Re: [PATCH v5 10/11] fuse: optimize direct io large folios processing

[David Hildenbrand <david@redhat.com>]

On 26.04.25 02:08, Joanne Koong wrote:
> Optimize processing folios larger than one page size for the direct io
> case. If contiguous pages are part of the same folio, collate the
> processing instead of processing each page in the folio separately.
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> Reviewed-by: Jeff Layton <jlayton@kernel.org>
> ---
>   fs/fuse/file.c | 55 +++++++++++++++++++++++++++++++++++++-------------
>   1 file changed, 41 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 9a31f2a516b9..61eaec1c993b 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1490,7 +1490,8 @@ static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
>   	}
>   
>   	while (nbytes < *nbytesp && nr_pages < max_pages) {
> -		unsigned nfolios, i;
> +		struct folio *prev_folio = NULL;
> +		unsigned npages, i;
>   		size_t start;
>   
>   		ret = iov_iter_extract_pages(ii, &pages,
> @@ -1502,23 +1503,49 @@ static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
>   
>   		nbytes += ret;
>   
> -		nfolios = DIV_ROUND_UP(ret + start, PAGE_SIZE);
> +		npages = DIV_ROUND_UP(ret + start, PAGE_SIZE);
>   
> -		for (i = 0; i < nfolios; i++) {
> -			struct folio *folio = page_folio(pages[i]);
> -			unsigned int offset = start +
> -				(folio_page_idx(folio, pages[i]) << PAGE_SHIFT);
> -			unsigned int len = min_t(unsigned int, ret, PAGE_SIZE - start);
> +		/*
> +		 * We must check each extracted page. We can't assume every page
> +		 * in a large folio is used. For example, userspace may mmap() a
> +		 * file PROT_WRITE, MAP_PRIVATE, and then store to the middle of
> +		 * a large folio, in which case the extracted pages could be
> +		 *
> +		 * folio A page 0
> +		 * folio A page 1
> +		 * folio B page 0
> +		 * folio A page 3
> +		 *
> +		 * where folio A belongs to the file and folio B is an anonymous
> +		 * COW page.
> +		 */
> +		for (i = 0; i < npages && ret; i++) {
> +			struct folio *folio;
> +			unsigned int offset;
> +			unsigned int len;
> +
> +			WARN_ON(!pages[i]);
> +			folio = page_folio(pages[i]);
> +
> +			len = min_t(unsigned int, ret, PAGE_SIZE - start);
> +
> +			if (folio == prev_folio && pages[i] != pages[i - 1]) {

I don't really understand the "pages[i] != pages[i - 1]" part.

Why would you have to equal page pointers in there?


Something that might be simpler to understand and implement would be using

	num_pages_contiguous()

from

	https://lore.kernel.org/all/20250704062602.33500-2-lizhe.67@bytedance.com/T/#u

and then just making sure that we don't exceed the current folio, if we 
ever get contiguous pages that cross a folio.


-- 
Cheers,

David / dhildenb