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 <[email protected]>
> 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 <[email protected]>
> ---
> 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
On Tue, Jan 26, 2010 at 4:29 PM, Jens Axboe <[email protected]> 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 <[email protected]>
>> 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 <[email protected]>
>> ---
>> 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?
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
>
>
Dmitry Monakhov <[email protected]> writes:
> On Tue, Jan 26, 2010 at 4:29 PM, Jens Axboe <[email protected]> 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 <[email protected]>
>>> 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 <[email protected]>
>>> ---
>>> 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.
On Thu, Jan 28 2010, Dmitry Monakhov wrote:
> Dmitry Monakhov <[email protected]> writes:
>
> > On Tue, Jan 26, 2010 at 4:29 PM, Jens Axboe <[email protected]> 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 <[email protected]>
> >>> 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 <[email protected]>
> >>> ---
> >>> ?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