Re: [PATCH v3] mm/mempolicy: fix mpol_rebind_nodemask() for MPOL_F_NUMA_BALANCING

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 15 Jan 2026 18:10:51 +0100 "David Hildenbrand (Red Hat)" <david@kernel.org> wrote:

> On 12/23/25 12:05, Jinjiang Tu wrote:
> > commit bda420b98505 ("numa balancing: migrate on fault among multiple
> > bound nodes") adds new flag MPOL_F_NUMA_BALANCING to enable NUMA balancing
> > for MPOL_BIND memory policy.
> > 
> > When the cpuset of tasks changes, the mempolicy of the task is rebound by
> > mpol_rebind_nodemask(). When MPOL_F_STATIC_NODES and MPOL_F_RELATIVE_NODES
> > are both not set, the behaviour of rebinding should be same whenever
> > MPOL_F_NUMA_BALANCING is set or not. So, when an application calls
> > set_mempolicy() with MPOL_F_NUMA_BALANCING set but both MPOL_F_STATIC_NODES
> > and MPOL_F_RELATIVE_NODES cleared, mempolicy.w.cpuset_mems_allowed should
> > be set to cpuset_current_mems_allowed nodemask. However, in current
> > implementation, mpol_store_user_nodemask() wrongly returns true, causing
> > mempolicy->w.user_nodemask to be incorrectly set to the user-specified
> > nodemask. Later, when the cpuset of the application changes,
> > mpol_rebind_nodemask() ends up rebinding based on the user-specified
> > nodemask rather than the cpuset_mems_allowed nodemask as intended.
> > 
> > To fix this, only set mempolicy->w.user_nodemask to the user-specified
> > nodemask if MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES is present.
> > 
> 
> ...
>
> I glimpsed over it and I think this is the right fix, thanks!
> 
> Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>

Cool.  I decided this was "not for backporting", but the description of
the userspace-visible runtime effects isn't very clear.  Jinjiang, can
you please advise?