[RFC PATCH v4 17/27] mm/oom: NP_OPS_OOM_ELIGIBLE - private node OOM participation

[Gregory Price <gourry@gourry.net>]

The OOM killer must know whether killing a task can actually free
memory such that pressure is reduced.

A private node only contributes to relieving pressure if it participates
in both reclaim and demotion. Without this check, the check, the OOM
killer may select an undeserving victim.

Introduce NP_OPS_OOM_ELIGIBLE and helpers node_oom_eligible() and
zone_oom_eligible().

Replace cpuset_mems_allowed_intersects() in oom_cpuset_eligible()
with oom_mems_intersect() that iterates N_MEMORY nodes and skips
ineligible private nodes.

Update constrained_alloc() to use zone_oom_eligible() for constraint
detection and node_oom_eligible() to exclude ineligible nodes from
totalpages accounting.

Remove cpuset_mems_allowed_intersects() as it has no remaining callers.

Signed-off-by: Gregory Price <gourry@gourry.net>
---
 include/linux/cpuset.h       |  9 -------
 include/linux/node_private.h |  3 +++
 kernel/cgroup/cpuset.c       | 17 ------------
 mm/oom_kill.c                | 52 ++++++++++++++++++++++++++++++++----
 4 files changed, 50 insertions(+), 31 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 7b2f3f6b68a9..53ccfb00b277 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -97,9 +97,6 @@ static inline bool cpuset_zone_allowed(struct zone *z, gfp_t gfp_mask)
 	return true;
 }
 
-extern int cpuset_mems_allowed_intersects(const struct task_struct *tsk1,
-					  const struct task_struct *tsk2);
-
 #ifdef CONFIG_CPUSETS_V1
 #define cpuset_memory_pressure_bump() 				\
 	do {							\
@@ -241,12 +238,6 @@ static inline bool cpuset_zone_allowed(struct zone *z, gfp_t gfp_mask)
 	return true;
 }
 
-static inline int cpuset_mems_allowed_intersects(const struct task_struct *tsk1,
-						 const struct task_struct *tsk2)
-{
-	return 1;
-}
-
 static inline void cpuset_memory_pressure_bump(void) {}
 
 static inline void cpuset_task_status_allowed(struct seq_file *m,
diff --git a/include/linux/node_private.h b/include/linux/node_private.h
index 34be52383255..34d862f09e24 100644
--- a/include/linux/node_private.h
+++ b/include/linux/node_private.h
@@ -141,6 +141,9 @@ struct node_private_ops {
 /* Kernel reclaim (kswapd, direct reclaim, OOM) operates on this node */
 #define NP_OPS_RECLAIM			BIT(4)
 
+/* Private node is OOM-eligible: reclaim can run and pages can be demoted here */
+#define NP_OPS_OOM_ELIGIBLE		(NP_OPS_RECLAIM | NP_OPS_DEMOTION)
+
 /**
  * struct node_private - Per-node container for N_MEMORY_PRIVATE nodes
  *
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 1a597f0c7c6c..29789d544fd5 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4530,23 +4530,6 @@ int cpuset_mem_spread_node(void)
 	return cpuset_spread_node(&current->cpuset_mem_spread_rotor);
 }
 
-/**
- * cpuset_mems_allowed_intersects - Does @tsk1's mems_allowed intersect @tsk2's?
- * @tsk1: pointer to task_struct of some task.
- * @tsk2: pointer to task_struct of some other task.
- *
- * Description: Return true if @tsk1's mems_allowed intersects the
- * mems_allowed of @tsk2.  Used by the OOM killer to determine if
- * one of the task's memory usage might impact the memory available
- * to the other.
- **/
-
-int cpuset_mems_allowed_intersects(const struct task_struct *tsk1,
-				   const struct task_struct *tsk2)
-{
-	return nodes_intersects(tsk1->mems_allowed, tsk2->mems_allowed);
-}
-
 /**
  * cpuset_print_current_mems_allowed - prints current's cpuset and mems_allowed
  *
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 5eb11fbba704..cd0d65ccd1e8 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -74,7 +74,45 @@ static inline bool is_memcg_oom(struct oom_control *oc)
 	return oc->memcg != NULL;
 }
 
+/* Private nodes are only eligible if they support both reclaim and demotion */
+static inline bool node_oom_eligible(int nid)
+{
+	if (!node_state(nid, N_MEMORY_PRIVATE))
+		return true;
+	return (node_private_flags(nid) & NP_OPS_OOM_ELIGIBLE) ==
+		NP_OPS_OOM_ELIGIBLE;
+}
+
+static inline bool zone_oom_eligible(struct zone *zone, gfp_t gfp_mask)
+{
+	if (!node_oom_eligible(zone_to_nid(zone)))
+		return false;
+	return cpuset_zone_allowed(zone, gfp_mask);
+}
+
 #ifdef CONFIG_NUMA
+/*
+ * Killing a task can only relieve system pressure if freed memory can be
+ * demoted there and reclaim can operate on the node's pages, so we
+ * omit private nodes that aren't eligible.
+ */
+static bool oom_mems_intersect(const struct task_struct *tsk1,
+			       const struct task_struct *tsk2)
+{
+	int nid;
+
+	for_each_node_state(nid, N_MEMORY) {
+		if (!node_isset(nid, tsk1->mems_allowed))
+			continue;
+		if (!node_isset(nid, tsk2->mems_allowed))
+			continue;
+		if (!node_oom_eligible(nid))
+			continue;
+		return true;
+	}
+	return false;
+}
+
 /**
  * oom_cpuset_eligible() - check task eligibility for kill
  * @start: task struct of which task to consider
@@ -107,9 +145,10 @@ static bool oom_cpuset_eligible(struct task_struct *start,
 		} else {
 			/*
 			 * This is not a mempolicy constrained oom, so only
-			 * check the mems of tsk's cpuset.
+			 * check the mems of tsk's cpuset, excluding private
+			 * nodes that do not participate in kernel reclaim.
 			 */
-			ret = cpuset_mems_allowed_intersects(current, tsk);
+			ret = oom_mems_intersect(current, tsk);
 		}
 		if (ret)
 			break;
@@ -291,16 +330,19 @@ static enum oom_constraint constrained_alloc(struct oom_control *oc)
 		return CONSTRAINT_MEMORY_POLICY;
 	}
 
-	/* Check this allocation failure is caused by cpuset's wall function */
+	/* Check this allocation failure is caused by cpuset or private node constraints */
 	for_each_zone_zonelist_nodemask(zone, z, oc->zonelist,
 			highest_zoneidx, oc->nodemask)
-		if (!cpuset_zone_allowed(zone, oc->gfp_mask))
+		if (!zone_oom_eligible(zone, oc->gfp_mask))
 			cpuset_limited = true;
 
 	if (cpuset_limited) {
 		oc->totalpages = total_swap_pages;
-		for_each_node_mask(nid, cpuset_current_mems_allowed)
+		for_each_node_mask(nid, cpuset_current_mems_allowed) {
+			if (!node_oom_eligible(nid))
+				continue;
 			oc->totalpages += node_present_pages(nid);
+		}
 		return CONSTRAINT_CPUSET;
 	}
 	return CONSTRAINT_NONE;
-- 
2.53.0