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

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Jun 29, 2026 at 08:33:32PM +0200, David Hildenbrand (Arm) wrote:
> On 6/29/26 18:33, 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.
> 
> Sounds bad enough to warrant CC: stable?

No objection from me. I was hesitant because it's old, and while these
are real workloads that see it they are hardware/kernel validation
runs. OTOH it's a straight-forward bug and should backport easily.

> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > ---
> >  mm/mempolicy.c | 21 ++++++---------------
> >  1 file changed, 6 insertions(+), 15 deletions(-)
> > 
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 36699fabd3c2..bba65898aee1 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -2057,24 +2057,15 @@ struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
> >  bool vma_policy_mof(struct vm_area_struct *vma)
> >  {
> >  	struct mempolicy *pol;
> > +	pgoff_t ilx;
> > +	bool mof;
> >  
> > -	if (vma->vm_ops && vma->vm_ops->get_policy) {
> > -		bool ret = false;
> > -		pgoff_t ilx;		/* ignored here */
> > -
> > -		pol = vma->vm_ops->get_policy(vma, vma->vm_start, &ilx);
> > -		if (pol && (pol->flags & MPOL_F_MOF))
> > -			ret = true;
> > -		mpol_cond_put(pol);
> > -
> > -		return ret;
> > -	}
> 
> Okay, we used the fallback of vma->vm_policy before (if vma->vm_ops->get_policy
> was not available), which is what __get_vma_policy() does at well.
> 
> But if vma->vm_ops->get_policy now returns NULL, we fallback to get_task_policy().

Yep.

> Makes sense to me although this is a source of confusion for me.

How so? Is there anything I can improve in the changelog?

> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thanks David!