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block: fix bio_add_page for non trivial merge_bvec_fn case

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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 fs_block_size == 1k and block device is raid0 with
small chunk_size = 8k. Then it is impossible to merge 7-th fs-block in
to bio which already has 6 fs-blocks.

Cc: <stable@kernel.org>
Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
This commit is contained in:
Dmitry Monakhov 2010-01-27 22:44:36 +03:00 committed by Jens Axboe
parent c84a301d4a
commit 1d6165851c
1 changed files with 6 additions and 1 deletions
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7
fs/bio.c
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@ -542,13 +542,18 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
if (page == prev->bv_page &&
offset == prev->bv_offset + prev->bv_len) {
unsigned int prev_bv_len = prev->bv_len;
prev->bv_len += len;
if (q->merge_bvec_fn) {
struct bvec_merge_data bvm = {
/* prev_bvec is already charged in
bi_size, discharge it in order to
simulate merging updated prev_bvec
as new bvec. */
.bi_bdev = bio->bi_bdev,
.bi_sector = bio->bi_sector,
.bi_size = bio->bi_size,
.bi_size = bio->bi_size - prev_bv_len,
.bi_rw = bio->bi_rw,
};