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

[Suren Baghdasaryan]

On Fri, Aug 4, 2023 at 5:49 PM Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
>
> On 8/5/23, Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > 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.
> >
>
> mm_users is definitely *not* stable -- it can be bumped by
> get_task_mm, which is only synchronized with task lock.

Ugh, you are of course correct. Poor choice for saying no new users
(threads) can appear from under us.

>
> However, the other users (that I know of ) go through the mmap
> semaphore to mess with anything which means they will wait for
> dup_mmap to finish (or do their work first). I would be surprised if
> there were any cases which don't take the semaphore, given that it was
> a requirement prior to the vma patchset (unless you patched some to no
> longer need it?). I would guess worst case the semaphore can be added
> if missing.

No, the only mmap_lock read-lock that is affected is during the page
fault, which is expected.

>
> What is guaranteed is that if the forking process is single-threaded,
> there will be no threads added out of nowhere -- the only thread which
> could do it is busy creating one in dup_mmap. If multithreaded
> operation of the forking process was the only problem, that's it.
>
> >>
> >> >
> >> > 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
> >
>
>
> --
> Mateusz Guzik <mjguzik gmail.com>