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

On Mon, Aug 03, 2026 at 11:12:01PM +0200, Jann Horn wrote:
> On Mon, Aug 3, 2026 at 6:08 PM Lorenzo Stoakes (ARM) <[email protected]> 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 <[email protected]> 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...

Thanks Jann, though also ugh god at that :)

(Really the ideal solution for nommu would be a 'virtual' MMU with noops for
everything IMO.)

Anyway, I think we could fix this with a:

static inline void vma_mark_attached(struct vm_area_struct *vma)
{
+	if (!IS_ENBLED(CONFIG_MMU))
+		return;

	vma_assert_write_locked(vma);
	vma_assert_detached(vma);
	refcount_set_release(&vma->vm_refcnt, 1);
}

Here?

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