linux-kernel - Re: [PATCH RESEND 3/5] block: Add a new helper to attempt to merge a bio

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Date:   Mon, 17 Aug 2020 14:24:40 +0200
From:   Christoph Hellwig <hch@....de>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     Christoph Hellwig <hch@....de>, axboe@...nel.dk,
        ming.lei@...hat.com, baolin.wang7@...il.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 3/5] block: Add a new helper to attempt to merge
 a bio

On Mon, Aug 17, 2020 at 08:10:02PM +0800, Baolin Wang wrote:
> On Mon, Aug 17, 2020 at 08:26:34AM +0200, Christoph Hellwig wrote:
> > On Mon, Aug 17, 2020 at 12:09:17PM +0800, Baolin Wang wrote:
> > > There are lots of duplicated code when trying to merge a bio from
> > > plug list and sw queue, we can introduce a new helper to attempt
> > > to merge a bio, which can simplify the blk_mq_bio_list_merge()
> > > and blk_attempt_plug_merge().
> > 
> > Looks sensible, but two comments:
> > 
> > > +enum bio_merge_status blk_attempt_bio_merge(struct request_queue *q,
> > > +					    struct request *rq,
> > > +					    struct bio *bio,
> > > +					    unsigned int nr_segs)
> > > +{
> > > +	bool merged = false;
> > > +
> > > +	if (!blk_rq_merge_ok(rq, bio))
> > > +		return BIO_MERGE_NONE;
> > > +
> > > +	switch (blk_try_merge(rq, bio)) {
> > > +	case ELEVATOR_BACK_MERGE:
> > > +		merged = bio_attempt_back_merge(rq, bio, nr_segs);
> > > +		break;
> > > +	case ELEVATOR_FRONT_MERGE:
> > > +		merged = bio_attempt_front_merge(rq, bio, nr_segs);
> > > +		break;
> > > +	case ELEVATOR_DISCARD_MERGE:
> > > +		merged = bio_attempt_discard_merge(q, rq, bio);
> > > +		break;
> > 
> > Can't we also switch the bio_attempt_*merge helpers to return
> > enum bio_merge_status to simplify this a bit?
> 
> Yes, will do.
> 
> > 
> > Also I think these helpers can be marked static now, although I didn't
> > actually apply your series, so I might have missed something.
> 
> Cause this function will be used by blk_mq_bio_list_merge() in
> blk-mq-sched.c, it should be exported.

Shouldn't blk_mq_bio_list_merge move to blk-merge.c as well?