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

[Dmitry Monakhov <dmonakhov@openvz.org>]

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Hi, year ago I've sent a patch which fix false bio merge rejects, but
seems patch was missed. Currently the issue is still present.


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>From 92a97ef181e15caa94bd56a1ade5c337db599b79 Mon Sep 17 00:00:00 2001
From: Dmitry Monakhov <dmonakhov@openvz.org>
Date: Tue, 26 Jan 2010 16:01:34 +0300
Subject: [PATCH] [PATCH] 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 Itanium: page_size == 16k, fs_block_size == 1k and block device
is raid with small chunk_size.

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

diff --git a/fs/bio.c b/fs/bio.c
index 76e6713..9f8e517 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -548,7 +548,8 @@ 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,
 				};
 
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1.6.3.3