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

[Gregory Price <gourry@gourry.net>]

On Thu, Jul 23, 2026 at 05:15:15PM -0700, Andrew Morton wrote:
> On Thu, 23 Jul 2026 10:54:06 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:
> 
> > > But in this case, IIUC, this change will "silently" enable NUMA balancing for
> > > shmem-based KVM setups where it was previously disabled (albeit unintentionally).
> > > 
> > > I'm not fundamentally opposed to the change, but I do worry that downstream KVM
> > > users could be in for a nasty surprise.
> 
> It is a bit concerning.
> 
> > This should be very visible in early kernel validation after an
> > upgrade. VM hosts tend to not do much else, with VM memory dominating
> > the host. You'd expect a significant uptick in numa_pte_updates,
> > numa_hint_faults, and a change in per-node nr_shmem stats. And the
> > patch subject makes it trivial to find in a commit delta scan.
> 
> Is there anything we can do to make this process less painful? 
> Documentation of course, but what about detecting the situation and
> emitting a once-off warning?

Maybe a rate-limited or one-time pr_info/warn on the first PROT_NONE
fault affecting KVM in the painful zap path?

Non-invasive, targeted, and helpful.

~Gregory