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

On Tue, Aug 4, 2026 at 1:56 AM Lorenzo Stoakes (ARM) [off-list ref] wrote:

On Mon, Aug 03, 2026 at 11:12:01PM +0200, Jann Horn wrote:

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On Mon, Aug 3, 2026 at 6:08 PM Lorenzo Stoakes (ARM) [off-list ref] wrote:

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On Mon, Aug 03, 2026 at 08:24:44AM -0700, Suren Baghdasaryan wrote:

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On Sun, Aug 2, 2026 at 2:55 PM Suren Baghdasaryan [off-list ref] wrote:

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-#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?

Yes, I think so.
I also need to make sure mmap_lock fallbacks we are removing are not
used in NOMMU, otherwise it will fail to lock_vma_under_rcu() and will
have no fallbacks.
TCP zerocopy and binder are gated on CONFIG_MMU, so that's fine. Need
to check the BPF part...

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

--
Cheers, Lorenzo