Re: [PATCH v2 3/3] fork: lock VMAs of the parent process when forking

[Suren Baghdasaryan <surenb@google.com>]

On Sat, Jul 8, 2023 at 3:54 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Sat, 8 Jul 2023 at 15:36, Suren Baghdasaryan <surenb@google.com> wrote:
> >
> > On Sat, Jul 8, 2023 at 2:18 PM Linus Torvalds
> > >
> > > Again - maybe I messed up, but it really feels like the missing
> > > vma_start_write() was more fundamental, and not some "TLB coherency"
> > > issue.
> >
> > Sounds plausible. I'll try to use the reproducer to verify if that's
> > indeed happening here.
>
> I really don't think that's what people are reporting, I was just
> trying to make up a completely different case that has nothing to do
> with any TLB issues.
>
> My real point was simply this one:
>
> > It's likely there are multiple problematic
> > scenarios due to this missing lock though.
>
> Right. That's my issue. I felt your explanation was *too* targeted at
> some TLB non-coherency thing, when I think the problem was actually a
> much larger "page faults simply must not happen while we're copying
> the page tables because data isn't coherent".
>
> The anon_vma case was just meant as another random example of the
> other kinds of things I suspect can go wrong, because we're simply not
> able to do this whole "copy vma while it's being modified by page
> faults".
>
> Now, I agree that the PTE problem is real, and probable the main
> thing, ie when we as part of fork() do this:
>
>         /*
>          * If it's a COW mapping, write protect it both
>          * in the parent and the child
>          */
>         if (is_cow_mapping(vm_flags) && pte_write(pte)) {
>                 ptep_set_wrprotect(src_mm, addr, src_pte);
>                 pte = pte_wrprotect(pte);
>         }
>
> and the thing that can go wrong before the TLB flush happens is that -
> because the TLB's haven't been flushed yet - some threads in the
> parent happily continue to write to the page and didn't see the
> wrprotect happening.
>
> And then you get into the situation where *some* thread see the page
> protections change (maybe they had a TLB flush event on that CPU for
> random reasons), and they will take a page fault and do the COW thing
> and create a new page.
>
> And all the while *other* threads still see the old writeable TLB
> state, and continue to write to the old page.
>
> So now you have a page that gets its data copied *while* somebody is
> still writing to it, and the end result is that some write easily gets
> lost, and so when that new copy is installed, you see it as data
> corruption.
>
> And I agree completely that that is probably the thing that most
> people actually saw and reacted to as corruption.
>
> But the reason I didn't like the explanation was that I think this is
> just one random example of the more fundamental issue of "we simply
> must not take page faults while copying".
>
> Your explanation made me think "stale TLB is the problem", and *that*
> was what I objected to. The stale TLB was just one random sign of the
> much larger problem.
>
> It might even have been the most common symptom, but I think it was
> just a *symptom*, not the *cause* of the problem.
>
> And I must have been bad at explaining that, because David Hildenbrand
> also reacted negatively to my change.
>
> So I'll happily take a patch that adds more commentary about this, and
> gives several examples of the things that go wrong.

How about adding your example to the original description as yet
another scenario which is broken without this change? I guess having
both issues described would not hurt.

>
>                 Linus