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

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Aug 9, 2023 at 2:07 PM Mateusz Guzik <mjguzik@gmail.com> wrote:
>
> On 8/5/23, Suren Baghdasaryan <surenb@google.com> wrote:
> > 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?
> >
>
> Sorry for the late reply!
>
> Looks there has been a bunch of weird talking past one another in this
> thread and I don't think trying to straighten it all out is worth any
> time.
>
> I think at least the two of us agree that if a single-threaded process
> enters dup_mmap an
> down_writes the mmap semaphore, then no new thread can pop up in said
> process, thus no surprise page faults from that angle. 3rd parties are
> supposed to interfaces like access_remote_vm, which down_read said
> semaphore and are consequently also not a problem. The only worry here
> is that someone is messing with another process memory without the
> semaphore, but is very unlikely and patchable in the worst case -- but
> someone(tm) has to audit. With all these conditions satisfied one can
> elide vma_start_write for a perf win.
>
> Finally, I think we agreed you are going to do the audit ;)

Ack. I'll look into this once the dust settles. Thanks!

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