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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: Commitfc3147245d("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 noticeable on tiered systems, where entire tmpfs workingsets can get stuck on lower-bandwidth memory. 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. Link: https://lore.kernel.org/20260629163337.1264881-1-hannes@cmpxchg.org Fixes:fc3147245d("mm: numa: Limit NUMA scanning to migrate-on-fault VMAs") Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Reported-by: Neha Gholkar <nehagholkar@gmail.com> Tested-by: Neha Gholkar <nehagholkar@gmail.com> Reviewed-by: Gregory Price <gourry@gourry.net> Acked-by: David Hildenbrand (Arm) <david@kernel.org> Acked-by: Balbir Singh <balbirs@nvidia.com> Cc: Alistair Popple <apopple@nvidia.com> Cc: Byungchul Park <byungchul@sk.com> Cc: "Huang, Ying" <ying.huang@linux.alibaba.com> Cc: Joshua Hahn <joshua.hahnjy@gmail.com> Cc: Matthew Brost <matthew.brost@intel.com> Cc: Rakie Kim <rakie.kim@sk.com> Cc: Zi Yan <ziy@nvidia.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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