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.
