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

[Gregory Price <gourry@gourry.net>]

On Sat, Dec 13, 2025 at 04:29:11PM +0800, Jinjiang Tu wrote:
> When mempolicy is rebound due to the process moves to a different cpuset
> context, or the set of nodes allowed by current cpuset context changes,
> mpol_rebind_nodemask() remaps the nodemask according to the old and new
> cpuset_mems_allowed by default. So, use mempolicy.w.cpuset_mems_allowed
> to store the old nodemask allowed by cpuset.
> 
> MPOL_F_STATIC_NODES suppresses the node remap and intersects the user's
> passed nodemask and nodes allowed by new cpuset context.
> For MPOL_F_RELATIVE_NODES, the user's passed nodemask means node IDs that
> are relative to the set of node IDs allowed by the process's current
> cpuset. So, use mempolicy.w.user_nodemask to store the user's passed
> nodemask.
> 
> 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, the behaviour of rebinding should be same with default
> befaviour. However, mpol_store_user_nodemask() returns true for
> MPOL_F_NUMA_BALANCING, leading to mempolicy.w.cpuset_mems_allowed stores
> the user's passed nodemask instead of cpuset_current_mems_allowed, and
> mpol_rebind_nodemask() remaps wrongly.
> 
> Fixes: bda420b98505 ("numa balancing: migrate on fault among multiple bound nodes")
> Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>

Fix looks good. Thank you!

With changelog updates discussed in the thread with Andrew:

Reviewed-by: Gregory Price <gourry@gourry.net>

~Gregory