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

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

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