On Wed, Apr 7, 2021 at 9:33 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > Trying my hand at backporting the patchsets Peter mentioned proved > this to be far from easy with many dependencies. Let me look into > Vlastimil's suggestion to backport only 17839856fd58 and it sounds > like 5.4 already followed that path. Well, in many ways 17839856fd58 was the "simple and obvious" fix, and I do think it's easily backportable. But it *did* cause problems too. Those problems may not be issues on those old kernels, though. In particular, commit 17839856fd58 caused uffd-wp to stop working right, and it caused some issues with debugging (I forget the exact details, but I think it was strace accessing PROT_NONE or write-only pages or something like that, and COW failed). But yes, in many ways that commit is a much simpler and more straightforward one (which is why I tried it once - we ended up with the much more subtle and far-reaching fixes after the UFFD issues crept up). The issues that 17839856fd58 caused may be entire non-events in old kernels. In fact, the uffd writeprotect API was added fairly recently (see commit 63b2d4174c4a that made it into v5.7), so the uffd-wp issue that was triggered probably cannot happen in the old kernels. The strace issue might not be relevant either, but I forget what the details were. Mikilas should know. See https://lore.kernel.org/lkml/alpine.LRH.2.02.2009031328040.6929@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ for Mikulas report. I never looked into it in detail, because by then the uffd-wp issue had already come up, so it was juat another nail in the coffin for that simpler approach. Mikulas, do you remember? Linus
