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author | Darrick J. Wong <darrick.wong@oracle.com> | 2013-04-05 14:36:32 (GMT) |
---|---|---|
committer | Alasdair G Kergon <agk@redhat.com> | 2013-04-05 14:36:32 (GMT) |
commit | b844fe691897221ad0d5e0279c8ea9e3e4a46982 (patch) | |
tree | aff4244920ea1fcc3247d1532c72ad416fd67972 /drivers | |
parent | 07961ac7c0ee8b546658717034fe692fd12eefa9 (diff) | |
download | linux-b844fe691897221ad0d5e0279c8ea9e3e4a46982.tar.xz |
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')
-rw-r--r-- | drivers/md/dm-cache-target.c | 4 |
1 files changed, 4 insertions, 0 deletions
diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c index 66120bd..1ab122a 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); |