[PATCH] mm/mempolicy: ignore a bind policy whose nodes have no memory

[Gregory Price (Meta)]

apply_policy_zone() assumes that policy->nodes intersects N_MEMORY, so
it reads a bind policy with no N_HIGH_MEMORY node as movable-only.

On systems with CONFIG_CPUSETS=n, nothing rebinds policies when memory
is removed, so offlining all memory on the nodes a bind policy names
leaves it with none. The policy is then treated as movable-only, and
movable allocations are restricted to nodes without memory.

This results in allocations failures, a VM_FAULT_OOM leak, and then
an infinitely retrying fault that hangs the process.

  page allocation failure: order:0, mode:0x140dca ... nodemask=2
  Huh VM_FAULT_OOM leaked out to the #PF handler. Retrying PF

Ignore a bind policy whose nodes have no memory. policy_nodemask()
already ignores a bind policy that does not intersect the cpuset.
This covers hugetlb's policy_mbind_nodemask() as well.

If the nodes regain memory, the policy applies again.

CPUSETS=n with HOTREMOVE=y is unlikely a common configuration, so this
isn't marked for stable.

Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
---
 mm/mempolicy.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index ca7e744727b1..5dc2d7ef1544 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2096,6 +2096,10 @@ bool apply_policy_zone(struct mempolicy *policy, enum zone_type zone)
 
 	BUG_ON(dynamic_policy_zone == ZONE_MOVABLE);
 
+	/* Memory hot-remove can leave policy->nodes without memory. */
+	if (!nodes_intersects(policy->nodes, node_states[N_MEMORY]))
+		return false;
+
 	/*
 	 * if policy->nodes has movable memory only,
 	 * we apply policy when gfp_zone(gfp) = ZONE_MOVABLE only.
-- 
2.56.0