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

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Jun 29, 2026 at 01:59:41PM -0400, Gregory Price wrote:
> On Mon, Jun 29, 2026 at 12:33:37PM -0400, Johannes Weiner wrote:
> > Neha reports that mapped shmem aren't considered for NUMA balancing,
> > noting convergence problems and bandwidth bottlenecking for cachelib
> > based workloads on tiered memory systems.
> > 
> > Looking at the code and going through the git history, this doesn't
> > actually seem intentional:
> > 
> > Commit fc3147245d19 ("mm: numa: Limit NUMA scanning to migrate-on-fault
> > VMAs") added a vma_policy_mof() gate to task_numa_work() so VMAs whose
> > policy lacks MPOL_F_MOF are skipped from NUMA balancing scans. The
> > motivation was a real usecase: Oracle was pinning shared segments with
> > mbind(MPOL_BIND) so trapping faults was both expensive and pointless.
> > 
> > The handling of NULL from vm_ops->get_policy, however, treated "user
> > explicitly opted out" the same as "user never specified anything." For
> > VMAs whose shared policy is absent - the common case for shmem - the
> > scan was disabled too.
> > 
> > This issue is old. It probably hurts less in conventional NUMA. But it's
> > very noticable on tiered systems, where entire tmpfs workingsets can get
> > stuck on lower-bandwidth memory.
> > 
> 
> Eugh.
> 
> Demotions don't care about mempolicy, so opting shmem out of NUMA
> balancing and mbind'ing on a tiered system is just full sadness.

Right, mbinding in tiered mode is a whole other ball of wax. I'm just
trying to make the default case work ;-)

> This is all just more evidence that demotion needs to be completely
> redone, it's creating a mess of undefined behavior for memory placement.

No argument from me.

> > Fix this by having vma_policy_mof() use __get_vma_policy() directly, and
> > thereby handle the fallback to task policy (-> preferred_node_policy()
> > has MPOL_F_MOF per default). Every other consumer of vm_ops->get_policy
> > already handles it this way, the scan-eligibility check was the outlier.
> > 
> > This preserves Mel's intended fix: don't scan stuff the user explicitly
> > pinned. But allow default policy vmas to participate in balancing.
> > 
> > Reported-by: Neha Gholkar <nehagholkar@gmail.com>
> > Tested-by: Neha Gholkar <nehagholkar@gmail.com>
> > Fixes: fc3147245d19 ("mm: numa: Limit NUMA scanning to migrate-on-fault VMAs")
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> 
> Reviewed-by: Gregory Price <gourry@gourry.net>

Thanks! Sorry for making you feel bad.