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

[Baolin Wang]

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.

> 
> > +++ b/block/blk-mq-sched.c
> > @@ -391,31 +391,17 @@ bool blk_mq_bio_list_merge(struct request_queue *q, struct list_head *list,
> >  {
> >  	struct request *rq;
> >  	int checked = 8;
> > +	enum bio_merge_status merge;
> >  
> >  	list_for_each_entry_reverse(rq, list, queuelist) {
> > -		bool merged = false;
> > -
> >  		if (!checked--)
> >  			break;
> >  
> > +		merge = blk_attempt_bio_merge(q, rq, bio, nr_segs);
> > +		if (merge == BIO_MERGE_NONE)
> >  			continue;
> >  
> > +		return merge == BIO_MERGE_OK ? true: false;
> 
> Maybe write this a little more explicit:
> 
> 		switch (blk_attempt_bio_merge(q, rq, bio, nr_segs)) {
> 		case BIO_MERGE_NONE:
> 			continue:
> 		case BIO_MERGE_OK:
> 			return true;
> 		case BIO_MERGE_FAILED:
> 			return false;
> 		}

Sure.

> 
> >  enum elv_merge blk_try_merge(struct request *rq, struct bio *bio);
> >  
> > +enum bio_merge_status {
> > +	BIO_MERGE_OK,
> > +	BIO_MERGE_NONE,
> > +	BIO_MERGE_FAILED,
> > +};
> > +
> > +enum bio_merge_status blk_attempt_bio_merge(struct request_queue *q,
> > +		struct request *rq, struct bio *bio, unsigned int nr_segs);
> > +
> >  int blk_dev_init(void);
> >  
> >  /*
> > -- 
> > 1.8.3.1
> ---end quoted text---