mm/mempolicy: refactor a read-once mechanism into a function for re-use

Move the use of barrier() to force policy->nodemask onto the stack into a
function `read_once_policy_nodemask` so that it may be re-used.

Link: https://lkml.kernel.org/r/20240202170238.90004-3-gregory.price@memverge.comSigned-off-by: Gregory Price <gregory.price@memverge.com>
Suggested-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Hasan Al Maruf <Hasan.Maruf@amd.com>
Cc: Honggyu Kim <honggyu.kim@sk.com>
Cc: Hyeongtak Ji <hyeongtak.ji@sk.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Ravi Jonnalagadda <ravis.opensrc@micron.com>
Cc: Srinivasulu Thanneeru <sthanneeru.opensrc@micron.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mempolicy.c

@@ -1905,6 +1905,20 @@ unsigned int mempolicy_slab_node(void)
 	}
 }
 
+static unsigned int read_once_policy_nodemask(struct mempolicy *pol,
+					      nodemask_t *mask)
+{
+	/*
+	 * barrier stabilizes the nodemask locally so that it can be iterated
+	 * over safely without concern for changes. Allocators validate node
+	 * selection does not violate mems_allowed, so this is safe.
+	 */
+	barrier();
+	memcpy(mask, &pol->nodes, sizeof(nodemask_t));
+	barrier();
+	return nodes_weight(*mask);
+}
+
 /*
  * Do static interleaving for interleave index @ilx.  Returns the ilx'th
  * node in pol->nodes (starting from ilx=0), wrapping around if ilx
@@ -1912,20 +1926,12 @@ unsigned int mempolicy_slab_node(void)
  */
 static unsigned int interleave_nid(struct mempolicy *pol, pgoff_t ilx)
 {
-	nodemask_t nodemask = pol->nodes;
+	nodemask_t nodemask;
 	unsigned int target, nnodes;
 	int i;
 	int nid;
-	/*
-	 * The barrier will stabilize the nodemask in a register or on
-	 * the stack so that it will stop changing under the code.
-	 *
-	 * Between first_node() and next_node(), pol->nodes could be changed
-	 * by other threads. So we put pol->nodes in a local stack.
-	 */
-	barrier();
 
-	nnodes = nodes_weight(nodemask);
+	nnodes = read_once_policy_nodemask(pol, &nodemask);
 	if (!nnodes)
 		return numa_node_id();
 	target = ilx % nnodes;