Re: [PATCH v2 0/4] sched/numa: stop VMA scan filters from gating promotion

[Gregory Price <gourry@gourry.net>]

On Wed, Sep 16, 2026 at 10:35:25PM -0700, Andrew Morton wrote:
> On Thu, 10 Sep 2026 20:18:22 -0400 Gregory Price <gourry@gourry.net> wrote:
> 
> > 
> > Functional observation:
> >   A 20 GB hash table VMA that previously remained entirely on CXL was
> >   split evenly between DRAM and CXL after the changes - and tier
> >   residency tracked hotness.  This was previously affected by the
> >   stavation issue caused by the "unaccessed VMA" filter.
> 
> This seems very significant?
> 
> Why cc:stable and Fixes:?  Is this something which ran at these sorts
> of speeds before the offending commits?
> 

It's actually that the changes functionally broke numa balancing to the
point that it just became completely ineffective for very common use
cases.

We only didn't notice because another bug:
https://lore.kernel.org/all/20260629163337.1264881-1-hannes@cmpxchg.org/

hid almost all these issues from being apparent on large workloads
(which was differently horrendous - that bug made ~70-90% of some
 database workload memory ineligible for tiering)

Once the shmem fix went in, all the starvation and filtering issues
become more apparent when the bandwidth numbers stopped making sense
(see above).

Basically numa balancing has been broken since 2022/2023, and anyone
who has done any testing with it since then has been working off of
bad data (incomplete VMA coverage).

~Gregory