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

[Gregory Price <gourry@gourry.net>]

On Wed, Sep 30, 2026 at 02:32:16PM +0200, David Hildenbrand (Arm) wrote:
> > 
> > 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.
> 

I would agree with you if demotion didn't entirely ignoring mempolicy.

On a tiered system, mempolicy *only* applies to initial (or fault-in)
placement, and otherwise is more or less entirely ignored.  This is
a case where it's not ignored - which is actually inconsistent with
the rest of the tiering tools (demotion, damon).

This is why I said this distinction may only make sense in TIERING
mode, while in NORMAL mode you likely just want to fault it back to
its original interleave location.

Also file interleave (indexed by offset) vs task (counter-incremented)
policies are affected by these changes very differently, which I asked
for some thought on.

As tiering develops, the less I think task-mempolicy as a whole makes
much sense (because it's at-best advisory).

~Gregory