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    SubjectRe: [PATCH] block: fix bio_add_page for non trivial merge_bvec_fn case
    On Thu, Jan 28 2010, Dmitry Monakhov wrote:
    > 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.

    Yes thanks, I agree with your evaluation, it's definitely correct. I'll
    add it.

    --
    Jens Axboe

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