Re: [RFC, PATCH 00/12] userfaultfd: working set tracking for VM guest memory

[David Hildenbrand (Arm)]

On 4/16/26 15:49, Kiryl Shutsemau wrote:
> On Tue, Apr 14, 2026 at 06:10:44PM +0100, Kiryl Shutsemau wrote:
>> On Tue, Apr 14, 2026 at 05:37:50PM +0200, David Hildenbrand (Arm) wrote:
>>>
>>> I would rather tackle this from the other direction: it's another form
>>> of protection (like WP), not really a "minor" mode.
>>>
>>> Could we add a UFFDIO_REGISTER_MODE_RWP (or however we would call it)
>>> and support it for anon+shmem, avoiding the zapping for shmem completely?
>>
>> I like this idea.
>>
>> It should be functionally equivalent, but your interface idea fits
>> better with the rest.
>>
>> Thanks! Will give it a try.
> 
> Here is an updated version:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git/log/?h=uffd/rfc-v2
> 
> will post after -rc1 is tagged.
> 
> I like it more. It got substantially cleaner.

I don't have time to look into the details just yet, but my thinking was
that

a) It would avoid the zap+refault

b) We could reuse the uffd-wp PTE bit + marker to indicate/remember the
   protection, making it co-exist with NUMA hinting naturally.

b) obviously means that we cannot use uffd-wp and uffd-rwp at the same
time in the same uffd area. I guess that should be acceptable for the
use cases we you should have in mind?

But I also haven't taken a closer look at this patch set, whether you
would already be using a PTE bit somehow (I suspect not :) )

-- 
Cheers,

David