mm/mempolicy: fix mpol_rebind_nodemask() for MPOL_F_NUMA_BALANCING

mm/mempolicy: fix mpol_rebind_nodemask() for MPOL_F_NUMA_BALANCING

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.

I can reproduce with the following steps in qemu with 4 NUMA nodes:
1. echo '+cpuset' > /sys/fs/cgroup/cgroup.subtree_control
2. mkdir /sys/fs/cgroup/test
3. ./reproducer &
4. cat /proc/$pid/numa_maps, the task is bound to NUMA 1
5. echo $pid > /sys/fs/cgroup/test/cgroup.procs
6. cat /proc/$pid/numa_maps, the task is bound to NUMA 0 now.

The reproducer code:

int main()
{
        struct bitmask *bmp;
        int ret;

        bmp = numa_parse_nodestring("1");
        ret = set_mempolicy(MPOL_BIND | MPOL_F_NUMA_BALANCING,
                bmp->maskp, bmp->size + 1);
        if (ret < 0) {
                perror("Failed to call set_mempolicy");
                exit(-1);
        }

        while (1);
        return 0;
}

If I call set_mempolicy() without MPOL_F_NUMA_BALANCING in the reproducer
code.  After step 5, the task is still bound to NUMA 1.

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.

Link: https://lkml.kernel.org/r/20260120011018.1256654-1-tujinjiang@huawei.com
Link: https://lkml.kernel.org/r/20251223110523.1161421-1-tujinjiang@huawei.com
Fixes: bda420b98505 ("numa balancing: migrate on fault among multiple bound nodes")
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Huang Ying <ying.huang@linux.alibaba.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Mel Gorman <mgorman <mgorman@suse.de>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>