Re: [PATCH v4 07/20] mm/vma: fix self-merge check in copy_vma()

On 8/11/26 18:51, Lorenzo Stoakes (ARM) wrote:

On Tue, Aug 11, 2026 at 06:44:01PM +0200, David Hildenbrand (Arm) wrote:

quoted

quoted

Since copy_vma_and_data() which calls copy_vma() intends to reference the
old VMA after the merge, it must have this pointer updated.

This kind of self-merge is not possible with a succeeding merge, as the
merge always prefers to expand the preceding VMA if possible.

copy_vma() accounts for this by explicitly checking to see if a self-merge
occurred and updating the vmap pointer if so. However it incorrect did so
even for a subsequent merge (this is simply a noop so it had no impact).

Took me longer than it should to find out that this is no real fix ;)

quoted

So change this to only check for the case which matters - a backwards
merge - and rearrange the parameters to make it clearer we're doing that -
i.e. check new_vma->vm_start < old_vma_start (having already renamed
vma_start to old_vma_start to make it clear this is the previous VMA).

Also update the existing wall-of-text comment to be a lot clearer.

While we're here, replace the VM_BUG_ON_VMA() with a VM_WARN_ON_ONCE_VMA()
and update the VMA userland tests accordingly.

No functional change intended.

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

I skimmed over it, looks correct, but the merging stuff is way over my head to
confidentially give a RB tag her :)

Well that's what A-b is for ;)

Assisted-by? Half-assed-by?

;)

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David