[PATCH 2/3] mm/mempolicy: refactor a read-once mechanism into a function for re-use

Gregory Price
2024-01-12T21:08:43Z

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.

Suggested-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Gregory Price <gregory.price@memverge.com>
---
mm/mempolicy.c | 26 ++++++++++++++++----------
1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 5da4fd79fd18..0abd3a3394ef 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1907,6 +1907,20 @@ 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();
+ __builtin_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
@@ -1914,20 +1928,12 @@ 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;
--
2.39.1