) {
 			xfs_alert(log->l_mp,
 		"log record CRC mismatch: found 0x%x, expected 0x%x.",
-					le32_to_cpu(rhead->h_crc),
+					le32_to_cpu(old_crc),
 					le32_to_cpu(crc));
 			xfs_hex_dump(dp, 32);
 		}