linux-kernel - Re: [PATCH 02/17] block: introduce bio_remove_last

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Date:   Thu, 16 Feb 2017 14:40:05 +0100
From:   Johannes Thumshirn <jthumshirn@...e.de>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     Shaohua Li <shli@...nel.org>, Jens Axboe <axboe@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:SOFTWARE RAID (Multiple Disks) SUPPORT" 
        <linux-raid@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        NeilBrown <neilb@...e.com>
Subject: Re: [PATCH 02/17] block: introduce bio_remove_last_page()

On 02/16/2017 02:30 PM, Ming Lei wrote:
> On Thu, Feb 16, 2017 at 8:08 PM, Johannes Thumshirn <jthumshirn@...e.de> 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@...il.com>
>>> ---
>>>  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--;
	}
}

-- 
Johannes Thumshirn                                          Storage
jthumshirn@...e.de                                +49 911 74053 689
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