Re: [PATCH v3 1/5] mm: Make per-VMA locks available universally

[Jann Horn]

On Mon, Aug 3, 2026 at 6:08 PM Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx> wrote:
> On Mon, Aug 03, 2026 at 08:24:44AM -0700, Suren Baghdasaryan wrote:
> > On Sun, Aug 2, 2026 at 2:55 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > > -#endif /* CONFIG_PER_VMA_LOCK */
> >
> > Now that I'm looking closer into this, I think we would break NOMMU
> > case because nommu.c does not take VMA write locks at all. So,
> > lock_vma_under_rcu() for example would always succeed.
>
> I don't think anything's broken actually.
>
> Per-VMA locks was gated on CONFIG_MMU so nothing there assumes per-VMA flags,
> but now you have stuff that happens that didn't before but:
>
> * vm_area_free() -> vma_assert_detached() - fine - it's always detached in nommu.

I don't think that's true, AFAICS vma_mark_attached() is called from
vma_iter_store_new() which is called from the nommu version of
do_mmap(). But I think you're right that one blunt workaround for this
would be to add a hack that prevents ever marking VMAs in nommu as
attached...

> * vm_area_dup() -> vma_lock_init() - no asserts, just sets refcount to 0 (correct).