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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 Closes: https://lore.kernel.org/lkml/CAAH8bW8C5humYnfpW3y5ypwx0E-09A3QxFE1JFzR66v+mO4XfA@mail.gmail.com/T/ Reported-by: Guenter Roeck Closes: https://lore.kernel.org/lkml/ZMHSNQfv39HN068m@yury-ThinkPad/T/#mf6431cb0b7f6f05193c41adeee444bc95bf2b1c4 Signed-off-by: Yury Norov --- This has been discovered and fixed by Yicong Yang: https://lore.kernel.org/lkml/CAAH8bW8C5humYnfpW3y5ypwx0E-09A3QxFE1JFzR66v+mO4XfA@mail.gmail.com/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);