mm: oom: deduplicate victim selection code for memcg and global oom

When selecting an oom victim, we use the same heuristic for both memory
cgroup and global oom.  The only difference is the scope of tasks to
select the victim from.  So we could just export an iterator over all
memcg tasks and keep all oom related logic in oom_kill.c, but instead we
duplicate pieces of it in memcontrol.c reusing some initially private
functions of oom_kill.c in order to not duplicate all of it.  That looks
ugly and error prone, because any modification of select_bad_process
should also be propagated to mem_cgroup_out_of_memory.

Let's rework this as follows: keep all oom heuristic related code private
to oom_kill.c and make oom_kill.c use exported memcg functions when it's
really necessary (like in case of iterating over memcg tasks).

Link: http://lkml.kernel.org/r/1470056933-7505-1-git-send-email-vdavydov@virtuozzo.com
Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 148 insertions, 166 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4be518d4e68a..48747ef5b88f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -921,6 +921,43 @@ static void invalidate_reclaim_iterators(struct mem_cgroup *dead_memcg)
 	     iter = mem_cgroup_iter(NULL, iter, NULL))
 
 /**
+ * mem_cgroup_scan_tasks - iterate over tasks of a memory cgroup hierarchy
+ * @memcg: hierarchy root
+ * @fn: function to call for each task
+ * @arg: argument passed to @fn
+ *
+ * This function iterates over tasks attached to @memcg or to any of its
+ * descendants and calls @fn for each task. If @fn returns a non-zero
+ * value, the function breaks the iteration loop and returns the value.
+ * Otherwise, it will iterate over all tasks and return 0.
+ *
+ * This function must not be called for the root memory cgroup.
+ */
+int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
+			  int (*fn)(struct task_struct *, void *), void *arg)
+{
+	struct mem_cgroup *iter;
+	int ret = 0;
+
+	BUG_ON(memcg == root_mem_cgroup);
+
+	for_each_mem_cgroup_tree(iter, memcg) {
+		struct css_task_iter it;
+		struct task_struct *task;
+
+		css_task_iter_start(&iter->css, &it);
+		while (!ret && (task = css_task_iter_next(&it)))
+			ret = fn(task, arg);
+		css_task_iter_end(&it);
+		if (ret) {
+			mem_cgroup_iter_break(memcg, iter);
+			break;
+		}
+	}
+	return ret;
+}
+
+/**
  * mem_cgroup_page_lruvec - return lruvec for isolating/putting an LRU page
  * @page: the page
  * @zone: zone of the page
@@ -1178,7 +1215,7 @@ static int mem_cgroup_count_children(struct mem_cgroup *memcg)
 /*
  * Return the memory (and swap, if configured) limit for a memcg.
  */
-static unsigned long mem_cgroup_get_limit(struct mem_cgroup *memcg)
+unsigned long mem_cgroup_get_limit(struct mem_cgroup *memcg)
 {
 	unsigned long limit;
 
@@ -1205,79 +1242,12 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
 		.gfp_mask = gfp_mask,
 		.order = order,
 	};
-	struct mem_cgroup *iter;
-	unsigned long chosen_points = 0;
-	unsigned long totalpages;
-	unsigned int points = 0;
-	struct task_struct *chosen = NULL;
+	bool ret;
 
 	mutex_lock(&oom_lock);
-
-	/*
-	 * If current has a pending SIGKILL or is exiting, then automatically
-	 * select it.  The goal is to allow it to allocate so that it may
-	 * quickly exit and free its memory.
-	 */
-	if (task_will_free_mem(current)) {
-		mark_oom_victim(current);
-		wake_oom_reaper(current);
-		goto unlock;
-	}
-
-	check_panic_on_oom(&oc, CONSTRAINT_MEMCG);
-	totalpages = mem_cgroup_get_limit(memcg) ? : 1;
-	for_each_mem_cgroup_tree(iter, memcg) {
-		struct css_task_iter it;
-		struct task_struct *task;
-
-		css_task_iter_start(&iter->css, &it);
-		while ((task = css_task_iter_next(&it))) {
-			switch (oom_scan_process_thread(&oc, task)) {
-			case OOM_SCAN_SELECT:
-				if (chosen)
-					put_task_struct(chosen);
-				chosen = task;
-				chosen_points = ULONG_MAX;
-				get_task_struct(chosen);
-				/* fall through */
-			case OOM_SCAN_CONTINUE:
-				continue;
-			case OOM_SCAN_ABORT:
-				css_task_iter_end(&it);
-				mem_cgroup_iter_break(memcg, iter);
-				if (chosen)
-					put_task_struct(chosen);
-				/* Set a dummy value to return "true". */
-				chosen = (void *) 1;
-				goto unlock;
-			case OOM_SCAN_OK:
-				break;
-			};
-			points = oom_badness(task, memcg, NULL, totalpages);
-			if (!points || points < chosen_points)
-				continue;
-			/* Prefer thread group leaders for display purposes */
-			if (points == chosen_points &&
-			    thread_group_leader(chosen))
-				continue;
-
-			if (chosen)
-				put_task_struct(chosen);
-			chosen = task;
-			chosen_points = points;
-			get_task_struct(chosen);
-		}
-		css_task_iter_end(&it);
-	}
-
-	if (chosen) {
-		points = chosen_points * 1000 / totalpages;
-		oom_kill_process(&oc, chosen, points, totalpages,
-				 "Memory cgroup out of memory");
-	}
-unlock:
+	ret = out_of_memory(&oc);
 	mutex_unlock(&oom_lock);
-	return chosen;
+	return ret;
 }
 
 #if MAX_NUMNODES > 1
@@ -1600,7 +1570,7 @@ bool mem_cgroup_oom_synchronize(bool handle)
 	if (!memcg)
 		return false;
 
-	if (!handle || oom_killer_disabled)
+	if (!handle)
 		goto cleanup;
 
 	owait.memcg = memcg;
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index d53a9aa00977..ef175518f05f 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -132,6 +132,11 @@ static inline bool is_sysrq_oom(struct oom_control *oc)
 	return oc->order == -1;
 }
 
+static inline bool is_memcg_oom(struct oom_control *oc)
+{
+	return oc->memcg != NULL;
+}
+
 /* return true if the task is not adequate as candidate victim task. */
 static bool oom_unkillable_task(struct task_struct *p,
 		struct mem_cgroup *memcg, const nodemask_t *nodemask)
@@ -213,12 +218,17 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 	return points > 0 ? points : 1;
 }
 
+enum oom_constraint {
+	CONSTRAINT_NONE,
+	CONSTRAINT_CPUSET,
+	CONSTRAINT_MEMORY_POLICY,
+	CONSTRAINT_MEMCG,
+};
+
 /*
  * Determine the type of allocation constraint.
  */
-#ifdef CONFIG_NUMA
-static enum oom_constraint constrained_alloc(struct oom_control *oc,
-					     unsigned long *totalpages)
+static enum oom_constraint constrained_alloc(struct oom_control *oc)
 {
 	struct zone *zone;
 	struct zoneref *z;
@@ -226,8 +236,16 @@ static enum oom_constraint constrained_alloc(struct oom_control *oc,
 	bool cpuset_limited = false;
 	int nid;
 
+	if (is_memcg_oom(oc)) {
+		oc->totalpages = mem_cgroup_get_limit(oc->memcg) ?: 1;
+		return CONSTRAINT_MEMCG;
+	}
+
 	/* Default to all available memory */
-	*totalpages = totalram_pages + total_swap_pages;
+	oc->totalpages = totalram_pages + total_swap_pages;
+
+	if (!IS_ENABLED(CONFIG_NUMA))
+		return CONSTRAINT_NONE;
 
 	if (!oc->zonelist)
 		return CONSTRAINT_NONE;
@@ -246,9 +264,9 @@ static enum oom_constraint constrained_alloc(struct oom_control *oc,
 	 */
 	if (oc->nodemask &&
 	    !nodes_subset(node_states[N_MEMORY], *oc->nodemask)) {
-		*totalpages = total_swap_pages;
+		oc->totalpages = total_swap_pages;
 		for_each_node_mask(nid, *oc->nodemask)
-			*totalpages += node_spanned_pages(nid);
+			oc->totalpages += node_spanned_pages(nid);
 		return CONSTRAINT_MEMORY_POLICY;
 	}
 
@@ -259,27 +277,21 @@ static enum oom_constraint constrained_alloc(struct oom_control *oc,
 			cpuset_limited = true;
 
 	if (cpuset_limited) {
-		*totalpages = total_swap_pages;
+		oc->totalpages = total_swap_pages;
 		for_each_node_mask(nid, cpuset_current_mems_allowed)
-			*totalpages += node_spanned_pages(nid);
+			oc->totalpages += node_spanned_pages(nid);
 		return CONSTRAINT_CPUSET;
 	}
 	return CONSTRAINT_NONE;
 }
-#else
-static enum oom_constraint constrained_alloc(struct oom_control *oc,
-					     unsigned long *totalpages)
-{
-	*totalpages = totalram_pages + total_swap_pages;
-	return CONSTRAINT_NONE;
-}
-#endif
 
-enum oom_scan_t oom_scan_process_thread(struct oom_control *oc,
-					struct task_struct *task)
+static int oom_evaluate_task(struct task_struct *task, void *arg)
 {
+	struct oom_control *oc = arg;
+	unsigned long points;
+
 	if (oom_unkillable_task(task, NULL, oc->nodemask))
-		return OOM_SCAN_CONTINUE;
+		goto next;
 
 	/*
 	 * This task already has access to memory reserves and is being killed.
@@ -289,68 +301,67 @@ enum oom_scan_t oom_scan_process_thread(struct oom_control *oc,
 	 */
 	if (!is_sysrq_oom(oc) && atomic_read(&task->signal->oom_victims)) {
 		struct task_struct *p = find_lock_task_mm(task);
-		enum oom_scan_t ret = OOM_SCAN_ABORT;
+		bool reaped = false;
 
 		if (p) {
-			if (test_bit(MMF_OOM_REAPED, &p->mm->flags))
-				ret = OOM_SCAN_CONTINUE;
+			reaped = test_bit(MMF_OOM_REAPED, &p->mm->flags);
 			task_unlock(p);
 		}
-
-		return ret;
+		if (reaped)
+			goto next;
+		goto abort;
 	}
 
 	/*
 	 * If task is allocating a lot of memory and has been marked to be
 	 * killed first if it triggers an oom, then select it.
 	 */
-	if (oom_task_origin(task))
-		return OOM_SCAN_SELECT;
+	if (oom_task_origin(task)) {
+		points = ULONG_MAX;
+		goto select;
+	}
 
-	return OOM_SCAN_OK;
+	points = oom_badness(task, NULL, oc->nodemask, oc->totalpages);
+	if (!points || points < oc->chosen_points)
+		goto next;
+
+	/* Prefer thread group leaders for display purposes */
+	if (points == oc->chosen_points && thread_group_leader(oc->chosen))
+		goto next;
+select:
+	if (oc->chosen)
+		put_task_struct(oc->chosen);
+	get_task_struct(task);
+	oc->chosen = task;
+	oc->chosen_points = points;
+next:
+	return 0;
+abort:
+	if (oc->chosen)
+		put_task_struct(oc->chosen);
+	oc->chosen = (void *)-1UL;
+	return 1;
 }
 
 /*
- * Simple selection loop. We chose the process with the highest
- * number of 'points'.  Returns -1 on scan abort.
+ * Simple selection loop. We choose the process with the highest number of
+ * 'points'. In case scan was aborted, oc->chosen is set to -1.
  */
-static struct task_struct *select_bad_process(struct oom_control *oc,
-		unsigned int *ppoints, unsigned long totalpages)
+static void select_bad_process(struct oom_control *oc)
 {
-	struct task_struct *p;
-	struct task_struct *chosen = NULL;
-	unsigned long chosen_points = 0;
-
-	rcu_read_lock();
-	for_each_process(p) {
-		unsigned int points;
-
-		switch (oom_scan_process_thread(oc, p)) {
-		case OOM_SCAN_SELECT:
-			chosen = p;
-			chosen_points = ULONG_MAX;
-			/* fall through */
-		case OOM_SCAN_CONTINUE:
-			continue;
-		case OOM_SCAN_ABORT:
-			rcu_read_unlock();
-			return (struct task_struct *)(-1UL);
-		case OOM_SCAN_OK:
-			break;
-		};
-		points = oom_badness(p, NULL, oc->nodemask, totalpages);
-		if (!points || points < chosen_points)
-			continue;
+	if (is_memcg_oom(oc))
+		mem_cgroup_scan_tasks(oc->memcg, oom_evaluate_task, oc);
+	else {
+		struct task_struct *p;
 
-		chosen = p;
-		chosen_points = points;
+		rcu_read_lock();
+		for_each_process(p)
+			if (oom_evaluate_task(p, oc))
+				break;
+		rcu_read_unlock();
 	}
-	if (chosen)
-		get_task_struct(chosen);
-	rcu_read_unlock();
 
-	*ppoints = chosen_points * 1000 / totalpages;
-	return chosen;
+	oc->chosen_points = oc->chosen_points * 1000 / oc->totalpages;
 }
 
 /**
@@ -419,7 +430,7 @@ static void dump_header(struct oom_control *oc, struct task_struct *p)
 static atomic_t oom_victims = ATOMIC_INIT(0);
 static DECLARE_WAIT_QUEUE_HEAD(oom_victims_wait);
 
-bool oom_killer_disabled __read_mostly;
+static bool oom_killer_disabled __read_mostly;
 
 #define K(x) ((x) << (PAGE_SHIFT-10))
 
@@ -627,7 +638,7 @@ static int oom_reaper(void *unused)
 	return 0;
 }
 
-void wake_oom_reaper(struct task_struct *tsk)
+static void wake_oom_reaper(struct task_struct *tsk)
 {
 	if (!oom_reaper_th)
 		return;
@@ -656,7 +667,11 @@ static int __init oom_init(void)
 	return 0;
 }
 subsys_initcall(oom_init)
-#endif
+#else
+static inline void wake_oom_reaper(struct task_struct *tsk)
+{
+}
+#endif /* CONFIG_MMU */
 
 /**
  * mark_oom_victim - mark the given task as OOM victim
@@ -665,7 +680,7 @@ subsys_initcall(oom_init)
  * Has to be called with oom_lock held and never after
  * oom has been disabled already.
  */
-void mark_oom_victim(struct task_struct *tsk)
+static void mark_oom_victim(struct task_struct *tsk)
 {
 	WARN_ON(oom_killer_disabled);
 	/* OOM killer might race with memcg OOM */
@@ -760,7 +775,7 @@ static inline bool __task_will_free_mem(struct task_struct *task)
  * Caller has to make sure that task->mm is stable (hold task_lock or
  * it operates on the current).
  */
-bool task_will_free_mem(struct task_struct *task)
+static bool task_will_free_mem(struct task_struct *task)
 {
 	struct mm_struct *mm = task->mm;
 	struct task_struct *p;
@@ -806,14 +821,10 @@ bool task_will_free_mem(struct task_struct *task)
 	return ret;
 }
 
-/*
- * Must be called while holding a reference to p, which will be released upon
- * returning.
- */
-void oom_kill_process(struct oom_control *oc, struct task_struct *p,
-		      unsigned int points, unsigned long totalpages,
-		      const char *message)
+static void oom_kill_process(struct oom_control *oc, const char *message)
 {
+	struct task_struct *p = oc->chosen;
+	unsigned int points = oc->chosen_points;
 	struct task_struct *victim = p;
 	struct task_struct *child;
 	struct task_struct *t;
@@ -860,7 +871,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 			 * oom_badness() returns 0 if the thread is unkillable
 			 */
 			child_points = oom_badness(child,
-					oc->memcg, oc->nodemask, totalpages);
+				oc->memcg, oc->nodemask, oc->totalpages);
 			if (child_points > victim_points) {
 				put_task_struct(victim);
 				victim = child;
@@ -942,7 +953,8 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 /*
  * Determines whether the kernel must panic because of the panic_on_oom sysctl.
  */
-void check_panic_on_oom(struct oom_control *oc, enum oom_constraint constraint)
+static void check_panic_on_oom(struct oom_control *oc,
+			       enum oom_constraint constraint)
 {
 	if (likely(!sysctl_panic_on_oom))
 		return;
@@ -988,19 +1000,18 @@ EXPORT_SYMBOL_GPL(unregister_oom_notifier);
  */
 bool out_of_memory(struct oom_control *oc)
 {
-	struct task_struct *p;
-	unsigned long totalpages;
 	unsigned long freed = 0;
-	unsigned int uninitialized_var(points);
 	enum oom_constraint constraint = CONSTRAINT_NONE;
 
 	if (oom_killer_disabled)
 		return false;
 
-	blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
-	if (freed > 0)
-		/* Got some memory back in the last second. */
-		return true;
+	if (!is_memcg_oom(oc)) {
+		blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
+		if (freed > 0)
+			/* Got some memory back in the last second. */
+			return true;
+	}
 
 	/*
 	 * If current has a pending SIGKILL or is exiting, then automatically
@@ -1024,37 +1035,38 @@ bool out_of_memory(struct oom_control *oc)
 
 	/*
 	 * Check if there were limitations on the allocation (only relevant for
-	 * NUMA) that may require different handling.
+	 * NUMA and memcg) that may require different handling.
 	 */
-	constraint = constrained_alloc(oc, &totalpages);
+	constraint = constrained_alloc(oc);
 	if (constraint != CONSTRAINT_MEMORY_POLICY)
 		oc->nodemask = NULL;
 	check_panic_on_oom(oc, constraint);
 
-	if (sysctl_oom_kill_allocating_task && current->mm &&
-	    !oom_unkillable_task(current, NULL, oc->nodemask) &&
+	if (!is_memcg_oom(oc) && sysctl_oom_kill_allocating_task &&
+	    current->mm && !oom_unkillable_task(current, NULL, oc->nodemask) &&
 	    current->signal->oom_score_adj != OOM_SCORE_ADJ_MIN) {
 		get_task_struct(current);
-		oom_kill_process(oc, current, 0, totalpages,
-				 "Out of memory (oom_kill_allocating_task)");
+		oc->chosen = current;
+		oom_kill_process(oc, "Out of memory (oom_kill_allocating_task)");
 		return true;
 	}
 
-	p = select_bad_process(oc, &points, totalpages);
+	select_bad_process(oc);
 	/* Found nothing?!?! Either we hang forever, or we panic. */
-	if (!p && !is_sysrq_oom(oc)) {
+	if (!oc->chosen && !is_sysrq_oom(oc) && !is_memcg_oom(oc)) {
 		dump_header(oc, NULL);
 		panic("Out of memory and no killable processes...\n");
 	}
-	if (p && p != (void *)-1UL) {
-		oom_kill_process(oc, p, points, totalpages, "Out of memory");
+	if (oc->chosen && oc->chosen != (void *)-1UL) {
+		oom_kill_process(oc, !is_memcg_oom(oc) ? "Out of memory" :
+				 "Memory cgroup out of memory");
 		/*
 		 * Give the killed process a good chance to exit before trying
 		 * to allocate memory again.
 		 */
 		schedule_timeout_killable(1);
 	}
-	return true;
+	return !!oc->chosen;
 }
 
 /*