+ mm-mempolicy-use-one-node-set-for-interleave-weights.patch added to mm-hotfixes-unstable branch

+ mm-mempolicy-use-one-node-set-for-interleave-weights.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/mempolicy: use one node set for interleave weights
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-mempolicy-use-one-node-set-for-interleave-weights.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-mempolicy-use-one-node-set-for-interleave-weights.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Andrew Morton <akpm@linux-foundation.org>
Subject: mm/mempolicy: use one node set for interleave weights
Date: Fri, 30 Oct 2026 12:10:46 -0400

Switching weighted interleave back to automatic mode while memory is being
onlined can cause a kernel divide error on x86, and a panic if
panic_on_oops is set.

reduce_interleave_weights() scans N_MEMORY three times, but hotplug can
cause N_MEMORY to change at runtime - and torn reads on that nodemask can
cause a total sum of zero bandwidth.  A node that was hot-unplugged and
then hotplugged back can then cause this loop to generate a div/0.

Record the nodes visited while summing their bandwidth, then use that same
set when scaling and reducing the weights.  This gives us a consistent
state even if N_MEMORY changes.

Link: https://lore.kernel.org/20261030161046.1497894-1-gourry@gourry.net
Fixes: e341f9c3c841 ("mm/mempolicy: Weighted Interleave Auto-tuning")
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
Cc: David Hildenbrand <david@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Harry Yoo <harry@kernel.org>
Cc: <stable@vger.kernel.org>
---

 mm/mempolicy.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

--- a/mm/mempolicy.c~mm-mempolicy-use-one-node-set-for-interleave-weights
+++ a/mm/mempolicy.c
@@ -207,15 +207,22 @@ static u8 get_il_weight(int node)
  */
 static void reduce_interleave_weights(unsigned int *bw, u8 *new_iw)
 {
+	nodemask_t nodes = NODE_MASK_NONE;
 	u64 sum_bw = 0;
 	unsigned int cast_sum_bw, scaling_factor = 1, iw_gcd = 0;
 	int nid;
 
-	for_each_node_state(nid, N_MEMORY)
+	/*
+	 * N_MEMORY can change while wi_state_lock is held. Remember the
+	 * nodes contributing to sum_bw so all passes use the same set.
+	 */
+	for_each_node_state(nid, N_MEMORY) {
+		node_set(nid, nodes);
 		sum_bw += bw[nid];
+	}
 
 	/* Scale bandwidths to whole numbers in the range [1, weightiness] */
-	for_each_node_state(nid, N_MEMORY) {
+	for_each_node_mask(nid, nodes) {
 		/*
 		 * Try not to perform 64-bit division.
 		 * If sum_bw < scaling_factor, then sum_bw < U32_MAX.
@@ -234,7 +241,7 @@ static void reduce_interleave_weights(un
 	}
 
 	/* 1:2 is strictly better than 16:32. Reduce by the weights' GCD. */
-	for_each_node_state(nid, N_MEMORY)
+	for_each_node_mask(nid, nodes)
 		new_iw[nid] /= iw_gcd;
 }
 
_

Patches currently in -mm which might be from akpm@linux-foundation.org are

openrisc-export-mulsi3.patch
mm-mempolicy-use-one-node-set-for-interleave-weights.patch
maintainers-add-ipc-maintainers.patch
profile-account-for-partial-proc-profile-reads.patch
profile-simplify-proc-profile-read-layout.patch
ipc-msg-clamp-negative-msg_info-usage-counts.patch
drivers-media-v4l2-core-v4l2-vp9c-reduce-inlining.patch