Re: [PATCH] mm/userfaultfd: only allocate anon_vma in CoW mappings

[Lorenzo Stoakes (ARM)]

On Wed, Oct 14, 2026 at 01:53:11PM +0200, David Hildenbrand (Arm) wrote:
> As uffd_lock_vma() is part of the same lookup machinery and
> also contains a misleading VM_SHARED check, change that one as well
> directly for consistency.

OK, I think that one isn't _strictly necessary_, but while we're there,
worth doing too, clearly.

We should also go through and convert the remaining !VM_SHARED checks ->
vma_is_cow_mapping() too if possible, but that's a follow up, obviously!

>
> A follow-up cleanup will stop taking the hugetlb_vma_lock_read(src_vma)
> lock in copy_hugetlb_page_range() entirely. Further, the usage of

Hmm really? Is it possible to do that without the hugetlb lock?

It'd be good in general if we could get rid of that lock as a whole :) but maybe
dreaming.

> VM_SHARED in userfaultfd code should be cleaned up later to focus on
> CoW vs. non-CoW mappings.

I think also anywhere else in the kernel where this is done. An audit on that in
general would be useful!

>
> Reported-by: Kyle Zeng <kylebot@xxxxxxxxxx>
> Fixes: 867a43a34ff8a ("userfaultfd: use per-vma locks in userfaultfd operations")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

This LGTM, thanks for handling this!

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

--
Cheers, Lorenzo