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

Joanne Koong
2024-12-11T21:04:56Z

On Mon, Dec 9, 2024 at 7:54 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, Dec 09, 2024 at 10:50:42AM -0500, Josef Bacik wrote:
> > 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,
>
> Some people have in the past thought that they could skip subsequent
> page lookup if the folio they get back is large. This is an incorrect
> optimisation. Userspace may mmap() a file PROT_WRITE, MAP_PRIVATE.
> If they store to the middle of a large folio (the file that is mmaped
> may be on a filesystem that does support large folios, rather than
> fuse), then we'll have, eg:
>
> 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.

Sounds good, I'll fix this up in v3. Thanks.