Re: [PATCH v2 10/12] fuse: support large folios for direct io

[Josef Bacik <josef@toxicpanda.com>]

On Mon, Nov 25, 2024 at 02:05:35PM -0800, Joanne Koong wrote:
> Add support for folios larger than one page size for direct io.
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/fuse/file.c | 34 ++++++++++++++++++++++------------
>  1 file changed, 22 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 590a3f2fa310..a907848f387a 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1482,7 +1482,8 @@ static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
>  		return -ENOMEM;
>  
>  	while (nbytes < *nbytesp && nr_pages < max_pages) {
> -		unsigned nfolios, i;
> +		unsigned npages;
> +		unsigned i = 0;
>  		size_t start;
>  
>  		ret = iov_iter_extract_pages(ii, &pages,
> @@ -1494,19 +1495,28 @@ static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
>  
>  		nbytes += ret;
>  
> -		ret += start;
> -		/* Currently, all folios in FUSE are one page */
> -		nfolios = DIV_ROUND_UP(ret, PAGE_SIZE);
> +		npages = DIV_ROUND_UP(ret + start, PAGE_SIZE);
>  
> -		ap->descs[ap->num_folios].offset = start;
> -		fuse_folio_descs_length_init(ap->descs, ap->num_folios, nfolios);
> -		for (i = 0; i < nfolios; i++)
> -			ap->folios[i + ap->num_folios] = page_folio(pages[i]);
> +		while (ret && i < npages) {
> +			struct folio *folio;
> +			unsigned int folio_offset;
> +			unsigned int len;
>  
> -		ap->num_folios += nfolios;
> -		ap->descs[ap->num_folios - 1].length -=
> -			(PAGE_SIZE - ret) & (PAGE_SIZE - 1);
> -		nr_pages += nfolios;
> +			folio = page_folio(pages[i]);
> +			folio_offset = ((size_t)folio_page_idx(folio, pages[i]) <<
> +				       PAGE_SHIFT) + start;
> +			len = min_t(ssize_t, ret, folio_size(folio) - folio_offset);
> +
> +			ap->folios[ap->num_folios] = folio;
> +			ap->descs[ap->num_folios].offset = folio_offset;
> +			ap->descs[ap->num_folios].length = len;
> +			ap->num_folios++;
> +
> +			ret -= len;
> +			i += DIV_ROUND_UP(start + len, PAGE_SIZE);
> +			start = 0;

As we've noticed in the upstream bug report for your initial work here, this
isn't quite correct, as we could have gotten a large folio in from userspace.  I
think the better thing here is to do the page extraction, and then keep track of
the last folio we saw, and simply skip any folios that are the same for the
pages we have.  This way we can handle large folios correctly.  Thanks,

Josef