Re: [PATCH v10 07/37] mm: thread user_addr through page allocator for cache-friendly zeroing

[David Hildenbrand (Arm)]

On 6/8/26 16:44, Matthew Wilcox wrote:
> On Mon, Jun 08, 2026 at 04:37:03PM +0200, David Hildenbrand (Arm) wrote:
>> On 6/8/26 16:31, Matthew Wilcox wrote:
>>>
>>> What I don't understand is how the kernel page allocator needs to know
>>> the user address in order to effectively zero it, but the hypervisor is
>>> able to zero the page without knowing the user address.  It feels like
>>> somebody has x86-centric thinking where cache colouring doesn't matter.
>>
>> (not commenting on the icache dache mess we have to drag along)
> 
> Well, that was kind of the point of this email ... I did ask the
> question you're answering in a different email so let me respond
> to that too.

Now I'm confused :)

> 
>> The thing is that with free-page-reporting the memory is already zeroed by the
>> hypervisor as part of discarding that memory previously (e.g., MADV_DONTNEED)
>> and allocating fresh pages on re-access.
>>
>> So it's not a question of "why is the hypervisor zeroing less efficiently", as
>> zeroing is just a side-product of reclaiming that memory in the first place.
> 
> We definitely have users who don't want the guest to trust the
> hypervisor.  So how do they disable this optimisation?

Right, I don't think we currently have a toggle to disable free page reporting.
So IIUC, this optimization would similarly automatically get enabled if the
hypervisor advertises it.

-- 
Cheers,

David