[PATCH 3/4] sched: fix sched_numa_find_nth_cpu() in CPU-less case

[Yury Norov]

When the node provided by user is CPU-less, corresponding record in
sched_domains_numa_masks is not set. Trying to dereference it in the
following code leads to kernel crash.

To avoid it, start searching from the nearest node with CPUs.

Fixes: cd7f55359c90 ("sched: add sched_numa_find_nth_cpu()")
Reported-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/CAAH8bW8C5humYnfpW3y5ypwx0E-09A3QxFE1JFzR66v+mO4XfA@xxxxxxxxxxxxxx/T/
Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/ZMHSNQfv39HN068m@yury-ThinkPad/T/#mf6431cb0b7f6f05193c41adeee444bc95bf2b1c4
Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
---

This has been discovered and fixed by Yicong Yang:

https://lore.kernel.org/lkml/CAAH8bW8C5humYnfpW3y5ypwx0E-09A3QxFE1JFzR66v+mO4XfA@xxxxxxxxxxxxxx/T/

When discovering Guenter's failure report for sparc64, I found it's due to
the same problem. And while fixing, I found an opportunity to generalize
nearest NUMA node search and avoid code duplication.

Yicong, if you like this approach, please feel free to add your co-developed-by
or any appropriate tags.

 kernel/sched/topology.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index d3a3b2646ec4..66b387172b6f 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2113,10 +2113,14 @@ static int hop_cmp(const void *a, const void *b)
  */
 int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
 {
-	struct __cmp_key k = { .cpus = cpus, .node = node, .cpu = cpu };
+	struct __cmp_key k = { .cpus = cpus, .cpu = cpu };
 	struct cpumask ***hop_masks;
 	int hop, ret = nr_cpu_ids;
 
+	/* CPU-less node entries are uninitialized in sched_domains_numa_masks */
+	node = numa_nearest_node(node, N_CPU);
+	k.node = node;
+
 	rcu_read_lock();
 
 	k.masks = rcu_dereference(sched_domains_numa_masks);
-- 
2.39.2