A cpuset update changes the task's mems_allowed and rebinds its mempolicy in separate steps under mems_allowed_seq, but the check reads both without it. This can cause a spurious numa placement miss under some cpuset/mempolicy combinations. A reader that loads one mask before the update and the other after it can see a spurious empty intersection and drop the nodemask. For example, a task with cpuset.mems={0} and MPOL_BIND={0}. If the cpuset moves to mems={1,2}, the policy is rebound to {1}. The allocator can race and compare the old mems_allowed {0} against the new policy {1} - which will result in a dropped nodemask and a misplaced allocation on node 2. Simply retry the intersection until mems_allowed_seq is stable. This covers the task policy, which is rebound under mems_allowed_seq. VMA policies are rebound afterwards by mpol_rebind_mm(), outside mems_allowed_seq, and are not covered by this change. Fixes: cc9a6c877661 ("cpuset: mm: reduce large amounts of memory barrier related damage v3") Signed-off-by: Gregory Price (Meta) --- kernel/cgroup/cpuset.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index 3f52717c19654..8ee239d5c6b48 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -4354,10 +4354,21 @@ nodemask_t cpuset_mems_allowed(struct task_struct *tsk) * @nodemask: the nodemask to be checked * * Are any of the nodes in the nodemask allowed in current->mems_allowed? + * + * A cpuset update rebinds the task's mempolicy and changes mems_allowed in + * separate steps, so retry if an update raced with the check. */ int cpuset_nodemask_valid_mems_allowed(const nodemask_t *nodemask) { - return nodes_intersects(*nodemask, current->mems_allowed); + unsigned int seq; + int ret; + + do { + seq = read_mems_allowed_begin(); + ret = nodes_intersects(*nodemask, current->mems_allowed); + } while (read_mems_allowed_retry(seq)); + + return ret; } /* -- 2.53.0-Meta