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

[Dmitry Monakhov <dmonakhov@openvz.org>]

[-- Attachment #1: Type: text/plain, Size: 2212 bytes --]

Dmitry Monakhov <dmonakhov@openvz.org> writes:

> On Tue, Jan 26, 2010 at 4:29 PM, Jens Axboe <jens.axboe@oracle.com> wrote:
>> On Tue, Jan 26 2010, Dmitry Monakhov wrote:
>>> 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.
>>>
>>
>>> 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,
>>>                               };
>>
>> Hmm confused. why isn't this just bio->bi_size - len?
I've attached more descriptive  version of the patch. Jens, please
clarify your opinion to the patch( do you like it or not?)
I don't want it miss again.

[-- Attachment #2: 0001-PATCH-block-fix-bio_add_page-for-non-trivial-merge_b.patch --]
[-- Type: text/plain, Size: 1521 bytes --]

>From 65ac8c9a2988310a79215b68e9c93b6dca6c6665 Mon Sep 17 00:00:00 2001
From: Dmitry Monakhov <dmonakhov@openvz.org>
Date: Wed, 27 Jan 2010 22:44:36 +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 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.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
---
 fs/bio.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/fs/bio.c b/fs/bio.c
index 76e6713..7448c7d 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,
 				};
 
-- 
1.6.3.3