On 09/12/2024 16:48, Josef Bacik wrote:
> On Mon, Dec 09, 2024 at 03:28:14PM +0000, Matthew Wilcox wrote:
>> On Mon, Dec 09, 2024 at 09:49:48AM -0500, Josef Bacik wrote:
>>>> Ha! This time I bisected from f03b296e8b51 to d1dfb5f52ffc. I ended up
>>>> with 3b97c3652d91 as the culprit.
>>> Willy, I've looked at this code and it does indeed look like a 1:1 conversion,
>>> EXCEPT I'm fuzzy about how how this works with large folios. Previously, if we
>>> got a hugepage in, we'd get each individual struct page back for the whole range
>>> of the hugepage, so if for example we had a 2M hugepage, we'd fill in the
>>> ->offset for each "middle" struct page as 0, since obviously we're consuming
>>> PAGE_SIZE chunks at a time.
>>>
>>> But now we're doing this
>>>
>>> for (i = 0; i < nfolios; i++)
>>> ap->folios[i + ap->num_folios] = page_folio(pages[i]);
>>>
>>> So if userspace handed us a 2M hugepage, page_folio() on each of the
>>> intermediary struct page's would return the same folio, correct? So we'd end up
>>> with the wrong offsets for our fuse request, because they should be based from
>>> the start of the folio, correct?
>> I think you're 100% right. We could put in some nice asserts to check
>> this is what's happening, but it does seem like a rather incautious
>> conversion. Yes, all folios _in the page cache_ for fuse are small, but
>> that's not guaranteed to be the case for folios found in userspace for
>> directio. At least the comment is wrong, and I'd suggest the code is too.
> Ok cool, Malte can you try the attached only compile tested patch and see if the
> problem goes away? Thanks,
>
> Josef
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 88d0946b5bc9..c4b93ead99a5 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1562,9 +1562,19 @@ static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
> nfolios = DIV_ROUND_UP(ret, 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]);
> + 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, folio_size(folio) - offset);
> +
> + len = min_t(unsigned int, len, PAGE_SIZE);
> +
> + ap->descs[ap->num_folios + i].offset = offset;
> + ap->descs[ap->num_folios + i].length = len;
> + ap->folios[i + ap->num_folios] = folio;
> + start = 0;
> + }
>
> ap->num_folios += nfolios;
> ap->descs[ap->num_folios - 1].length -=
The problem persists with this patch.
/Malte
