Re: [PATCH 02/17] block: introduce bio_remove_last_page()

[Johannes Thumshirn]

On 02/16/2017 02:59 PM, Ming Lei wrote:
> On Thu, Feb 16, 2017 at 9:40 PM, Johannes Thumshirn <jthumshirn@xxxxxxx> wrote:
>> On 02/16/2017 02:30 PM, Ming Lei wrote:
>>> On Thu, Feb 16, 2017 at 8:08 PM, Johannes Thumshirn <jthumshirn@xxxxxxx> wrote:
>>>> On 02/16/2017 12:45 PM, Ming Lei wrote:
>>>>> MD need this helper to remove the last added page, so introduce
>>>>> it.
>>>>>
>>>>> Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx>
>>>>> ---
>>>>>  block/bio.c         | 23 +++++++++++++++++++++++
>>>>>  include/linux/bio.h |  1 +
>>>>>  2 files changed, 24 insertions(+)
>>>>>
>>>>> diff --git a/block/bio.c b/block/bio.c
>>>>> index 5eec5e08417f..0ce7ffcd7939 100644
>>>>> --- a/block/bio.c
>>>>> +++ b/block/bio.c
>>>>> @@ -837,6 +837,29 @@ int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
>>>>>  EXPORT_SYMBOL(bio_add_pc_page);
>>>>>
>>>>>  /**
>>>>> + *   bio_remove_last_page    -       remove the last added page
>>>>> + *   @bio: destination bio
>>>>> + *
>>>>> + *   Attempt to remove the last added page from the bio_vec maplist.
>>>>> + */
>>>>> +void bio_remove_last_page(struct bio *bio)
>>>>> +{
>>>>> +     /*
>>>>> +      * cloned bio must not modify vec list
>>>>> +      */
>>>>> +     if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
>>>>> +             return;
>>>>> +
>>>>> +     if (bio->bi_vcnt > 0) {
>>>>
>>>> In patch 1 you introduce bio_segments_all() with the log message 'So
>>>> that we can replace the direct access to .bi_vcnt.' Here you introduce a
>>>> new direct access to it (plus the duplicated WARN_ON_ONCE()).
>>>>
>>>> Maybe use the helper directly here (I admit I haven't gone through the
>>>> whole series yet, so I can't see if the change is made later).
>>>
>>> Firstly MD does need one helper to remove the last added page, as you
>>> can see there are three such uses in patch3.
>>>
>>> Secondly both the two helpers will be changed once multipage bvec
>>> is supported, that means we have to change MD too after multipage bvec
>>> if just using bio_segments_all() to replace .bi_vcnt for removing
>>> the last added page.
>>
>> I'm not sure if we're talking past each other here, I assumed you'd do
>> something like:
>>
>> void bio_remove_last_page(struct bio *bio)
>> {
>>         int vcnt = bio_segments_all(bio);
>>
>>         if (bio_flagged(bio, BIO_CLONED))
>>                 return;
>>
>>         if (vcnt > 0) {
>>                 struct bio_vec *bv = &bio->bi_io_vec[vcnt - 1];
>>
>>                 bio->bi_iter.bi_size -= bv->bv_len;
>>                 bio->bi_vcnt--;
>>         }
>> }
> 
> What we are doing is to remove the external direct access to bvec
> table in drivers or filesystems because they may misuse that, for
> example, drivers often use .bi_vcnt to get the page count in the bio,
> but the actual meaning is just bvec's count.
> 
> And the implementation in block layer still need to play the table directly,
> especially for sake of efficiency, cause we understand the details and won't
> misuse that.

Ah OK, so bio_segments_all() is intended for drivers.

Thanks for the clarification,
	Johannes


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