Re: [PATCH V11 14/19] block: handle non-cluster bio out of blk_bio_segment_split

Ming Lei
2018-11-21T15:38:04Z

On Wed, Nov 21, 2018 at 03:33:55PM +0100, Christoph Hellwig wrote:
> > + non-cluster.o
>
> Do we really need a new source file for these few functions?
>
> > default:
> > + if (!blk_queue_cluster(q)) {
> > + blk_queue_non_cluster_bio(q, bio);
> > + return;
>
> I'd name this blk_bio_segment_split_singlepage or similar.

OK.

>
> > +static __init int init_non_cluster_bioset(void)
> > +{
> > + WARN_ON(bioset_init(&non_cluster_bio_set, BIO_POOL_SIZE, 0,
> > + BIOSET_NEED_BVECS));
> > + WARN_ON(bioset_integrity_create(&non_cluster_bio_set, BIO_POOL_SIZE));
> > + WARN_ON(bioset_init(&non_cluster_bio_split, BIO_POOL_SIZE, 0, 0));
>
> Please only allocate the resources once a queue without the cluster
> flag is registered, there are only very few modern drivers that do that.

OK.

>
> > +static void non_cluster_end_io(struct bio *bio)
> > +{
> > + struct bio *bio_orig = bio->bi_private;
> > +
> > + bio_orig->bi_status = bio->bi_status;
> > + bio_endio(bio_orig);
> > + bio_put(bio);
> > +}
>
> Why can't we use bio_chain for the split bios?

The parent bio is multi-page bvec, we can't submit it for non-cluster.

>
> > + bio_for_each_segment(from, *bio_orig, iter) {
> > + if (i++ < max_segs)
> > + sectors += from.bv_len >> 9;
> > + else
> > + break;
> > + }
>
> The easy to read way would be:
>
> bio_for_each_segment(from, *bio_orig, iter) {
> if (i++ == max_segs)
> break;
> sectors += from.bv_len >> 9;
> }

OK.

>
> > + if (sectors < bio_sectors(*bio_orig)) {
> > + bio = bio_split(*bio_orig, sectors, GFP_NOIO,
> > + &non_cluster_bio_split);
> > + bio_chain(bio, *bio_orig);
> > + generic_make_request(*bio_orig);
> > + *bio_orig = bio;
>
> I don't think this is very efficient, as this means we now
> clone the bio twice, first to split it at the sector boundary,
> and then again when converting it to single-page bio_vec.

That is exactly what bounce code does. The problem for both bounce
and non-cluster is same actually because the bvec table itself has
to be changed.

>
> I think this could be something like this (totally untested):
>
> diff --git a/block/non-cluster.c b/block/non-cluster.c
> index 9c2910be9404..60389f275c43 100644
> --- a/block/non-cluster.c
> +++ b/block/non-cluster.c
> @@ -13,58 +13,59 @@
>
> #include "blk.h"
>
> -static struct bio_set non_cluster_bio_set, non_cluster_bio_split;
> +static struct bio_set non_cluster_bio_set;
>
> static __init int init_non_cluster_bioset(void)
> {
> WARN_ON(bioset_init(&non_cluster_bio_set, BIO_POOL_SIZE, 0,
> BIOSET_NEED_BVECS));
> WARN_ON(bioset_integrity_create(&non_cluster_bio_set, BIO_POOL_SIZE));
> - WARN_ON(bioset_init(&non_cluster_bio_split, BIO_POOL_SIZE, 0, 0));
>
> return 0;
> }
> __initcall(init_non_cluster_bioset);
>
> -static void non_cluster_end_io(struct bio *bio)
> -{
> - struct bio *bio_orig = bio->bi_private;
> -
> - bio_orig->bi_status = bio->bi_status;
> - bio_endio(bio_orig);
> - bio_put(bio);
> -}
> -
> void blk_queue_non_cluster_bio(struct request_queue *q, struct bio **bio_orig)
> {
> - struct bio *bio;
> struct bvec_iter iter;
> - struct bio_vec from;
> - unsigned i = 0;
> - unsigned sectors = 0;
> - unsigned short max_segs = min_t(unsigned short, BIO_MAX_PAGES,
> - queue_max_segments(q));
> + struct bio *bio;
> + struct bio_vec bv;
> + unsigned short max_segs, segs = 0;
> +
> + bio = bio_alloc_bioset(GFP_NOIO, bio_segments(*bio_orig),
> + &non_cluster_bio_set);

bio_segments(*bio_orig) may be > 256, so bio_alloc_bioset() may fail.

Thanks,
Ming