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

[Gregory Price <gourry@gourry.net>]

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.

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

> 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>