On Thu, Sep 23, 2021 at 09:47:42AM -0700, Axel Rasmussen wrote: > My thinking there was a THP collapse bug was really just based on > speculation, not a real reproducer, so it's very possible my > speculation was wrong. It will take some more thinking and reading to > convince myself one way or the other. :) Thanks to you and Hugh for > all the details. > > I'd prefer not to add this fix "just in case", if it isn't a real > problem, as it seems like it may confuse future readers of the code. It's not "just in case" to me - IMHO it's theoretically causing more false positives as I used to mention, at least that's my understanding so far. So if the theory is correct it'll 100% happen when khugepaged merged some minor-registered regions. Uffd-wp could have many false positives like this if we don't support swap - at last we decided to fully support swap then we removed all the false positives regarding swapping. I think it's similar here, but khugepaged should trigger much less frequently on the false positives upon uffd-minor, than swapping upon uffd-wp. But yes, there's definitely no rush on thinking or anything - it'll never hurt to think more. And more importantly, verify it with some test program would be great; after all theoretically it'll just work like a charm to me. > > I'll send out a patch for it if / when I manage to build a real > reproducer. Or, in the meantime, some of my Google colleagues are > testing this code via their live migration implementation, so if there > is a bug here there's a good chance we'll find it that way too. Sounds like a good plan. Thanks, -- Peter Xu
