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

[Jinjiang Tu <tujinjiang@huawei.com>]

在 2025/12/21 15:06, Huang, Ying 写道:
> Hi, Jinjiang,
>
> Jinjiang Tu <tujinjiang@huawei.com> writes:
>
>> 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.
> Good catch!  Thanks for fixing this.
>
>> Fixes: bda420b98505 ("numa balancing: migrate on fault among multiple bound nodes")
>> Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
>> ---
>>   include/uapi/linux/mempolicy.h | 6 ++++++
>>   mm/mempolicy.c                 | 2 +-
>>   2 files changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/uapi/linux/mempolicy.h b/include/uapi/linux/mempolicy.h
>> index 8fbbe613611a..1802b6c89603 100644
>> --- a/include/uapi/linux/mempolicy.h
>> +++ b/include/uapi/linux/mempolicy.h
>> @@ -39,6 +39,12 @@ enum {
>>   #define MPOL_MODE_FLAGS							\
>>   	(MPOL_F_STATIC_NODES | MPOL_F_RELATIVE_NODES | MPOL_F_NUMA_BALANCING)
>>   
>> +/*
>> + * MPOL_USER_NODEMASK_FLAGS is used to determine if nodemask passed by
>> + * users should be used in mpol_rebind_nodemask().
>> + */
> This sounds a little internal in a user API header file.  How about
> something like below.

Thanks, I have updated it in v2.

>
> /* Whether is a nodemask specified by user */
>
>> +#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,
> ---
> Best Regards,
> Huang, Ying