linux-kernel - Re: [PATCH] block: fix bio_add_page for non trivial merge_bvec

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Date:	Thu, 28 Jan 2010 00:11:37 +0300
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: fix bio_add_page for non trivial merge_bvec_fn  case

Dmitry Monakhov <dmonakhov@...nvz.org> writes:

> On Tue, Jan 26, 2010 at 4:29 PM, Jens Axboe <jens.axboe@...cle.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@...nvz.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@...nvz.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.

View attachment "0001-PATCH-block-fix-bio_add_page-for-non-trivial-merge_b.patch" of type "text/plain" (1522 bytes)