mm: memcontrol: take a css reference for each charged page

Charges currently pin the css indirectly by playing tricks during
css_offline(): user pages stall the offlining process until all of them
have been reparented, whereas kmemcg acquires a keep-alive reference if
outstanding kernel pages are detected at that point.

In preparation for removing all this complexity, make the pinning explicit
and acquire a css references for every charged page.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/cgroup.h

@@ -112,6 +112,19 @@ static inline void css_get(struct cgroup_subsys_state *css)
 		percpu_ref_get(&css->refcnt);
 }
 
+/**
+ * css_get_many - obtain references on the specified css
+ * @css: target css
+ * @n: number of references to get
+ *
+ * The caller must already have a reference.
+ */
+static inline void css_get_many(struct cgroup_subsys_state *css, unsigned int n)
+{
+	if (!(css->flags & CSS_NO_REF))
+		percpu_ref_get_many(&css->refcnt, n);
+}
+
 /**
  * css_tryget - try to obtain a reference on the specified css
  * @css: target css
@@ -159,6 +172,19 @@ static inline void css_put(struct cgroup_subsys_state *css)
 		percpu_ref_put(&css->refcnt);
 }
 
+/**
+ * css_put_many - put css references
+ * @css: target css
+ * @n: number of references to put
+ *
+ * Put references obtained via css_get() and css_tryget_online().
+ */
+static inline void css_put_many(struct cgroup_subsys_state *css, unsigned int n)
+{
+	if (!(css->flags & CSS_NO_REF))
+		percpu_ref_put_many(&css->refcnt, n);
+}
+
 /* bits in struct cgroup flags field */
 enum {
 	/* Control Group requires release notifications to userspace */

include/linux/percpu-refcount.h

@@ -146,6 +146,29 @@ static inline bool __ref_is_percpu(struct percpu_ref *ref,
 	return true;
 }
 
+/**
+ * percpu_ref_get_many - increment a percpu refcount
+ * @ref: percpu_ref to get
+ * @nr: number of references to get
+ *
+ * Analogous to atomic_long_add().
+ *
+ * This function is safe to call as long as @ref is between init and exit.
+ */
+static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
+{
+	unsigned long __percpu *percpu_count;
+
+	rcu_read_lock_sched();
+
+	if (__ref_is_percpu(ref, &percpu_count))
+		this_cpu_add(*percpu_count, nr);
+	else
+		atomic_long_add(nr, &ref->count);
+
+	rcu_read_unlock_sched();
+}
+
 /**
  * percpu_ref_get - increment a percpu refcount
  * @ref: percpu_ref to get
@@ -156,16 +179,7 @@ static inline bool __ref_is_percpu(struct percpu_ref *ref,
  */
 static inline void percpu_ref_get(struct percpu_ref *ref)
 {
-	unsigned long __percpu *percpu_count;
+	percpu_ref_get_many(ref, 1);
-
-	rcu_read_lock_sched();
-
-	if (__ref_is_percpu(ref, &percpu_count))
-		this_cpu_inc(*percpu_count);
-	else
-		atomic_long_inc(&ref->count);
-
-	rcu_read_unlock_sched();
 }
 
 /**
@@ -230,6 +244,30 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 	return ret;
 }
 
+/**
+ * percpu_ref_put_many - decrement a percpu refcount
+ * @ref: percpu_ref to put
+ * @nr: number of references to put
+ *
+ * Decrement the refcount, and if 0, call the release function (which was passed
+ * to percpu_ref_init())
+ *
+ * This function is safe to call as long as @ref is between init and exit.
+ */
+static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
+{
+	unsigned long __percpu *percpu_count;
+
+	rcu_read_lock_sched();
+
+	if (__ref_is_percpu(ref, &percpu_count))
+		this_cpu_sub(*percpu_count, nr);
+	else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
+		ref->release(ref);
+
+	rcu_read_unlock_sched();
+}
+
 /**
  * percpu_ref_put - decrement a percpu refcount
  * @ref: percpu_ref to put
@@ -241,16 +279,7 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
  */
 static inline void percpu_ref_put(struct percpu_ref *ref)
 {
-	unsigned long __percpu *percpu_count;
+	percpu_ref_put_many(ref, 1);
-
-	rcu_read_lock_sched();
-
-	if (__ref_is_percpu(ref, &percpu_count))
-		this_cpu_dec(*percpu_count);
-	else if (unlikely(atomic_long_dec_and_test(&ref->count)))
-		ref->release(ref);
-
-	rcu_read_unlock_sched();
 }
 
 /**

mm/memcontrol.c

@@ -2273,6 +2273,7 @@ static void drain_stock(struct memcg_stock_pcp *stock)
 		page_counter_uncharge(&old->memory, stock->nr_pages);
 		if (do_swap_account)
 			page_counter_uncharge(&old->memsw, stock->nr_pages);
+		css_put_many(&old->css, stock->nr_pages);
 		stock->nr_pages = 0;
 	}
 	stock->cached = NULL;
@@ -2530,6 +2531,7 @@ bypass:
 	return -EINTR;
 
 done_restock:
+	css_get_many(&memcg->css, batch);
 	if (batch > nr_pages)
 		refill_stock(memcg, batch - nr_pages);
 done:
@@ -2544,6 +2546,8 @@ static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
 	page_counter_uncharge(&memcg->memory, nr_pages);
 	if (do_swap_account)
 		page_counter_uncharge(&memcg->memsw, nr_pages);
+
+	css_put_many(&memcg->css, nr_pages);
 }
 
 /*
@@ -2739,6 +2743,7 @@ static int memcg_charge_kmem(struct mem_cgroup *memcg, gfp_t gfp,
 		page_counter_charge(&memcg->memory, nr_pages);
 		if (do_swap_account)
 			page_counter_charge(&memcg->memsw, nr_pages);
+		css_get_many(&memcg->css, nr_pages);
 		ret = 0;
 	} else if (ret)
 		page_counter_uncharge(&memcg->kmem, nr_pages);
@@ -2754,8 +2759,10 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg,
 		page_counter_uncharge(&memcg->memsw, nr_pages);
 
 	/* Not down to 0 */
-	if (page_counter_uncharge(&memcg->kmem, nr_pages))
+	if (page_counter_uncharge(&memcg->kmem, nr_pages)) {
+		css_put_many(&memcg->css, nr_pages);
 		return;
+	}
 
 	/*
 	 * Releases a reference taken in kmem_cgroup_css_offline in case
@@ -2767,6 +2774,8 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg,
 	 */
 	if (memcg_kmem_test_and_clear_dead(memcg))
 		css_put(&memcg->css);
+
+	css_put_many(&memcg->css, nr_pages);
 }
 
 /*
@@ -3394,10 +3403,13 @@ static int mem_cgroup_move_parent(struct page *page,
 	ret = mem_cgroup_move_account(page, nr_pages,
 				pc, child, parent);
 	if (!ret) {
+		if (!mem_cgroup_is_root(parent))
+			css_get_many(&parent->css, nr_pages);
 		/* Take charge off the local counters */
 		page_counter_cancel(&child->memory, nr_pages);
 		if (do_swap_account)
 			page_counter_cancel(&child->memsw, nr_pages);
+		css_put_many(&child->css, nr_pages);
 	}
 
 	if (nr_pages > 1)
@@ -5767,7 +5779,6 @@ static void __mem_cgroup_clear_mc(void)
 {
 	struct mem_cgroup *from = mc.from;
 	struct mem_cgroup *to = mc.to;
-	int i;
 
 	/* we must uncharge all the leftover precharges from mc.to */
 	if (mc.precharge) {
@@ -5795,8 +5806,7 @@ static void __mem_cgroup_clear_mc(void)
 		if (!mem_cgroup_is_root(mc.to))
 			page_counter_uncharge(&mc.to->memory, mc.moved_swap);
 
-		for (i = 0; i < mc.moved_swap; i++)
+		css_put_many(&mc.from->css, mc.moved_swap);
-			css_put(&mc.from->css);
 
 		/* we've already done css_get(mc.to) */
 		mc.moved_swap = 0;
@@ -6343,6 +6353,9 @@ static void uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
 	__this_cpu_add(memcg->stat->nr_page_events, nr_anon + nr_file);
 	memcg_check_events(memcg, dummy_page);
 	local_irq_restore(flags);
+
+	if (!mem_cgroup_is_root(memcg))
+		css_put_many(&memcg->css, max(nr_mem, nr_memsw));
 }
 
 static void uncharge_list(struct list_head *page_list)