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

[David Hildenbrand (Arm)]

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

-- 
Cheers,

David