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

[Suren Baghdasaryan]

On Fri, Aug 4, 2023 at 5:26 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Fri, Aug 4, 2023 at 5:15 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Fri, 4 Aug 2023 at 16:25, Mateusz Guzik <mjguzik@xxxxxxxxx> 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.

The optimization Mateusz describes looks valid to me. If there is
nobody else to fault a page and mm_users is stable (which I think it
is because we are holding mmap_lock for write) then we can skip vma
locking, I think.

>
> >
> > 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@xxxxxxxxxxxxxx/
> >
> > and it's a separate issue.
> >
> >           Linus