mm/mempolicy: fix lock contention on mems_allowed

The mems_allowed field can be modified by other tasks, so it isn't safe to
access it with alloc_lock unlocked even in the current process context.

Say there are two tasks: A from cpusetA is performing set_mempolicy(2),
and B is changing cpusetA's cpuset.mems:

  A (set_mempolicy)		B (echo xx > cpuset.mems)
  -------------------------------------------------------
  pol = mpol_new();
				update_tasks_nodemask(cpusetA) {
				  foreach t in cpusetA {
				    cpuset_change_task_nodemask(t) {
  mpol_set_nodemask(pol) {
				      task_lock(t); // t could be A
    new = f(A->mems_allowed);
				      update t->mems_allowed;
    pol.create(pol, new);
				      task_unlock(t);
  }
				    }
				  }
				}
  task_lock(A);
  A->mempolicy = pol;
  task_unlock(A);

In this case A's pol->nodes is computed by old mems_allowed, and could
be inconsistent with A's new mems_allowed.

While it is different when replacing vmas' policy: the pol->nodes is
gone wild only when current_cpuset_is_being_rebound():

  A (mbind)			B (echo xx > cpuset.mems)
  -------------------------------------------------------
  pol = mpol_new();
  mmap_write_lock(A->mm);
				cpuset_being_rebound = cpusetA;
				update_tasks_nodemask(cpusetA) {
				  foreach t in cpusetA {
				    cpuset_change_task_nodemask(t) {
  mpol_set_nodemask(pol) {
				      task_lock(t); // t could be A
    mask = f(A->mems_allowed);
				      update t->mems_allowed;
    pol.create(pol, mask);
				      task_unlock(t);
  }
				    }
  foreach v in A->mm {
    if (cpuset_being_rebound == cpusetA)
      pol.rebind(pol, cpuset.mems);
    v->vma_policy = pol;
  }
  mmap_write_unlock(A->mm);
				    mmap_write_lock(t->mm);
				    mpol_rebind_mm(t->mm);
				    mmap_write_unlock(t->mm);
				  }
				}
				cpuset_being_rebound = NULL;

In this case, the cpuset.mems, which has already done updating, is finally
used for calculating pol->nodes, rather than A->mems_allowed.  So it is OK
to call mpol_set_nodemask() with alloc_lock unlocked when doing mbind(2).

Link: https://lkml.kernel.org/r/20220811124157.74888-1-wuyun.abel@bytedance.com
Fixes: 78b132e9ba ("mm/mempolicy: remove or narrow the lock on current")
Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mempolicy.c

@@ -853,12 +853,14 @@ static long do_set_mempolicy(unsigned short mode, unsigned short flags,
 		goto out;
 	}
 
+	task_lock(current);
 	ret = mpol_set_nodemask(new, nodes, scratch);
 	if (ret) {
+		task_unlock(current);
 		mpol_put(new);
 		goto out;
 	}
-	task_lock(current);
+
 	old = current->mempolicy;
 	current->mempolicy = new;
 	if (new && new->mode == MPOL_INTERLEAVE)