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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

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.
> 
> Fixes: bda420b98505 ("numa balancing: migrate on fault among multiple bound nodes")
> Reviewed-by: Gregory Price <gourry@gourry.net>
> Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>
> Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
> ---
> Change in v3:
>   * update changelog
>   * collect RB from Huang Ying
> 
>   include/uapi/linux/mempolicy.h | 3 +++
>   mm/mempolicy.c                 | 2 +-
>   2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/mempolicy.h b/include/uapi/linux/mempolicy.h
> index 8fbbe613611a..6c962d866e86 100644
> --- a/include/uapi/linux/mempolicy.h
> +++ b/include/uapi/linux/mempolicy.h
> @@ -39,6 +39,9 @@ enum {
>   #define MPOL_MODE_FLAGS							\
>   	(MPOL_F_STATIC_NODES | MPOL_F_RELATIVE_NODES | MPOL_F_NUMA_BALANCING)
>   
> +/* Whether the nodemask is specified by users */
> +#define MPOL_USER_NODEMASK_FLAGS (MPOL_F_STATIC_NODES | MPOL_F_RELATIVE_NODES)
> +
>   /* Flags for get_mempolicy */
>   #define MPOL_F_NODE	(1<<0)	/* return next IL mode instead of node mask */
>   #define MPOL_F_ADDR	(1<<1)	/* look up vma using address */
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 68a98ba57882..76da50425712 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -365,7 +365,7 @@ static const struct mempolicy_operations {
>   
>   static inline int mpol_store_user_nodemask(const struct mempolicy *pol)
>   {
> -	return pol->flags & MPOL_MODE_FLAGS;
> +	return pol->flags & MPOL_USER_NODEMASK_FLAGS;
>   }
>   
>   static void mpol_relative_nodemask(nodemask_t *ret, const nodemask_t *orig,

I glimpsed over it and I think this is the right fix, thanks!

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

-- 
Cheers

David