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

[Mateusz Guzik <mjguzik@gmail.com>]

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.

-- 
Mateusz Guzik <mjguzik gmail.com>