Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Dec 19, 2024 at 12:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Dec 18, 2024 at 09:58:12AM -0800, Suren Baghdasaryan wrote:
>
> > And remove_vma will be just freeing the vmas. Is that correct?
>
> Yep.
>
> > I'm a bit confused because the original thinking was that
> > vma_mark_detached() would drop the last refcnt and if it's 0 we would
> > free the vma right there. If that's still what we want to do then I
> > think the above sequence should look like this:
>
> Right; sorry about that. So my initial objection to that extra sync was
> based on the reasons presented -- but having had to look at the unmap
> path again (my mm-foo is somewhat rusty, I've not done much the past few
> years) I realized that keeping a VMA alive beyond unmapping PTEs is just
> plain daft.
>
> So yes, back to your original semantics, but cleaned up to not need that
> extra sync point -- instead relying on the natural placement of
> vma_start_write() after unhooking from the mm. And not for reasons of
> the race, but for reasons of integrity -- VMA without PTEs is asking for
> more trouble.

Ack. Thanks for clarification!