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

David Hildenbrand (Arm)
2026-04-25T06:05:29Z

On 4/24/26 15:49, Kiryl Shutsemau wrote:
> On Fri, Apr 24, 2026 at 07:51:44AM -0400, Peter Xu wrote:
>> On Fri, Apr 24, 2026 at 11:34:48AM +0100, Kiryl Shutsemau wrote:
>>> Both page_idle and the LRUs (legacy or MGLRU) track accesses on physical
>>> memory. We need visibility in the virtual address space domain.
>>
>> Yes they are, but ACCESS bit isn't.
>
> A-bit is not a reliable signal for userspace working-set tracking
> because the kernel itself is a concurrent consumer.
Right, I don't think we want to rely on either the A bit just like we don't want
to rely on the Dirty bit in other code. (and even SoftDirty bit is a flawed concept)

I do see some value in a reliable RWP mechanism based on uffd. The real question
is, how much benefit it would bring (which other use cases could benefit from it).

--
Cheers,

David