linux-kernel - [PATCH v3 2/4] mm/mempolicy: refactor a read-once mechanism into a function for re-use

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Message-Id: <20240125184345.47074-3-gregory.price@memverge.com>
Date: Thu, 25 Jan 2024 13:43:43 -0500
From: Gregory Price <gourry.memverge@...il.com>
To: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
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Subject: [PATCH v3 2/4] 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.

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

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index f1627d45b0c8..b13c45a0bfcb 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1894,6 +1894,20 @@ 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
@@ -1901,20 +1915,12 @@ 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