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@xxxxxxxxxxxxx> 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?
