Re: [PATCH] mm/numa_balancing: allow migrate on protnone reference with MPOL_WEIGHTED_INTERLEAVE policy

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

On 9/30/26 14:03, Gregory Price wrote:
> On Wed, Sep 30, 2026 at 01:26:17PM +0200, David Hildenbrand (Arm) wrote:
>>>
>>> Initial placement is only a starting point.  Pages initially allocated on
>>> fast memory are not necessarily the long-term hot pages, and pages
>>> initially allocated on slower memory may become hot as the workload's hot
>>> set changes.  The policy therefore needs to be able to combine weighted
>>> initial placement with memory tiering's NUMA fault based hot-page
>>> promotion.
>>
>> Ok, so memory allocation will respect the weights but balancing will ignore
>> them? That really sounds rather odd to me.
>>
>> And I assume that was the reason why we might have disallowed the combination:
>> it turns a weighted mechanism into an unweighted mechanism.
>>
>> So are we really sure these semantics that you would essentially set in stone
>> here are the semantics we want? (ignoring weights)
>>
> 
> Right, but you're requesting this configuration.  If you didn't set
> F_NUMA_BALANCING then you get the existing semantics.
> 
> In the existing semantics, if we're INTERLEAVE and WEIGHTED_INTERLEAVE
> we just always say "no tiering for you".
> 
> But as an opt-in option?  I don't quite see the argument for saying
> interleaved regions (or tasks) to be opted-out if the user asks for it -
> that just seems like an arbitrary limitation.

Just to be clear, what I am saying is: the proposed semantics are inconsistent
(weighted when mechanism A honors them and mechanism B doesn't honor them), but
once we set these semantics in stone like that, we cannot easily change them
later because some user might depend on that behavior.

So you'll need yet another flag to say "NUMA balancing really also honors the
weights". And that's where it all gets ugly.

-- 
Cheers,

David