[dm-devel] [PATCH] dm writecache: fix performance degradation in ssd mode

[Mikulas Patocka]

On Fri, 15 Jan 2021, J. Bruce Fields wrote:

> Any recommendations?  I spent some time with blktrace/blkparse/btt and
> can't make head or tails of them, I'm afraid.
> 
> --b.

Hi

Try this patch:


From: Mikulas Patocka <mpatocka at redhat.com>

Fix a thinko in ssd_commit_superblock. region.count is in sectors, not
bytes. This bug doesn't corrupt data, but it causes performance
degradation.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
Fixes: dc8a01ae1dbd ("dm writecache: optimize superblock write")
Cc: stable at vger.kernel.org	# v5.7
Reported-by: J. Bruce Fields <bfields at redhat.com>

---
 drivers/md/dm-writecache.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6/drivers/md/dm-writecache.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-writecache.c	2021-01-23 10:51:31.000000000 +0100
+++ linux-2.6/drivers/md/dm-writecache.c	2021-01-23 15:16:53.000000000 +0100
@@ -523,7 +523,7 @@ static void ssd_commit_superblock(struct
 
 	region.bdev = wc->ssd_dev->bdev;
 	region.sector = 0;
-	region.count = PAGE_SIZE;
+	region.count = PAGE_SIZE >> SECTOR_SHIFT;
 
 	if (unlikely(region.sector + region.count > wc->metadata_sectors))
 		region.count = wc->metadata_sectors - region.sector;