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authorDarrick J. Wong <darrick.wong@oracle.com>2013-04-05 15:36:32 +0100
committerAlasdair G Kergon <agk@redhat.com>2013-04-05 15:36:32 +0100
commitb844fe691897221ad0d5e0279c8ea9e3e4a46982 (patch)
treeaff4244920ea1fcc3247d1532c72ad416fd67972 /drivers/md
parent07961ac7c0ee8b546658717034fe692fd12eefa9 (diff)
dm cache: fix writes to cache device in writethrough mode
The dm-cache writethrough strategy introduced by commit e2e74d617eadc15 ("dm cache: fix race in writethrough implementation") issues a bio to the origin device, remaps and then issues the bio to the cache device. This more conservative in-series approach was selected to favor correctness over performance (of the previous parallel writethrough). However, this in-series implementation that reuses the same bio to write both the origin and cache device didn't take into account that the block layer's req_bio_endio() modifies a completing bio's bi_sector and bi_size. So the new writethrough strategy needs to preserve these bio fields, and restore them before submission to the cache device, otherwise nothing gets written to the cache (because bi_size is 0). This patch adds a struct dm_bio_details field to struct per_bio_data, and uses dm_bio_record() and dm_bio_restore() to ensure the bio is restored before reissuing to the cache device. Adding such a large structure to the per_bio_data is not ideal but we can improve this later, for now correctness is the important thing. This problem initially went unnoticed because the dm-cache test-suite uses a linear DM device for the dm-cache device's origin device. Writethrough worked as expected because DM submits a *clone* of the original bio, so the original bio which was reused for the cache was never touched. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Diffstat (limited to 'drivers/md')
-rw-r--r--drivers/md/dm-cache-target.c4
1 files changed, 4 insertions, 0 deletions
diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 66120bd46d1..1ab122a7576 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -6,6 +6,7 @@
#include "dm.h"
#include "dm-bio-prison.h"
+#include "dm-bio-record.h"
#include "dm-cache-metadata.h"
#include <linux/dm-io.h>
@@ -205,6 +206,7 @@ struct per_bio_data {
struct cache *cache;
dm_cblock_t cblock;
bio_end_io_t *saved_bi_end_io;
+ struct dm_bio_details bio_details;
};
struct dm_cache_migration {
@@ -643,6 +645,7 @@ static void writethrough_endio(struct bio *bio, int err)
return;
}
+ dm_bio_restore(&pb->bio_details, bio);
remap_to_cache(pb->cache, bio, pb->cblock);
/*
@@ -667,6 +670,7 @@ static void remap_to_origin_then_cache(struct cache *cache, struct bio *bio,
pb->cache = cache;
pb->cblock = cblock;
pb->saved_bi_end_io = bio->bi_end_io;
+ dm_bio_record(&pb->bio_details, bio);
bio->bi_end_io = writethrough_endio;
remap_to_origin_clear_discard(pb->cache, bio, oblock);