Re: [PATCH 05/13] block: Improvements to bounce-buffer handling

[Ming Lei]

On Tue, May 02, 2017 at 01:42:24PM +1000, NeilBrown wrote:
> Since commit 23688bf4f830 ("block: ensure to split after potentially
> bouncing a bio") blk_queue_bounce() is called *before*
> blk_queue_split().
> This means that:
>  1/ the comments blk_queue_split() about bounce buffers are
>     irrelevant, and
>  2/ a very large bio (more than BIO_MAX_PAGES) will no longer be
>     split before it arrives at blk_queue_bounce(), leading to the
>     possibility that bio_clone_bioset() will fail and a NULL
>     will be dereferenced.
> 
> Separately, blk_queue_bounce() shouldn't use fs_bio_set as the bio
> being copied could be from the same set, and this could lead to a
> deadlock.
> 
> So:
>  - allocate 2 private biosets for blk_queue_bounce, one for
>    splitting enormous bios and one for cloning bios.
>  - add code to split a bio that exceeds BIO_MAX_PAGES.
>  - Fix up the comments in blk_queue_split()
> 
> Credit-to: Ming Lei <tom.leiming@xxxxxxxxx> (suggested using single bio_for_each_segment loop)
> Signed-off-by: NeilBrown <neilb@xxxxxxxx>
> ---
>  block/blk-merge.c |   14 ++++----------
>  block/bounce.c    |   32 ++++++++++++++++++++++++++------
>  2 files changed, 30 insertions(+), 16 deletions(-)
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index d59074556703..51c84540d3bb 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -117,17 +117,11 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
>  		 * each holds at most BIO_MAX_PAGES bvecs because
>  		 * bio_clone() can fail to allocate big bvecs.
>  		 *
> -		 * It should have been better to apply the limit per
> -		 * request queue in which bio_clone() is involved,
> -		 * instead of globally. The biggest blocker is the
> -		 * bio_clone() in bio bounce.
> +		 * Those drivers which will need to use bio_clone()
> +		 * should tell us in some way.  For now, impose the
> +		 * BIO_MAX_PAGES limit on all queues.
>  		 *
> -		 * If bio is splitted by this reason, we should have
> -		 * allowed to continue bios merging, but don't do
> -		 * that now for making the change simple.
> -		 *
> -		 * TODO: deal with bio bounce's bio_clone() gracefully
> -		 * and convert the global limit into per-queue limit.
> +		 * TODO: handle users of bio_clone() differently.
>  		 */
>  		if (bvecs++ >= BIO_MAX_PAGES)
>  			goto split;
> diff --git a/block/bounce.c b/block/bounce.c
> index 1cb5dd3a5da1..087ecc2dc66c 100644
> --- a/block/bounce.c
> +++ b/block/bounce.c
> @@ -26,6 +26,7 @@
>  #define POOL_SIZE	64
>  #define ISA_POOL_SIZE	16
>  
> +struct bio_set *bounce_bio_set, *bounce_bio_split;
>  static mempool_t *page_pool, *isa_page_pool;
>  
>  #if defined(CONFIG_HIGHMEM) || defined(CONFIG_NEED_BOUNCE_POOL)
> @@ -40,6 +41,14 @@ static __init int init_emergency_pool(void)
>  	BUG_ON(!page_pool);
>  	pr_info("pool size: %d pages\n", POOL_SIZE);
>  
> +	bounce_bio_set = bioset_create(BIO_POOL_SIZE, 0, BIOSET_NEED_BVECS);
> +	BUG_ON(!bounce_bio_set);
> +	if (bioset_integrity_create(bounce_bio_set, BIO_POOL_SIZE))
> +		BUG_ON(1);
> +
> +	bounce_bio_split = bioset_create(BIO_POOL_SIZE, 0, 0);
> +	BUG_ON(!bounce_bio_split);
> +
>  	return 0;
>  }
>  
> @@ -186,15 +195,26 @@ static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
>  	int rw = bio_data_dir(*bio_orig);
>  	struct bio_vec *to, from;
>  	struct bvec_iter iter;
> -	unsigned i;
> +	unsigned i = 0;
> +	bool bounce = false;
> +	int sectors = 0;
>  
> -	bio_for_each_segment(from, *bio_orig, iter)
> +	bio_for_each_segment(from, *bio_orig, iter) {
> +		if (i++ < BIO_MAX_PAGES)
> +			sectors += from.bv_len >> 9;
>  		if (page_to_pfn(from.bv_page) > queue_bounce_pfn(q))
> -			goto bounce;
> +			bounce = true;
> +	}
> +	if (!bounce)
> +		return;
>  
> -	return;
> -bounce:
> -	bio = bio_clone_bioset(*bio_orig, GFP_NOIO, fs_bio_set);
> +	if (sectors < bio_sectors(*bio_orig)) {
> +		bio = bio_split(*bio_orig, sectors, GFP_NOIO, bounce_bio_split);
> +		bio_chain(bio, *bio_orig);
> +		generic_make_request(*bio_orig);
> +		*bio_orig = bio;
> +	}
> +	bio = bio_clone_bioset(*bio_orig, GFP_NOIO, bounce_bio_set);
>  
>  	bio_for_each_segment_all(to, bio, i) {
>  		struct page *page = to->bv_page;

Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>

Thanks,
Ming