[PATCH] memcg: defer page_cgroup initialization

[Glauber Costa <glommer@parallels.com>]

We have now reached the point in which there is no real need to allocate
page_cgroup upon system boot. We can defer it to the first memcg
initialization, and if it fails, we treat it like any other memcg memory
failures (like for instance, if the mem_cgroup structure itself failed).
In the future, we may want to defer this to the first non-root cgroup
initialization, but we are not there yet. With that, page_cgroup can be
more silent in its initialization.

Unfortunately, doing that for flatmem models would lead to significant
vmalloc-area waste. Since big-memory 32-bit machines are quite common,
this would be reality for most of them. This means that we will leave
FLATMEM alone, and fix only the SPARSEMEM case. We modify the message
slightly so that in future reports we know precisely if this message is
from a flatmem kernel or a older kernel initializing page_cgroup early.

Signed-off-by: Glauber Costa <glommer@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
---
 include/linux/page_cgroup.h | 13 +++++++------
 init/main.c                 |  1 -
 mm/memcontrol.c             |  2 ++
 mm/page_cgroup.c            | 19 ++++++++-----------
 4 files changed, 17 insertions(+), 18 deletions(-)

diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
index 777a524..bfb43f0 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -33,11 +33,16 @@ void __meminit pgdat_page_cgroup_init(struct pglist_data *pgdat);
 static inline void __init page_cgroup_init_flatmem(void)
 {
 }
-extern void __init page_cgroup_init(void);
+extern bool page_cgroup_init(void);
 #else
 void __init page_cgroup_init_flatmem(void);
-static inline void __init page_cgroup_init(void)
+/*
+ * If we reach here, we would have already initialized flatmem mappings.
+ * So just always succeed
+ */
+static inline bool page_cgroup_init(void)
 {
+	return 0;
 }
 #endif
 
@@ -94,10 +99,6 @@ static inline struct page_cgroup *lookup_page_cgroup(struct page *page)
 	return NULL;
 }
 
-static inline void page_cgroup_init(void)
-{
-}
-
 static inline void __init page_cgroup_init_flatmem(void)
 {
 }
diff --git a/init/main.c b/init/main.c
index cee4b5c..49aa019 100644
--- a/init/main.c
+++ b/init/main.c
@@ -592,7 +592,6 @@ asmlinkage void __init start_kernel(void)
 		initrd_start = 0;
 	}
 #endif
-	page_cgroup_init();
 	debug_objects_mem_init();
 	kmemleak_init();
 	setup_per_cpu_pageset();
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f608546..59a5b1f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6357,6 +6357,8 @@ mem_cgroup_css_alloc(struct cgroup *cont)
 		res_counter_init(&memcg->res, NULL);
 		res_counter_init(&memcg->memsw, NULL);
 		res_counter_init(&memcg->kmem, NULL);
+		if (page_cgroup_init())
+			goto free_out;
 	}
 
 	memcg->last_scanned_node = MAX_NUMNODES;
diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index 6d757e3..679de38 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -78,7 +78,8 @@ void __init page_cgroup_init_flatmem(void)
 	}
 	printk(KERN_INFO "allocated %ld bytes of page_cgroup\n", total_usage);
 	printk(KERN_INFO "please try 'cgroup_disable=memory' option if you"
-	" don't want memory cgroups\n");
+	" don't want memory cgroups. Alternatively, use SPARSEMEM mappings"
+	" to defer initialization until actual use.");
 	return;
 fail:
 	printk(KERN_CRIT "allocation of page_cgroup failed.\n");
@@ -266,13 +267,13 @@ static int __meminit page_cgroup_callback(struct notifier_block *self,
 
 #endif
 
-void __init page_cgroup_init(void)
+bool page_cgroup_init(void)
 {
 	unsigned long pfn;
 	int nid;
 
 	if (mem_cgroup_disabled())
-		return;
+		return 0;
 
 	for_each_node_state(nid, N_MEMORY) {
 		unsigned long start_pfn, end_pfn;
@@ -299,17 +300,13 @@ void __init page_cgroup_init(void)
 			if (pfn_to_nid(pfn) != nid)
 				continue;
 			if (init_section_page_cgroup(pfn, nid))
-				goto oom;
+				return 1;
 		}
 	}
+#ifdef CONFIG_MEMORY_HOTPLUG
 	hotplug_memory_notifier(page_cgroup_callback, 0);
-	printk(KERN_INFO "allocated %ld bytes of page_cgroup\n", total_usage);
-	printk(KERN_INFO "please try 'cgroup_disable=memory' option if you "
-			 "don't want memory cgroups\n");
-	return;
-oom:
-	printk(KERN_CRIT "try 'cgroup_disable=memory' boot option\n");
-	panic("Out of memory");
+#endif
+	return 0;
 }
 
 void __meminit pgdat_page_cgroup_init(struct pglist_data *pgdat)
-- 
1.8.1.4

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