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

On Thu, Sep 24, 2026 at 03:28:10PM -0400, Zi Yan wrote:

On Thu Sep 17, 2026 at 12:22 PM EDT, Lorenzo Stoakes (ARM) 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>
---
 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,

Is it better to say "for VMAs ... after mmap_action_prepare()"? When I
first read this, I wonder why the check is done after
mmap_action_prepare(), which does some work based on action.type. Then,
I realize mmap_action_prepare() changes desc->vma_flags and affect its
mergeablitiy.

Ack, good point will update!

quoted

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

Otherwise, LGTM.

Reviewed-by: Zi Yan <ziy@nvidia.com>

Thanks!

--
Best Regards,
Yan, Zi

--
Cheers, Lorenzo