[PATCH 3/4] memcg: punt high overage reclaim to return-to-userland path

[Tejun Heo <tj@kernel.org>]

Currently, try_charge() tries to reclaim memory directly when the high
limit is breached; however, this has a couple issues.

* try_charge() can be invoked from any in-kernel allocation site and
  reclaim path may use considerable amount of stack.  This can lead to
  stack overflows which are extremely difficult to reproduce.

* If the allocation doesn't have __GFP_WAIT, direct reclaim is
  skipped.  If a process performs only speculative allocations, it can
  blow way past the high limit.  This is actually easily reproducible
  by simply doing "find /".  VFS tries speculative !__GFP_WAIT
  allocations first, so as long as there's memory which can be
  consumed without blocking, it can keep allocating memory regardless
  of the high limit.

This patch makes try_charge() always punt the direct reclaim to the
return-to-userland path.  If try_charge() detects that high limit is
breached, it sets current->memcg_over_high to the offending memcg and
schedules execution of mem_cgroup_handle_over_high() which performs
the direct reclaim from the return-to-userland path.

As long as kernel doesn't have a run-away allocation spree, this
should provide enough protection while making kmemcg behave more
consistently.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 include/linux/memcontrol.h |  6 +++++
 include/linux/sched.h      |  1 +
 include/linux/tracehook.h  |  3 +++
 mm/memcontrol.c            | 66 +++++++++++++++++++++++++++++++++++++---------
 4 files changed, 64 insertions(+), 12 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 3d28656..8d345a7 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -402,6 +402,8 @@ static inline int mem_cgroup_inactive_anon_is_low(struct lruvec *lruvec)
 	return inactive * inactive_ratio < active;
 }
 
+void mem_cgroup_handle_over_high(void);
+
 void mem_cgroup_print_oom_info(struct mem_cgroup *memcg,
 				struct task_struct *p);
 
@@ -621,6 +623,10 @@ static inline void mem_cgroup_end_page_stat(struct mem_cgroup *memcg)
 {
 }
 
+static inline void mem_cgroup_handle_over_high(void)
+{
+}
+
 static inline void mem_cgroup_oom_enable(void)
 {
 }
diff --git a/include/linux/sched.h b/include/linux/sched.h
index ef73b54..c76b71d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1785,6 +1785,7 @@ struct task_struct {
 #endif /* CONFIG_TRACING */
 #ifdef CONFIG_MEMCG
 	struct mem_cgroup *memcg_in_oom;
+	struct mem_cgroup *memcg_over_high; /* reclaim on returning to user */
 	gfp_t memcg_oom_gfp_mask;
 	int memcg_oom_order;
 #endif
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 84d4972..26c1521 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -50,6 +50,7 @@
 #include <linux/ptrace.h>
 #include <linux/security.h>
 #include <linux/task_work.h>
+#include <linux/memcontrol.h>
 struct linux_binprm;
 
 /*
@@ -188,6 +189,8 @@ static inline void tracehook_notify_resume(struct pt_regs *regs)
 	smp_mb__after_atomic();
 	if (unlikely(current->task_works))
 		task_work_run();
+
+	mem_cgroup_handle_over_high();
 }
 
 #endif	/* <linux/tracehook.h> */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 74abb31..c94b686 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -62,6 +62,7 @@
 #include <linux/oom.h>
 #include <linux/lockdep.h>
 #include <linux/file.h>
+#include <linux/tracehook.h>
 #include "internal.h"
 #include <net/sock.h>
 #include <net/ip.h>
@@ -1963,6 +1964,33 @@ static int memcg_cpu_hotplug_callback(struct notifier_block *nb,
 	return NOTIFY_OK;
 }
 
+/*
+ * Scheduled by try_charge() to be executed from the userland return path
+ * and reclaims memory over the high limit.
+ */
+void mem_cgroup_handle_over_high(void)
+{
+	struct mem_cgroup *memcg = current->memcg_over_high;
+
+	if (likely(!memcg))
+		return;
+
+	do {
+		unsigned long usage = page_counter_read(&memcg->memory);
+		unsigned long high = ACCESS_ONCE(memcg->high);
+
+		if (usage <= high)
+			continue;
+
+		mem_cgroup_events(memcg, MEMCG_HIGH, 1);
+		try_to_free_mem_cgroup_pages(memcg, usage - high,
+					     GFP_KERNEL, true);
+	} while ((memcg = parent_mem_cgroup(memcg)));
+
+	css_put(&current->memcg_over_high->css);
+	current->memcg_over_high = NULL;
+}
+
 static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 		      unsigned int nr_pages)
 {
@@ -2071,21 +2099,27 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	css_get_many(&memcg->css, batch);
 	if (batch > nr_pages)
 		refill_stock(memcg, batch - nr_pages);
-	if (!(gfp_mask & __GFP_WAIT))
-		goto done;
+
 	/*
-	 * If the hierarchy is above the normal consumption range,
-	 * make the charging task trim their excess contribution.
+	 * If the hierarchy is above the normal consumption range, schedule
+	 * direct reclaim on returning to userland.  We can perform direct
+	 * reclaim here if __GFP_WAIT; however, punting has the benefit of
+	 * avoiding surprise high stack usages and it's fine to breach the
+	 * high limit temporarily while control stays in kernel.
 	 */
-	do {
-		unsigned long usage = page_counter_read(&memcg->memory);
-		unsigned long high = ACCESS_ONCE(memcg->high);
+	if (!current->memcg_over_high) {
+		struct mem_cgroup *pos = memcg;
 
-		if (usage <= high)
-			continue;
-		mem_cgroup_events(memcg, MEMCG_HIGH, 1);
-		try_to_free_mem_cgroup_pages(memcg, high - usage, gfp_mask, true);
-	} while ((memcg = parent_mem_cgroup(memcg)));
+		do {
+			if (page_counter_read(&pos->memory) > pos->high) {
+				/* make user return path rescan from leaf */
+				css_get(&memcg->css);
+				current->memcg_over_high = memcg;
+				set_notify_resume(current);
+				break;
+			}
+		} while ((pos = parent_mem_cgroup(pos)));
+	}
 done:
 	return ret;
 }
@@ -5053,6 +5087,13 @@ static void mem_cgroup_move_task(struct cgroup_subsys_state *css,
 }
 #endif
 
+static void mem_cgroup_exit(struct cgroup_subsys_state *css,
+			    struct cgroup_subsys_state *old_css,
+			    struct task_struct *task)
+{
+	mem_cgroup_handle_over_high();
+}
+
 /*
  * Cgroup retains root cgroups across [un]mount cycles making it necessary
  * to verify whether we're attached to the default hierarchy on each mount
@@ -5223,6 +5264,7 @@ struct cgroup_subsys memory_cgrp_subsys = {
 	.can_attach = mem_cgroup_can_attach,
 	.cancel_attach = mem_cgroup_cancel_attach,
 	.attach = mem_cgroup_move_task,
+	.exit = mem_cgroup_exit,
 	.bind = mem_cgroup_bind,
 	.dfl_cftypes = memory_files,
 	.legacy_cftypes = mem_cgroup_legacy_files,
-- 
2.4.3

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