On Wed, 7 Apr 2021, Linus Torvalds wrote: > 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 Hi I think that we never found a root cause for this bug. I was testing if the whole system can run from persistent memory and found out that strace didn't work. I bisected it, reported it and when I received Peter Xu's patches (which fixed it), I stopped bothering about it. So, we fixed it, but we don't know why. Peter Xu's patchset that fixed it is here: https://lore.kernel.org/lkml/20200821234958.7896-1-peterx@xxxxxxxxxx/ Mikulas
