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

Mateusz Guzik mjguzik at gmail.com
Sat Aug 5 11:06:13 AEST 2023

On 8/5/23, Linus Torvalds <torvalds at linux-foundation.org> wrote:
> On Fri, 4 Aug 2023 at 16:25, Mateusz Guzik <mjguzik at 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) {

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>

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