[PATCH 2/2] oom: split out forced OOM killer

[Michal Hocko]

The forced OOM killing is currently wired into out_of_memory() call
even though their objective is different which makes the code ugly
and harder to follow. Generic out_of_memory path has to deal with
configuration settings and heuristics which are completely irrelevant
to the forced OOM killer (e.g. sysctl_oom_kill_allocating_task or
OOM killer prevention for already dying tasks). All of them are
either relying on explicit force_kill check or indirectly by checking
current->mm which is always NULL for sysrq+f. This is not nice, hard
to follow and error prone.

Let's pull forced OOM killer code out into a separate function
(force_out_of_memory) which is really trivial now.
As a bonus we can clearly state that this is a forced OOM killer
in the OOM message which is helpful to distinguish it from the
regular OOM killer.

Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 drivers/tty/sysrq.c |  3 +--
 include/linux/oom.h |  3 ++-
 mm/oom_kill.c       | 57 ++++++++++++++++++++++++++++++++---------------------
 mm/page_alloc.c     |  2 +-
 4 files changed, 39 insertions(+), 26 deletions(-)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 3a42b7187b8e..06a95a8ed701 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -356,8 +356,7 @@ static struct sysrq_key_op sysrq_term_op = {
 static void moom_callback(struct work_struct *ignored)
 {
 	mutex_lock(&oom_lock);
-	if (!out_of_memory(node_zonelist(first_memory_node, GFP_KERNEL),
-			   GFP_KERNEL, 0, NULL, true))
+	if (!force_out_of_memory())
 		pr_info("OOM request ignored because killer is disabled\n");
 	mutex_unlock(&oom_lock);
 }
diff --git a/include/linux/oom.h b/include/linux/oom.h
index 7deecb7bca5e..061e0ffd3493 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -70,8 +70,9 @@ extern enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
 		unsigned long totalpages, const nodemask_t *nodemask,
 		bool force_kill);
 
+extern bool force_out_of_memory(void);
 extern bool out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
-		int order, nodemask_t *mask, bool force_kill);
+		int order, nodemask_t *mask);
 
 extern void exit_oom_victim(void);
 
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 0c312eaac834..050936f35944 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -635,12 +635,38 @@ int unregister_oom_notifier(struct notifier_block *nb)
 EXPORT_SYMBOL_GPL(unregister_oom_notifier);
 
 /**
- * __out_of_memory - kill the "best" process when we run out of memory
+ * force_out_of_memory - forces OOM killer
+ *
+ * External trigger for the OOM killer. The system doesn't have to be under
+ * OOM condition (e.g. sysrq+f).
+ */
+bool force_out_of_memory(void)
+{
+	struct zonelist *zonelist = node_zonelist(first_memory_node, GFP_KERNEL);
+	struct task_struct *p;
+	unsigned long totalpages;
+	unsigned int points;
+
+	if (oom_killer_disabled)
+		return false;
+
+	constrained_alloc(zonelist, GFP_KERNEL, NULL, &totalpages);
+	p = select_bad_process(&points, totalpages, NULL, true);
+	if (p != (void *)-1UL)
+		oom_kill_process(p, GFP_KERNEL, 0, points, totalpages, NULL,
+				 NULL, "Forced out of memory killer");
+	else
+		pr_warn("Forced out of memory. No killable task found...\n");
+
+	return true;
+}
+
+/**
+ * out_of_memory - kill the "best" process when we run out of memory
  * @zonelist: zonelist pointer
  * @gfp_mask: memory allocation flags
  * @order: amount of memory being requested as a power of 2
  * @nodemask: nodemask passed to page allocator
- * @force_kill: true if a task must be killed, even if others are exiting
  *
  * If we run out of memory, we have the choice between either
  * killing a random task (bad), letting the system crash (worse)
@@ -648,7 +674,7 @@ EXPORT_SYMBOL_GPL(unregister_oom_notifier);
  * don't have to be perfect here, we just have to be good.
  */
 bool out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
-		   int order, nodemask_t *nodemask, bool force_kill)
+		   int order, nodemask_t *nodemask)
 {
 	const nodemask_t *mpol_mask;
 	struct task_struct *p;
@@ -687,14 +713,8 @@ bool out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
 	constraint = constrained_alloc(zonelist, gfp_mask, nodemask,
 						&totalpages);
 	mpol_mask = (constraint == CONSTRAINT_MEMORY_POLICY) ? nodemask : NULL;
-	/* Ignore panic_on_oom when the OOM killer is sysrq triggered */
-	if (!force_kill)
-		check_panic_on_oom(constraint, gfp_mask, order, mpol_mask, NULL);
+	check_panic_on_oom(constraint, gfp_mask, order, mpol_mask, NULL);
 
-	/*
-	 * not affecting force_kill because sysrq triggered OOM killer runs from
-	 * the workqueue context so current->mm will be NULL
-	 */
 	if (sysctl_oom_kill_allocating_task && current->mm &&
 	    !oom_unkillable_task(current, NULL, nodemask) &&
 	    current->signal->oom_score_adj != OOM_SCORE_ADJ_MIN) {
@@ -705,18 +725,11 @@ bool out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
 		goto out;
 	}
 
-	p = select_bad_process(&points, totalpages, mpol_mask, force_kill);
-	/*
-	 * Found nothing?!?! Either we hang forever, or we panic.
-	 * Do not panic when the OOM killer is sysrq triggered.
-	 */
+	p = select_bad_process(&points, totalpages, mpol_mask, false);
+	/* Found nothing?!?! Either we hang forever, or we panic. */
 	if (!p) {
-		if (!force_kill) {
-			dump_header(NULL, gfp_mask, order, NULL, mpol_mask);
-			panic("Out of memory and no killable processes...\n");
-		} else {
-			pr_info("Forced out of memory. No killable task found...\n");
-		}
+		dump_header(NULL, gfp_mask, order, NULL, mpol_mask);
+		panic("Out of memory and no killable processes...\n");
 	}
 	if (p != (void *)-1UL) {
 		oom_kill_process(p, gfp_mask, order, points, totalpages, NULL,
@@ -747,7 +760,7 @@ void pagefault_out_of_memory(void)
 	if (!mutex_trylock(&oom_lock))
 		return;
 
-	if (!out_of_memory(NULL, 0, 0, NULL, false)) {
+	if (!out_of_memory(NULL, 0, 0, NULL)) {
 		/*
 		 * There shouldn't be any user tasks runnable while the
 		 * OOM killer is disabled, so the current task has to
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1f9ffbb087cb..014806d13138 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2731,7 +2731,7 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
 			goto out;
 	}
 	/* Exhausted what can be done so it's blamo time */
-	if (out_of_memory(ac->zonelist, gfp_mask, order, ac->nodemask, false)
+	if (out_of_memory(ac->zonelist, gfp_mask, order, ac->nodemask)
 			|| WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL))
 		*did_some_progress = 1;
 out:
-- 
2.1.4

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