Re: [PATCH 0/5] 4.14 backports of fixes for "CoW after fork() issue"

[Linus Torvalds]

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