[PATCH] mm/mempolicy: take a cpuset cookie for the interleave node count

[Gregory Price <gourry@gourry.net>]

alloc_pages_bulk_interleave() counts pol->nodes without a cpuset cookie:

	nodes = nodes_weight(pol->nodes);
	nr_pages_per_node = nr_pages / nodes;

nodemask_t spans several words once MAX_NUMNODES exceeds BITS_PER_LONG, so
a concurrent cpuset rebind can tear that read and yield an empty mask even
though neither version of it was empty.  The call then allocates nothing
and returns 0.

Some compilers will hoist the loop entry test above the division,
because nr_pages_per_node is dead when the loop does not run.

	682e:	call	...		<- nodes_weight()
	6838:	test	%eax,%eax
	683a:	jle	692d		<- nodes <= 0 skips the loop
	684a:	div	%rcx

So in most deployments, this div/0 is unreachable - but nothing in the
source guarantees that, it's just not easily exercised.

Take the cookie around the count and bail if the mask really is empty.
Only the count needs it, interleave_nodes() takes the cookie itself so
so a torn read there is already retried.

A rebind landing mid-loop can still leave the count disagreeing with the
mask, so the loop may revisit a node or skip one - but a rebind where
nodes change causes migration, so a handful of misplaced pages isn't
catastrophic in any sense.

Measured on a 72 node VM (NODES_SHIFT=10) with a cgroup v2 cpuset flipping
cpuset.mems between a word 0 and a word 1 node set, and the two word read
artificially widened: 330 zero counts in 130414 calls without the cookie,
and 401 retries with it.

Reported-by: Chelsy Ratnawat <chelsyratnawat2001@gmail.com>
Link: https://lore.kernel.org/all/20250907160829.91628-1-chelsyratnawat2001@gmail.com/
Fixes: c00b6b961099 ("mm/vmalloc: introduce alloc_pages_bulk_array_mempolicy to accelerate memory allocation")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
---
 mm/mempolicy.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 79053ece02cd..060a0eb26917 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2592,6 +2592,7 @@ static unsigned long alloc_pages_bulk_interleave(gfp_t gfp,
 		struct mempolicy *pol, unsigned long nr_pages,
 		struct page **page_array)
 {
+	unsigned int cpuset_mems_cookie;
 	int nodes;
 	unsigned long nr_pages_per_node;
 	int delta;
@@ -2599,7 +2600,16 @@ static unsigned long alloc_pages_bulk_interleave(gfp_t gfp,
 	unsigned long nr_allocated;
 	unsigned long total_allocated = 0;
 
-	nodes = nodes_weight(pol->nodes);
+	/* count the nodes, retry if a rebind happened during the read */
+	do {
+		cpuset_mems_cookie = read_mems_allowed_begin();
+		nodes = nodes_weight(pol->nodes);
+	} while (read_mems_allowed_retry(cpuset_mems_cookie));
+
+	/* if the nodemask has become invalid, we cannot do anything */
+	if (!nodes)
+		return 0;
+
 	nr_pages_per_node = nr_pages / nodes;
 	delta = nr_pages - nodes * nr_pages_per_node;
 
-- 
2.55.0