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

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Aug 4, 2023 at 6:06 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
>
> On 8/5/23, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > On Fri, 4 Aug 2023 at 16:25, Mateusz Guzik <mjguzik@gmail.com> wrote:
> >>
> >> I know of these guys, I think they are excluded as is -- they go
> >> through access_remote_vm, starting with:
> >>         if (mmap_read_lock_killable(mm))
> >>                 return 0;
> >>
> >> while dup_mmap already write locks the parent's mm.
> >
> > Oh, you're only worried about vma_start_write()?
> >
> > That's a non-issue. It doesn't take the lock normally, since it starts off
> > with
> >
> >         if (__is_vma_write_locked(vma, &mm_lock_seq))
> >                 return;
> >
> > which catches on the lock sequence number already being set.
> >
> > So no extra locking there.
> >
> > Well, technically there's extra locking because the code stupidly
> > doesn't initialize new vma allocations to the right sequence number,
> > but that was talked about here:
> >
> >
> > https://lore.kernel.org/all/CAHk-=wiCrWAoEesBuoGoqqufvesicbGp3cX0LyKgEvsFaZNpDA@mail.gmail.com/
> >
> > and it's a separate issue.
> >
>
> I'm going to bet one beer this is the issue.
>
> The patch I'm responding to only consists of adding the call to
> vma_start_write and claims the 5% slowdown from it, while fixing
> crashes if the forking process is multithreaded.
>
> For the fix to work it has to lock something against the parent.
>
>         VMA_ITERATOR(old_vmi, oldmm, 0);
> [..]
>         for_each_vma(old_vmi, mpnt) {
> [..]
>                 vma_start_write(mpnt);
>
> the added line locks an obj in the parent's vm space.
>
> The problem you linked looks like pessimization for freshly allocated
> vmas, but that's what is being operated on here.

Sorry, now I'm having trouble understanding the problem you are
describing. We are locking the parent's vma before copying it and the
newly created vma is locked before it's added into the vma tree. What
is the problem then?

>
> --
> Mateusz Guzik <mjguzik gmail.com>