Re: [PATCH 1/2] loop: use blk_rq_nr_phys_segments() instead of iterating bvecs

[Ming Lei <ming.lei@redhat.com>]

On Mon, Nov 10, 2025 at 08:47:46AM -0800, Caleb Sander Mateos wrote:
> On Sun, Nov 9, 2025 at 4:20 AM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > On Sat, Nov 08, 2025 at 04:01:00PM -0700, Caleb Sander Mateos wrote:
> > > The number of bvecs can be obtained directly from struct request's
> > > nr_phys_segments field via blk_rq_nr_phys_segments(), so use that
> > > instead of iterating over the bvecs an extra time.
> > >
> > > Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
> > > ---
> > >  drivers/block/loop.c | 5 +----
> > >  1 file changed, 1 insertion(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > > index 13ce229d450c..8096478fad45 100644
> > > --- a/drivers/block/loop.c
> > > +++ b/drivers/block/loop.c
> > > @@ -346,16 +346,13 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
> > >       struct request *rq = blk_mq_rq_from_pdu(cmd);
> > >       struct bio *bio = rq->bio;
> > >       struct file *file = lo->lo_backing_file;
> > >       struct bio_vec tmp;
> > >       unsigned int offset;
> > > -     int nr_bvec = 0;
> > > +     unsigned short nr_bvec = blk_rq_nr_phys_segments(rq);
> > >       int ret;
> > >
> > > -     rq_for_each_bvec(tmp, rq, rq_iter)
> > > -             nr_bvec++;
> > > -
> >
> > The two may not be same, since one bvec can be splitted into multiple segments.
> 
> Hmm, io_buffer_register_bvec() already assumes
> blk_rq_nr_phys_segments() returns the number of bvecs iterated by
> rq_for_each_bvec(). I asked about this on the patch adding it, but
> Keith assures me they match:
> https://lore.kernel.org/io-uring/Z7TmrB4_aBnZdFbo@kbusch-mbp/.

blk_rq_nr_phys_segments() >= nr_bvec, would you like to cook a fix?

Thanks, 
Ming