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>

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);