Re: [PATCH v2] mm/gup: Allow real explicit breaking of COW

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, Aug 10, 2020 at 4:39 PM Jann Horn <jannh@google.com> wrote:
>
> The way I understand Peter, he doesn't want to avoid doing COW; he
> wants to decouple userfaultfd-WP's fault handling from COW, so that
> userfaultfd-wp notifies only when a previously-write-protected page is
> actually written to. In other words, he wants the COW to basically
> happen as it happens now, but it should only create a readonly PTE;
> and if someone later triggers a real write fault, the fault handling
> path would run again, and this time userfaultfd-wp would be notified
> before that readonly PTE is turned into a writable one.

Ahh.

A light goes on.

Thank you.

And apologies to Peter - I misread that patch entirely.

That said, now that I (finally) understand what Peter wants to do, I
don't think the patch does what you say.

Because the GUP will now indeed avoid userfaultfd-wp unless it's
_actually_ a write, but then any reads will cause a COW that turns
things writable. There is no second fault.

So now later writes will never cause any userfaultfd-wp notifications at all.

Which for all I know might be acceptable and ok, but it seems to be
against userfaultfd rules, and against the whole synchronization idea.

So I think the patch is broken, but I'm less fundamentally concerned about it.

Because at that point, it's "only" userfaultfd that might break.

                  Linus