Re: [PATCH v2 5/5] block: use largest_zero_folio in __blkdev_issue_zero_pages()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Jul 07, 2025 at 04:23:19PM +0200, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
>
> Use largest_zero_folio() in __blkdev_issue_zero_pages().
>
> On systems with CONFIG_STATIC_PMD_ZERO_PAGE enabled, we will end up
> sending larger bvecs instead of multiple small ones.
>
> Noticed a 4% increase in performance on a commercial NVMe SSD which does
> not support OP_WRITE_ZEROES. The device's MDTS was 128K. The performance
> gains might be bigger if the device supports bigger MDTS.
>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
>  block/blk-lib.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index 4c9f20a689f7..70a5700b6717 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -196,6 +196,10 @@ static void __blkdev_issue_zero_pages(struct block_device *bdev,
>  		sector_t sector, sector_t nr_sects, gfp_t gfp_mask,
>  		struct bio **biop, unsigned int flags)
>  {
> +	struct folio *zero_folio;
> +
> +	zero_folio = largest_zero_folio();

Just assign this in the decl :)

> +
>  	while (nr_sects) {
>  		unsigned int nr_vecs = __blkdev_sectors_to_bio_pages(nr_sects);
>  		struct bio *bio;
> @@ -208,15 +212,14 @@ static void __blkdev_issue_zero_pages(struct block_device *bdev,
>  			break;
>
>  		do {
> -			unsigned int len, added;
> +			unsigned int len;
>
> -			len = min_t(sector_t,
> -				PAGE_SIZE, nr_sects << SECTOR_SHIFT);
> -			added = bio_add_page(bio, ZERO_PAGE(0), len, 0);
> -			if (added < len)
> +			len = min_t(sector_t, folio_size(zero_folio),
> +				    nr_sects << SECTOR_SHIFT);
> +			if (!bio_add_folio(bio, zero_folio, len, 0))

Hmm, will this work if nr_sects << SECTOR_SHIFT size isn't PMD-aligned?

I guess it actually just copies individual pages in the folio as needed?

Does this actually result in a significant performance improvement? Do we
have numbers for this to justify the series?

>  				break;
> -			nr_sects -= added >> SECTOR_SHIFT;
> -			sector += added >> SECTOR_SHIFT;
> +			nr_sects -= len >> SECTOR_SHIFT;
> +			sector += len >> SECTOR_SHIFT;
>  		} while (nr_sects);
>
>  		*biop = bio_chain_and_submit(*biop, bio);
> --
> 2.49.0
>