[dm-devel] [PATCH V10 03/19] block: use bio_for_each_bvec() to compute multi-page bvec count

[Mike Snitzer]

On Thu, Nov 15 2018 at  3:20pm -0500,
Omar Sandoval <osandov at osandov.com> 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 at redhat.com>
> > Cc: Kent Overstreet <kent.overstreet at gmail.com>
> > Cc: Mike Snitzer <snitzer at redhat.com>
> > Cc: dm-devel at redhat.com
> > Cc: Alexander Viro <viro at zeniv.linux.org.uk>
> > Cc: linux-fsdevel at vger.kernel.org
> > Cc: Shaohua Li <shli at kernel.org>
> > Cc: linux-raid at vger.kernel.org
> > Cc: linux-erofs at lists.ozlabs.org
> > Cc: David Sterba <dsterba at suse.com>
> > Cc: linux-btrfs at vger.kernel.org
> > Cc: Darrick J. Wong <darrick.wong at oracle.com>
> > Cc: linux-xfs at vger.kernel.org
> > Cc: Gao Xiang <gaoxiang25 at huawei.com>
> > Cc: Christoph Hellwig <hch at lst.de>
> > Cc: Theodore Ts'o <tytso at mit.edu>
> > Cc: linux-ext4 at vger.kernel.org
> > Cc: Coly Li <colyli at suse.de>
> > Cc: linux-bcache at vger.kernel.org
> > Cc: Boaz Harrosh <ooo at electrozaur.com>
> > Cc: Bob Peterson <rpeterso at redhat.com>
> > Cc: cluster-devel at redhat.com
> > Signed-off-by: Ming Lei <ming.lei at redhat.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.

Curious why?  I've wondered what govens use of "unsigned" vs "unsigned
int" recently and haven't found _the_ reason to pick one over the other.

Thanks,
Mike