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

Omar Sandoval
2018-11-15T22:18:53Z

On Thu, Nov 15, 2018 at 04:05:10PM -0500, Mike Snitzer wrote:
> On Thu, Nov 15 2018 at 3:20pm -0500,
> Omar Sandoval <osandov@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@redhat.com>
> > > Cc: Kent Overstreet <kent.overstreet@gmail.com>
> > > Cc: Mike Snitzer <snitzer@redhat.com>
> > > Cc: dm-devel@redhat.com
> > > Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> > > Cc: linux-fsdevel@vger.kernel.org
> > > Cc: Shaohua Li <shli@kernel.org>
> > > Cc: linux-raid@vger.kernel.org
> > > Cc: linux-erofs@lists.ozlabs.org
> > > Cc: David Sterba <dsterba@suse.com>
> > > Cc: linux-btrfs@vger.kernel.org
> > > Cc: Darrick J. Wong <darrick.wong@oracle.com>
> > > Cc: linux-xfs@vger.kernel.org
> > > Cc: Gao Xiang <gaoxiang25@huawei.com>
> > > Cc: Christoph Hellwig <hch@lst.de>
> > > Cc: Theodore Ts'o <tytso@mit.edu>
> > > Cc: linux-ext4@vger.kernel.org
> > > Cc: Coly Li <colyli@suse.de>
> > > Cc: linux-bcache@vger.kernel.org
> > > Cc: Boaz Harrosh <ooo@electrozaur.com>
> > > Cc: Bob Peterson <rpeterso@redhat.com>
> > > Cc: cluster-devel@redhat.com
> > > Signed-off-by: Ming Lei <ming.lei@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.

My only reason to prefer unsigned int is consistency. unsigned int is
much more common in the kernel:

$ ag --cc -s 'unsigned\s+int' | wc -l
129632
$ ag --cc -s 'unsigned\s+(?!char|short|int|long)' | wc -l
22435

checkpatch also warns on plain unsigned.