On Thu, Aug 24, 2017 at 10:57:39AM -0700, Omar Sandoval wrote:
> On Wed, Aug 23, 2017 at 04:49:24PM -0700, Shaohua Li wrote:
> > From: Shaohua Li <shli@xxxxxx>
> >
> > Currently loop disables merge. While it makes sense for buffer IO mode,
> > directio mode can benefit from request merge. Without merge, loop could
> > send small size IO to underlayer disk and harm performance.
>
> A couple of nits on comments below, besides that
>
> Reviewed-by: Omar Sandoval <osandov@xxxxxx>
>
> > Signed-off-by: Shaohua Li <shli@xxxxxx>
> > ---
> > drivers/block/loop.c | 43 +++++++++++++++++++++++++++++++++++--------
> > drivers/block/loop.h | 1 +
> > 2 files changed, 36 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index 428da07..1bfa3ff 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -213,10 +213,13 @@ static void __loop_update_dio(struct loop_device *lo, bool dio)
> > */
> > blk_mq_freeze_queue(lo->lo_queue);
> > lo->use_dio = use_dio;
> > - if (use_dio)
> > + if (use_dio) {
> > + queue_flag_clear_unlocked(QUEUE_FLAG_NOMERGES, lo->lo_queue);
> > lo->lo_flags |= LO_FLAGS_DIRECT_IO;
> > - else
> > + } else {
> > + queue_flag_set_unlocked(QUEUE_FLAG_NOMERGES, lo->lo_queue);
> > lo->lo_flags &= ~LO_FLAGS_DIRECT_IO;
> > + }
>
> Could you please also update the comment in loop_add() where we set
> QUEUE_FLAG_NOMERGES to say something like
>
> /*
> * By default we do buffered I/O, so it doesn't make sense to
> * enable merging because the I/O submitted to backing file is
> * handled page by page. We enable merging when switching to
> * direct I/O mode.
> */
sure, will add
> > blk_mq_unfreeze_queue(lo->lo_queue);
> > }
> >
> > @@ -464,6 +467,8 @@ static void lo_rw_aio_complete(struct kiocb *iocb, long ret, long ret2)
> > {
> > struct loop_cmd *cmd = container_of(iocb, struct loop_cmd, iocb);
> >
> > + kfree(cmd->bvec);
> > + cmd->bvec = NULL;
> > cmd->ret = ret;
> > blk_mq_complete_request(cmd->rq);
> > }
> > @@ -473,22 +478,44 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
> > {
> > struct iov_iter iter;
> > struct bio_vec *bvec;
> > - struct bio *bio = cmd->rq->bio;
> > + struct request *rq = cmd->rq;
> > + struct bio *bio = rq->bio;
> > struct file *file = lo->lo_backing_file;
> > + unsigned int offset;
> > + int segments = 0;
> > int ret;
> >
> > - /* nomerge for loop request queue */
> > - WARN_ON(cmd->rq->bio != cmd->rq->biotail);
> > + if (rq->bio != rq->biotail) {
> > + struct req_iterator iter;
> > + struct bio_vec tmp;
> > +
> > + __rq_for_each_bio(bio, rq)
> > + segments += bio_segments(bio);
> > + bvec = kmalloc(sizeof(struct bio_vec) * segments, GFP_KERNEL);
> > + if (!bvec)
> > + return -EIO;
> > + cmd->bvec = bvec;
> > +
> > + rq_for_each_segment(tmp, rq, iter) {
> > + *bvec = tmp;
> > + bvec++;
> > + }
> > + bvec = cmd->bvec;
> > + offset = 0;
> > + } else {
> > + offset = bio->bi_iter.bi_bvec_done;
> > + bvec = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
> > + segments = bio_segments(bio);
> > + }
> >
> > - bvec = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
> > iov_iter_bvec(&iter, ITER_BVEC | rw, bvec,
> > - bio_segments(bio), blk_rq_bytes(cmd->rq));
> > + segments, blk_rq_bytes(rq));
> > /*
> > * This bio may be started from the middle of the 'bvec'
> > * because of bio splitting, so offset from the bvec must
> > * be passed to iov iterator
> > */
>
> This comment should be moved above to where you do
>
> offset = bio->bi_iter.bi_bvec_done;
This comment actually applies to 'rq->bio != rq->biotail' case too for
each bio of the request. That's why I don't just copy bio->bi_io_vec.
I'll explain this in different way.
Thanks,
Shaohua
