[PATCH] block: fix bio_add_page for non trivial merge_bvec_fn case(v2)

[Dmitri Monakhov <dmonakhov@xxxxxxxxxx>]

We have to properly decrease bi_size in order to merge_bvec_fn return
right result.  Otherwise this result in false merge rejects for two
absolutely valid bio_vecs.  This may cause significant performance penalty
for example Itanium: page_size ==16k, fs_block_size == 1k and block device
is raid with small chunk_size.

Changes against v1:
 According to axboe@ and agk@ we don't have to decrease bi_vcnt and segments
 counters, so patch now looks like one line fix.

Signed-off-by: Dmitri Monakhov <dmonakhov@xxxxxxxxxx>
---
 fs/bio.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/fs/bio.c b/fs/bio.c
index 25f1af0..0e6ed2a 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -330,7 +330,8 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 				struct bvec_merge_data bvm = {
 					.bi_bdev = bio->bi_bdev,
 					.bi_sector = bio->bi_sector,
-					.bi_size = bio->bi_size,
+					.bi_size = bio->bi_size -
+							(prev->bv_len - len),
 					.bi_rw = bio->bi_rw,
 				};
 
-- 
1.5.4.rc4

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