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

[Dmitry Monakhov]

On Tue, Jan 26, 2010 at 4:29 PM, Jens Axboe <jens.axboe@xxxxxxxxxx> 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@xxxxxxxxxx>
>> 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. ÂOtherwise this result in false merge rejects for two
>> absolutely valid bio_vecs. ÂThis 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@xxxxxxxxxx>
>> ---
>> Âfs/bio.c | Â Â3 ++-
>> Â1 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
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â struct bvec_merge_data bvm = {
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â .bi_bdev = bio->bi_bdev,
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â .bi_sector = bio->bi_sector,
>> - Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â .bi_size = bio->bi_size,
>> + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â .bi_size = bio->bi_size -
>> + Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â (prev->bv_len - len),
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â .bi_rw = bio->bi_rw,
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â };
>
> Hmm confused. why isn't this just bio->bi_size - len?
because we have following scenario:
0) old_bv_len = prev->bv_len ;
1) prev->bv_len += len;
2)->merge_bvec_fn()
      usually it looks like follows
      if (bio->bv_len + bvm->bi_size > max_chunk_size)
          goto fail
     So formally we detach last bvec increase it size and try to
attach it to bio.

     but old_bv_len is already accounted in bi_size, so we have to decrease
    bi_size to this value, and old_bv_len = (prev->bv_len - len) at
that moment.
    I've used math manipuation in order to avoid temporary variable
    because stack size is matter on block layer.
>
> --
> Jens Axboe
>
>
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