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Date:   Mon, 19 Nov 2018 15:50:25 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Omar Sandoval <osandov@...ndov.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Dave Chinner <dchinner@...hat.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Shaohua Li <shli@...nel.org>,
        linux-raid@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, Gao Xiang <gaoxiang25@...wei.com>,
        Christoph Hellwig <hch@....de>, Theodore Ts'o <tytso@....edu>,
        linux-ext4@...r.kernel.org, Coly Li <colyli@...e.de>,
        linux-bcache@...r.kernel.org, Boaz Harrosh <ooo@...ctrozaur.com>,
        Bob Peterson <rpeterso@...hat.com>, cluster-devel@...hat.com
Subject: Re: [PATCH V10 03/19] block: use bio_for_each_bvec() to compute
 multi-page bvec count

On Thu, Nov 15, 2018 at 12:20:28PM -0800, Omar Sandoval wrote:
> On Thu, Nov 15, 2018 at 04:52:50PM +0800, Ming Lei wrote:
> > First it is more efficient to use bio_for_each_bvec() in both
> > blk_bio_segment_split() and __blk_recalc_rq_segments() to compute how
> > many multi-page bvecs there are in the bio.
> > 
> > Secondly once bio_for_each_bvec() is used, the bvec may need to be
> > splitted because its length can be very longer than max segment size,
> > so we have to split the big bvec into several segments.
> > 
> > Thirdly when splitting multi-page bvec into segments, the max segment
> > limit may be reached, so the bio split need to be considered under
> > this situation too.
> > 
> > Cc: Dave Chinner <dchinner@...hat.com>
> > Cc: Kent Overstreet <kent.overstreet@...il.com>
> > Cc: Mike Snitzer <snitzer@...hat.com>
> > Cc: dm-devel@...hat.com
> > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > Cc: linux-fsdevel@...r.kernel.org
> > Cc: Shaohua Li <shli@...nel.org>
> > Cc: linux-raid@...r.kernel.org
> > Cc: linux-erofs@...ts.ozlabs.org
> > Cc: David Sterba <dsterba@...e.com>
> > Cc: linux-btrfs@...r.kernel.org
> > Cc: Darrick J. Wong <darrick.wong@...cle.com>
> > Cc: linux-xfs@...r.kernel.org
> > Cc: Gao Xiang <gaoxiang25@...wei.com>
> > Cc: Christoph Hellwig <hch@....de>
> > Cc: Theodore Ts'o <tytso@....edu>
> > Cc: linux-ext4@...r.kernel.org
> > Cc: Coly Li <colyli@...e.de>
> > Cc: linux-bcache@...r.kernel.org
> > Cc: Boaz Harrosh <ooo@...ctrozaur.com>
> > Cc: Bob Peterson <rpeterso@...hat.com>
> > Cc: cluster-devel@...hat.com
> > Signed-off-by: Ming Lei <ming.lei@...hat.com>
> > ---
> >  block/blk-merge.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++---------
> >  1 file changed, 76 insertions(+), 14 deletions(-)
> > 
> > diff --git a/block/blk-merge.c b/block/blk-merge.c
> > index 91b2af332a84..6f7deb94a23f 100644
> > --- a/block/blk-merge.c
> > +++ b/block/blk-merge.c
> > @@ -160,6 +160,62 @@ static inline unsigned get_max_io_size(struct request_queue *q,
> >  	return sectors;
> >  }
> >  
> > +/*
> > + * Split the bvec @bv into segments, and update all kinds of
> > + * variables.
> > + */
> > +static bool bvec_split_segs(struct request_queue *q, struct bio_vec *bv,
> > +		unsigned *nsegs, unsigned *last_seg_size,
> > +		unsigned *front_seg_size, unsigned *sectors)
> > +{
> > +	bool need_split = false;
> > +	unsigned len = bv->bv_len;
> > +	unsigned total_len = 0;
> > +	unsigned new_nsegs = 0, seg_size = 0;
> 
> "unsigned int" here and everywhere else.
> 
> > +	if ((*nsegs >= queue_max_segments(q)) || !len)
> > +		return need_split;
> > +
> > +	/*
> > +	 * Multipage bvec may be too big to hold in one segment,
> > +	 * so the current bvec has to be splitted as multiple
> > +	 * segments.
> > +	 */
> > +	while (new_nsegs + *nsegs < queue_max_segments(q)) {
> > +		seg_size = min(queue_max_segment_size(q), len);
> > +
> > +		new_nsegs++;
> > +		total_len += seg_size;
> > +		len -= seg_size;
> > +
> > +		if ((queue_virt_boundary(q) && ((bv->bv_offset +
> > +		    total_len) & queue_virt_boundary(q))) || !len)
> > +			break;
> 
> Checking queue_virt_boundary(q) != 0 is superfluous, and the len check
> could just control the loop, i.e.,
> 
> 	while (len && new_nsegs + *nsegs < queue_max_segments(q)) {
> 		seg_size = min(queue_max_segment_size(q), len);
> 
> 		new_nsegs++;
> 		total_len += seg_size;
> 		len -= seg_size;
> 
> 		if ((bv->bv_offset + total_len) & queue_virt_boundary(q))
> 			break;
> 	}
> 
> And if you rewrite it this way, I _think_ you can get rid of this
> special case:
> 
> 	if ((*nsegs >= queue_max_segments(q)) || !len)
> 		return need_split;
> 
> above.

Good point, will do in next version.

> 
> > +	}
> > +
> > +	/* split in the middle of the bvec */
> > +	if (len)
> > +		need_split = true;
> 
> need_split is unnecessary, just return len != 0.

OK.

> 
> > +
> > +	/* update front segment size */
> > +	if (!*nsegs) {
> > +		unsigned first_seg_size = seg_size;
> > +
> > +		if (new_nsegs > 1)
> > +			first_seg_size = queue_max_segment_size(q);
> > +		if (*front_seg_size < first_seg_size)
> > +			*front_seg_size = first_seg_size;
> > +	}
> > +
> > +	/* update other varibles */
> > +	*last_seg_size = seg_size;
> > +	*nsegs += new_nsegs;
> > +	if (sectors)
> > +		*sectors += total_len >> 9;
> > +
> > +	return need_split;
> > +}
> > +
> >  static struct bio *blk_bio_segment_split(struct request_queue *q,
> >  					 struct bio *bio,
> >  					 struct bio_set *bs,
> > @@ -173,7 +229,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> >  	struct bio *new = NULL;
> >  	const unsigned max_sectors = get_max_io_size(q, bio);
> >  
> > -	bio_for_each_segment(bv, bio, iter) {
> > +	bio_for_each_bvec(bv, bio, iter) {
> >  		/*
> >  		 * If the queue doesn't support SG gaps and adding this
> >  		 * offset would create a gap, disallow it.
> > @@ -188,8 +244,12 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> >  			 */
> >  			if (nsegs < queue_max_segments(q) &&
> >  			    sectors < max_sectors) {
> > -				nsegs++;
> > -				sectors = max_sectors;
> > +				/* split in the middle of bvec */
> > +				bv.bv_len = (max_sectors - sectors) << 9;
> > +				bvec_split_segs(q, &bv, &nsegs,
> > +						&seg_size,
> > +						&front_seg_size,
> > +						&sectors);
> >  			}
> >  			goto split;
> >  		}
> > @@ -214,11 +274,12 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> >  		if (nsegs == 1 && seg_size > front_seg_size)
> >  			front_seg_size = seg_size;
> 
> Hm, do we still need to check this here now that we're updating
> front_seg_size inside of bvec_split_segs()?

Right, the check & update can be removed.

> 
> >  
> > -		nsegs++;
> >  		bvprv = bv;
> >  		bvprvp = &bvprv;
> > -		seg_size = bv.bv_len;
> > -		sectors += bv.bv_len >> 9;
> > +
> > +		if (bvec_split_segs(q, &bv, &nsegs, &seg_size,
> > +					&front_seg_size, &sectors))
> 
> What happened to the indent alignment here?

Will fix it in next version.

> 
> > +			goto split;
> >  
> >  	}
> >  
> > @@ -296,6 +357,7 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
> >  	struct bio_vec bv, bvprv = { NULL };
> >  	int cluster, prev = 0;
> >  	unsigned int seg_size, nr_phys_segs;
> > +	unsigned front_seg_size = bio->bi_seg_front_size;
> >  	struct bio *fbio, *bbio;
> >  	struct bvec_iter iter;
> >  
> > @@ -316,7 +378,7 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
> >  	seg_size = 0;
> >  	nr_phys_segs = 0;
> >  	for_each_bio(bio) {
> > -		bio_for_each_segment(bv, bio, iter) {
> > +		bio_for_each_bvec(bv, bio, iter) {
> >  			/*
> >  			 * If SG merging is disabled, each bio vector is
> >  			 * a segment
> > @@ -336,20 +398,20 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
> >  				continue;
> >  			}
> >  new_segment:
> > -			if (nr_phys_segs == 1 && seg_size >
> > -			    fbio->bi_seg_front_size)
> > -				fbio->bi_seg_front_size = seg_size;
> > +			if (nr_phys_segs == 1 && seg_size > front_seg_size)
> > +				front_seg_size = seg_size;
> 
> Same comment as in blk_bio_segment_split(), do we still need to check
> this if we're updating front_seg_size in bvec_split_segs()?

I think we can remove it too.


Thanks,
Ming

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