From 1d6165851cd8e3f919d446cd6da35dee44e8837e Mon Sep 17 00:00:00 2001
From: Dmitry Monakhov <dmonakhov@openvz.org>
Date: Wed, 27 Jan 2010 22:44:36 +0300
Subject: block: fix bio_add_page for non trivial merge_bvec_fn case
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>
---
fs/bio.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/fs/bio.c b/fs/bio.c
index 12429c9553e..88094afc29e 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -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,
};
--
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