Re: [PATCH] mm: mempolicy: fix automatic numa balancing for shmem

[Gregory Price <gourry@gourry.net>]

On Tue, Jun 30, 2026 at 07:20:50PM +0800, Huang, Ying wrote:
> Gregory Price <gourry@gourry.net> writes:
> 
> [snip]
> 
> > Demotions don't care about mempolicy, so opting shmem out of NUMA
> > balancing and mbind'ing on a tiered system is just full sadness.
> >
> > This is all just more evidence that demotion needs to be completely
> > redone, it's creating a mess of undefined behavior for memory placement.
> 
> It's hard to respect mempolicy during demotion in the current
> implementation.  Do you have any ideas on how to improve this?
> 

I think it's feasible we could respect per-vma mempolicies, but not
per-task.  That would at least make this particular interaction less
painful and mbind() would do what you'd expect.  It is a bit racy,
but with MPOL_MF_MOVE_ALL the user can get what they actually want.

I think task-wide mempolicy is problematic and generally a bad idea
on tiered systems, maybe it's ok if we simply document task policies
are not respected on tiered systems?

~Gregory