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

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

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

>
> --
> Cheers,
>
> David

--
Cheers, Lorenzo