[PATCH] block: fix bio_add_page for non trivial merge_bvec_fn case
Jens Axboe
> 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 <dmon...@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 <dmon...@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?
--
Jens Axboe
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Dmitry Monakhov
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
> 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.