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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 10/14/26 20:04, Kyle Zeng wrote:
> On Wed, Oct 14, 2026 at 03:51:22PM +0100, Lorenzo Stoakes (ARM) wrote:
>> 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@openai.com>
>>> Fixes: 867a43a34ff8a ("userfaultfd: use per-vma locks in userfaultfd operations")
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>>
>> This LGTM, thanks for handling this!
>>
>> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>>
>> --
>> Cheers, Lorenzo
> 
> I ran the patch through my system and did not find any issues with this
> patch. Thanks for addressing this!
> 
> Reviewed-by: Kyle Zeng <kylebot@openai.com>

Thanks a bunch, and thanks for reporting all these issues!

-- 
Cheers,

David