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.com
Signed-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>

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index b4fccc921b62382c33d8d09299eadc6b3d2a7b3d..1bdc7d0d1b0b22d1c2ee54b9241db23563d0b575 100644 (file)
--- a/mm/mempolicy.c
+++ b/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;