Re: [PATCH v3 05/40] mm/vma: ensure mmap_prepare doesn't set actions on a mergeable vma

On Fri, Sep 25, 2026 at 12:28:49AM -0700, Suren Baghdasaryan wrote:

On Thu, Sep 17, 2026 at 9:25 AM Lorenzo Stoakes (ARM) [off-list ref] wrote:

quoted

When a user requests an mmap_action be performed in mmap_prepare, this
involves populating the VMA range with data.

However, if the VMA is mergeable, it might then mistakenly be merged with
another VMA without having populated the range.

Every mmap action currently available sets VMA flags such that the VMA
cannot be merged.

However, to ensure that no future mmap action falls foul of this, assert
that this is the case upon mmap_prepare validation.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

Thanks!

quoted

---
 mm/vma.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/vma.c b/mm/vma.c
index d6ed10cefc8f..62f2ce1ad5a1 100644
--- a/mm/vma.c
+++ b/mm/vma.c

@@ -2809,6 +2809,15 @@ static int mmap_validate(unsigned long prev_start, unsigned long prev_end,
 int mmap_prepare_validate(const struct vm_area_desc *prev_desc,
                          const struct vm_area_desc *desc)
 {
+       /*
+        * It is not valid to execute mmap actions for VMAs which can be merged,
+        * as any such merge would leave portions of the mapping incorrectly
+        * unmapped.
+        */
+       if (vma_flags_can_merge(&desc->vma_flags) &&
+           WARN_ON_ONCE(desc->action.type != MMAP_NOTHING))

Any reason you chose this "if (A && WARN_ON_ONCE(B))" pattern instead
of a simpler "if (WARN_ON_ONCE(A && B))"? Unless there are races
between A and B updates, I think these would be equivalent, right?

They are equivalent yes!

So it's more of a style thing, as that reads less clearly to me:

	if (WARN_ON_ONCE(vma_flags_can_merge(&desc->vma_flags) &&
			 desc->action.type != MMAP_NOTHING))

And this version highlights what is the broken thing here (action type set) a
little more clearly though obviously that's only if mergeable so arguable.

I don't have a really strong opinion though I can change it if you prefer?

quoted

+               return -EINVAL;
+
        return mmap_validate(prev_desc->start, prev_desc->end,
                             desc->start, desc->end,
                             &prev_desc->vma_flags, &desc->vma_flags);

--
2.55.0

--
Cheers, Lorenzo