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

Matthew Wilcox
2024-12-11T21:11:51Z

On Wed, Dec 11, 2024 at 01:04:45PM -0800, Joanne Koong wrote:
> 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.

Hm? I didn't notice this bug in your code, just mentioning something
I've seen other people do and wanted to make suree you didn't. Did I
miss a bug in your code?