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 5:15 PM 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.

That check will prevent re-locking but if vma is not already locked
then the call will proceed with obtaining the lock and setting
vma->vm_lock_seq to mm->mm_lock_seq.

>
> 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.
>
>           Linus