Re: [RFC] iomap: use huge zero folio in iomap_dio_zero

[Zi Yan <ziy@nvidia.com>]

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On 7 May 2024, at 10:58, Pankaj Raghav (Samsung) wrote:

> From: Pankaj Raghav <p.raghav@samsung.com>
>
> Instead of looping with ZERO_PAGE, use a huge zero folio to zero pad the
> block. Fallback to ZERO_PAGE if mm_get_huge_zero_folio() fails.
>
> Suggested-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
> I rebased on top of mm-unstable to get mm_get_huge_zero_folio().
>
> @Christoph is this inline with what you had in mind?
>
>  fs/iomap/direct-io.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 5f481068de5b..7f584f9ff2c5 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -236,11 +236,18 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
>  		loff_t pos, unsigned len)
>  {
>  	struct inode *inode = file_inode(dio->iocb->ki_filp);
> -	struct page *page = ZERO_PAGE(0);
> +	struct folio *zero_page_folio = page_folio(ZERO_PAGE(0));
> +	struct folio *folio = zero_page_folio;
>  	struct bio *bio;
>
>  	WARN_ON_ONCE(len > (BIO_MAX_VECS * PAGE_SIZE));
>
> +	if (len > PAGE_SIZE) {
> +		folio = mm_get_huge_zero_folio(current->mm);

The huge zero folio is order-9, but it seems here you only require len to be
bigger than PAGE_SIZE. I am not sure if it works when len is smaller than
HPAGE_PMD_SIZE.

> +		if (!folio)
> +			folio = zero_page_folio;
> +	}
> +
>  	bio = iomap_dio_alloc_bio(iter, dio, BIO_MAX_VECS,
>  				  REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
>  	fscrypt_set_bio_crypt_ctx(bio, inode, pos >> inode->i_blkbits,
> @@ -251,10 +258,10 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
>  	bio->bi_end_io = iomap_dio_bio_end_io;
>
>  	while (len) {
> -		unsigned int io_len = min_t(unsigned int, len, PAGE_SIZE);
> +		unsigned int size = min(len, folio_size(folio));
>
> -		__bio_add_page(bio, page, io_len, 0);
> -		len -= io_len;
> +		bio_add_folio_nofail(bio, folio, size, 0);
> +		len -= size;

Maybe use huge zero folio when len > HPAGE_PMD_SIZE and use ZERO_PAGE(0)
for len % HPAGE_PMD_SIZE.

>  	}
>  	iomap_dio_submit_bio(iter, dio, bio, pos);
>  }
> -- 
> 2.34.1


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Best Regards,
Yan, Zi

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