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mem-hotplug: fix potential race while building zonelist for new populated zone

PATCH In review Haicheng Li 3 participants 7 messages May 17, 2010 – May 19, 2010

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Haicheng Li

author
#0 lore raw

PATCH 3/3 mem-hotplug: fix potential race while building zonelist for new populated zone

Make "zone->present_pages gets increased" and "building zonelist" an atomic operation to prevent possible race.

It is merely a theoretical race: after new zone gets populated, its pages might be allocated by others before itself building zonelist.

Besides, atomic operation ensures alloc_percpu() will never fail since there is a new fresh memory block added.

Signed-off-by Haicheng Li <[email protected]>
Signed-off-by Wu Fengguang <[email protected]>
Reviewed-by Andi Kleen <[email protected]>
---
 include/linux/memory_hotplug.h |    8 ++++++++
 mm/memory_hotplug.c            |   15 +++++++++------
 mm/page_alloc.c                |   20 ++++++++++++++++----
 3 files changed, 33 insertions(+), 10 deletions(-)
include/linux/memory_hotplug.h +8 -0
index 35b07b7..42b1416 100644
@@ -157,6 +157,14 @@ extern void register_page_bootmem_info_node(struct pglist_data *pgdat);
 extern void put_page_bootmem(struct page *page);
 #endif
 
+/* online_pages() will pass such zone info to build_all_zonelists()
+ * when it needs to initialize a new zone.
+ */
+struct zone_online_info {
+	struct zone *zone;
+	unsigned long onlined_pages;
+};
+
 #else /* ! CONFIG_MEMORY_HOTPLUG */
 /*
  * Stub functions for when hotplug is off
mm/memory_hotplug.c +9 -6
index b564b6a..06738b2 100644
@@ -389,6 +389,7 @@ int online_pages(unsigned long pfn, unsigned long nr_pages)
 	int nid;
 	int ret;
 	struct memory_notify arg;
+	struct zone_online_info zone_online_info;
 	/*
 	 * mutex to protect zone->pageset when it's still shared
 	 * in onlined_pages()

@@ -434,13 +435,15 @@ int online_pages(unsigned long pfn, unsigned long nr_pages)
 		return ret;
 	}
 
-	zone->present_pages += onlined_pages;
-	zone->zone_pgdat->node_present_pages += onlined_pages;
-	if (need_zonelists_rebuild)
-		build_all_zonelists(zone);
-	else
+	if (need_zonelists_rebuild) {
+		zone_online_info.zone = zone;
+		zone_online_info.onlined_pages = onlined_pages;
+		build_all_zonelists(&zone_online_info);
+	} else {
+		zone->present_pages += onlined_pages;
+		zone->zone_pgdat->node_present_pages += onlined_pages;
 		zone_pcp_update(zone);
-
+	}
 	mutex_unlock(&zone_pageset_mutex);
 	setup_per_zone_wmarks();
 	calculate_zone_inactive_ratio(zone);
mm/page_alloc.c +16 -4
index 72c1211..0729a82 100644
@@ -2783,6 +2783,20 @@ static __init_refok int __build_all_zonelists(void *data)
 {
 	int nid;
 	int cpu;
+#ifdef CONFIG_MEMORY_HOTPLUG
+	struct zone_online_info *new = (struct zone_online_info *)data;
+
+	/*
+	 * Populate the new zone before build zonelists, which could
+	 * happen only when onlining a new node after system is booted.
+	 */
+	if (new) {
+		/* We are expecting a new memory block here. */
+		WARN_ON(!new->onlined_pages);
+		new->zone->present_pages += new->onlined_pages;
+		new->zone->zone_pgdat->node_present_pages += new->onlined_pages;
+	}
+#endif
 
 #ifdef CONFIG_NUMA
 	memset(node_load, 0, sizeof(node_load));

@@ -2796,10 +2810,8 @@ static __init_refok int __build_all_zonelists(void *data)
 
 #ifdef CONFIG_MEMORY_HOTPLUG
 	/* Setup real pagesets for the new zone */
-	if (data) {
-		struct zone *zone = data;
-		setup_zone_pageset(zone);
-	}
+	if (new)
+		setup_zone_pageset(new->zone);
 #endif
 
 	/*
Signature · 8 lines
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#1 lore raw
Quoted 14 lines · Haicheng Li
mm/page_alloc.c +5 -0
index 72c1211..0729a82 100644
@@ -2783,6 +2783,20 @@ static __init_refok int __build_all_zonelists(void
 *data)
 {
 	int nid;
 	int cpu;
+#ifdef CONFIG_MEMORY_HOTPLUG
+	struct zone_online_info *new = (struct zone_online_info *)data;
+
+	/*
+	 * Populate the new zone before build zonelists, which could

mm/page_alloc.c static __init_refok int __build_all_zonelists(void · line 2801

Quoted 10 lines
mm/page_alloc.c +9 -0
+	 * happen only when onlining a new node after system is booted.
+	 */
+	if (new) {
+		/* We are expecting a new memory block here. */
+		WARN_ON(!new->onlined_pages);
+		new->zone->present_pages += new->onlined_pages;
+		new->zone->zone_pgdat->node_present_pages +=
 new->onlined_pages;
+	}
+#endif

Building a zonelist now has the potential side effect of changes to the size of the zone?

Can we have a global mutex that protects against size modification of zonelists instead? And it could also serialize the pageset setup?

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#2 lore raw
Quoted 22 lines · Christoph Lameter
Quoted 20 lines · Haicheng Li
mm/page_alloc.c +11 -0
index 72c1211..0729a82 100644
@@ -2783,6 +2783,20 @@ static __init_refok int __build_all_zonelists(void
 *data)
 {
 	int nid;
 	int cpu;
+#ifdef CONFIG_MEMORY_HOTPLUG
+	struct zone_online_info *new = (struct zone_online_info *)data;
+
+	/*
+	 * Populate the new zone before build zonelists, which could
+	 * happen only when onlining a new node after system is booted.
+	 */
+	if (new) {
+		/* We are expecting a new memory block here. */
+		WARN_ON(!new->onlined_pages);
+		new->zone->present_pages += new->onlined_pages;
Quoted 8 lines

mm/page_alloc.c static __init_refok int __build_all_zonelists(void *data) · line 2799

Quoted 4 lines
mm/page_alloc.c +3 -0
+		new->zone->zone_pgdat->node_present_pages +=
 new->onlined_pages;
+	}
+#endif

Building a zonelist now has the potential side effect of changes to the size of the zone?

Yeah, this sounds a bit hacky.

Quoted 2 lines

Can we have a global mutex that protects against size modification of zonelists instead? And it could also serialize the pageset setup?

Good suggestion. We could make zone_pageset_mutex a global mutex and take it in all the functions that call build_all_zonelists() -- currently only online_pages() and numa_zonelist_order_handler().

This can equally fix the possible race:

CPU0 CPU1 CPU2
(1) zone->present_pages += online_pages;
(2) build_all_zonelists();
(3) alloc_page();
(4) free_page();
(5) build_all_zonelists();
(6) __build_all_zonelists();
(7) zone->pageset = alloc_percpu();

In step (3,4), zone->pageset still points to boot_pageset, so bad things may happen if 2+ nodes are in this state. Even if only 1 node is accessing the boot_pageset, (3) may still consume too much memory to fail the memory allocations in step (7).

Thanks,
Fengguang

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author
#3 lore raw

PATCH RESEND 3/3 mem-hotplug: fix potential race while building zonelist for new populated zone

Quoted 4 lines · Wu Fengguang
Quoted 2 lines · Christoph Lameter

Building a zonelist now has the potential side effect of changes to the size of the zone?

Quoted 8 lines

Yeah, this sounds a bit hacky.

Quoted 2 lines

Can we have a global mutex that protects against size modification of zonelists instead? And it could also serialize the pageset setup?

Good suggestion. We could make zone_pageset_mutex a global mutex and take it in all the functions that call build_all_zonelists() -- currently only online_pages() and numa_zonelist_order_handler().

Yes, if we don't mind adding a global mutex, it's another way to fix the race.

Quoted 11 lines

This can equally fix the possible race:

CPU0 CPU1 CPU2
(1) zone->present_pages += online_pages;
(2) build_all_zonelists();
(3) alloc_page();
(4) free_page();
(5) build_all_zonelists();
(6) __build_all_zonelists();
(7) zone->pageset = alloc_percpu();

Quoted 4 lines

In step (3,4), zone->pageset still points to boot_pageset, so bad things may happen if 2+ nodes are in this state. Even if only 1 node is accessing the boot_pageset, (3) may still consume too much memory to fail the memory allocations in step (7).

Fengguang, this is a nice description of the possible race, I'd include it into the changelog of this patch.

So how about the revised patch below?

---
From 0b1e776dc6c135b1d0c9a0bfb35daa9e9cfb046d Mon Sep 17 00:00:00 2001
From: Haicheng Li <[email protected]>
Date: Tue, 18 May 2010 15:53:28 +0800
Subject: [PATCH] mem-hotplug: fix potential setup_zone_pageset race

Add global mutex zonelists_pageset_mutex to fix the possible race:

CPU0 CPU1 CPU2
(1) zone->present_pages += online_pages;
(2) build_all_zonelists();
(3) alloc_page();
(4) free_page();
(5) build_all_zonelists();
(6) __build_all_zonelists();
(7) zone->pageset = alloc_percpu();

In step (3,4), zone->pageset still points to boot_pageset, so bad things may happen if 2+ nodes are in this state. Even if only 1 node is accessing the boot_pageset, (3) may still consume too much memory to fail the memory allocations in step (7).

Signed-off-by Haicheng Li <[email protected]>
Signed-off-by Wu Fengguang <[email protected]>
Reviewed-by Andi Kleen <[email protected]>
---
 include/linux/mmzone.h |    1 +
 mm/memory_hotplug.c    |   11 +++--------
 mm/page_alloc.c        |   15 ++++++++++++++-
 3 files changed, 18 insertions(+), 9 deletions(-)
include/linux/mmzone.h +1 -0
index dbbcd50..5103747 100644
@@ -641,6 +641,7 @@ typedef struct pglist_data {
 
 #include <linux/memory_hotplug.h>
 
+extern struct mutex zonelists_pageset_mutex;
 void get_zone_counts(unsigned long *active, unsigned long *inactive,
 			unsigned long *free);
 void build_all_zonelists(void *data);
mm/memory_hotplug.c +3 -8
index b564b6a..b32f3eb 100644
@@ -389,11 +389,6 @@ int online_pages(unsigned long pfn, unsigned long nr_pages)
 	int nid;
 	int ret;
 	struct memory_notify arg;
-	/*
-	 * mutex to protect zone->pageset when it's still shared
-	 * in onlined_pages()
-	 */
-	static DEFINE_MUTEX(zone_pageset_mutex);
 
 	arg.start_pfn = pfn;
 	arg.nr_pages = nr_pages;

@@ -420,14 +415,14 @@ int online_pages(unsigned long pfn, unsigned long nr_pages)
 	 * This means the page allocator ignores this zone.
 	 * So, zonelist must be updated after online.
 	 */
-	mutex_lock(&zone_pageset_mutex);
+	mutex_lock(&zonelists_pageset_mutex);
 	if (!populated_zone(zone))
 		need_zonelists_rebuild = 1;
 
 	ret = walk_system_ram_range(pfn, nr_pages, &onlined_pages,
 		online_pages_range);
 	if (ret) {
-		mutex_unlock(&zone_pageset_mutex);
+		mutex_unlock(&zonelists_pageset_mutex);
 		printk(KERN_DEBUG "online_pages %lx at %lx failed\n",
 			nr_pages, pfn);
 		memory_notify(MEM_CANCEL_ONLINE, &arg);

@@ -441,7 +436,7 @@ int online_pages(unsigned long pfn, unsigned long nr_pages)
 	else
 		zone_pcp_update(zone);
 
-	mutex_unlock(&zone_pageset_mutex);
+	mutex_unlock(&zonelists_pageset_mutex);
 	setup_per_zone_wmarks();
 	calculate_zone_inactive_ratio(zone);
 	if (onlined_pages) {
mm/page_alloc.c +14 -1
index 72c1211..602c4b7 100644
@@ -2434,8 +2434,11 @@ int numa_zonelist_order_handler(ctl_table *table, int write,
 			strncpy((char*)table->data, saved_string,
 				NUMA_ZONELIST_ORDER_LEN);
 			user_zonelist_order = oldval;
-		} else if (oldval != user_zonelist_order)
+		} else if (oldval != user_zonelist_order) {
+			mutex_lock(&zonelists_pageset_mutex);
 			build_all_zonelists(NULL);
+			mutex_unlock(&zonelists_pageset_mutex);
+		}
 	}
 out:
 	mutex_unlock(&zl_order_mutex);

@@ -2778,6 +2781,12 @@ static void setup_pageset(struct per_cpu_pageset *p, unsigned long batch);
 static DEFINE_PER_CPU(struct per_cpu_pageset, boot_pageset);
 static void setup_zone_pageset(struct zone *zone);
 
+/*
+ * Global mutex to protect size modification of zonelists
+ * as well as to serialize pageset setup for a new populated zone.
+ */
+DEFINE_MUTEX(zonelists_pageset_mutex);
+
 /* return values int ....just for stop_machine() */
 static __init_refok int __build_all_zonelists(void *data)
 {

@@ -2821,6 +2830,10 @@ static __init_refok int __build_all_zonelists(void *data)
 	return 0;
 }
 
+/*
+ * Called with zonelists_pageset_mutex held always
+ * unless system_state == SYSTEM_BOOTING.
+ */
 void build_all_zonelists(void *data)
 {
 	set_zonelist_order();
Signature · 8 lines
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#4 lore raw
Quoted 1 line · Haicheng Li

+extern struct mutex zonelists_pageset_mutex;

The mutext is used for multiple serializations having to do with zones. "pageset" suggests its only for pagesets.

So

zones_mutex?

or

zonelists_mutex?

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author
#5 lore raw

PATCH RESEND 3/3 mem-hotplug: fix potential race while building zonelist for new populated zone

Quoted 6 lines · Christoph Lameter
Quoted 1 line · Haicheng Li

+extern struct mutex zonelists_pageset_mutex;

The mutext is used for multiple serializations having to do with zones. "pageset" suggests its only for pagesets.

hmm yes, "pageset" sounds a little bit confusing.

Quoted 7 lines

So

zones_mutex?

or

zonelists_mutex?

I prefer zonelists_mutex.

Christoph, how about below patch? Thanks.

---
From 3ccfd04d6ae9127bf0f5472db0b266e7b3f158bd Mon Sep 17 00:00:00 2001
From: Haicheng Li <[email protected]>
Date: Wed, 19 May 2010 11:07:21 +0800
Subject: [PATCH] mem-hotplug: fix potential race while building zonelist for new populated zone

Add global mutex zonelists_mutex to fix the possible race:

CPU0 CPU1 CPU2
(1) zone->present_pages += online_pages;
(2) build_all_zonelists();
(3) alloc_page();
(4) free_page();
(5) build_all_zonelists();
(6) __build_all_zonelists();
(7) zone->pageset = alloc_percpu();

In step (3,4), zone->pageset still points to boot_pageset, so bad things may happen if 2+ nodes are in this state. Even if only 1 node is accessing the boot_pageset, (3) may still consume too much memory to fail the memory allocations in step (7).

Besides, atomic operation ensures alloc_percpu() in step (7) will never fail since there is a new fresh memory block added in step(6).

Signed-off-by Haicheng Li <[email protected]>
Signed-off-by Wu Fengguang <[email protected]>
Reviewed-by Andi Kleen <[email protected]>
---
 include/linux/mmzone.h |    1 +
 mm/memory_hotplug.c    |   11 +++--------
 mm/page_alloc.c        |   15 ++++++++++++++-
 3 files changed, 18 insertions(+), 9 deletions(-)
include/linux/mmzone.h +1 -0
index dbbcd50..4d87baa 100644
@@ -641,6 +641,7 @@ typedef struct pglist_data {
 
 #include <linux/memory_hotplug.h>
 
+extern struct mutex zonelists_mutex;
 void get_zone_counts(unsigned long *active, unsigned long *inactive,
 			unsigned long *free);
 void build_all_zonelists(void *data);
mm/memory_hotplug.c +3 -8
index b564b6a..bc4a942 100644
@@ -389,11 +389,6 @@ int online_pages(unsigned long pfn, unsigned long nr_pages)
 	int nid;
 	int ret;
 	struct memory_notify arg;
-	/*
-	 * mutex to protect zone->pageset when it's still shared
-	 * in onlined_pages()
-	 */
-	static DEFINE_MUTEX(zone_pageset_mutex);
 
 	arg.start_pfn = pfn;
 	arg.nr_pages = nr_pages;

@@ -420,14 +415,14 @@ int online_pages(unsigned long pfn, unsigned long nr_pages)
 	 * This means the page allocator ignores this zone.
 	 * So, zonelist must be updated after online.
 	 */
-	mutex_lock(&zone_pageset_mutex);
+	mutex_lock(&zonelists_mutex);
 	if (!populated_zone(zone))
 		need_zonelists_rebuild = 1;
 
 	ret = walk_system_ram_range(pfn, nr_pages, &onlined_pages,
 		online_pages_range);
 	if (ret) {
-		mutex_unlock(&zone_pageset_mutex);
+		mutex_unlock(&zonelists_mutex);
 		printk(KERN_DEBUG "online_pages %lx at %lx failed\n",
 			nr_pages, pfn);
 		memory_notify(MEM_CANCEL_ONLINE, &arg);

@@ -441,7 +436,7 @@ int online_pages(unsigned long pfn, unsigned long nr_pages)
 	else
 		zone_pcp_update(zone);
 
-	mutex_unlock(&zone_pageset_mutex);
+	mutex_unlock(&zonelists_mutex);
 	setup_per_zone_wmarks();
 	calculate_zone_inactive_ratio(zone);
 	if (onlined_pages) {
mm/page_alloc.c +14 -1
index 72c1211..c8e6146 100644
@@ -2434,8 +2434,11 @@ int numa_zonelist_order_handler(ctl_table *table, int write,
 			strncpy((char*)table->data, saved_string,
 				NUMA_ZONELIST_ORDER_LEN);
 			user_zonelist_order = oldval;
-		} else if (oldval != user_zonelist_order)
+		} else if (oldval != user_zonelist_order) {
+			mutex_lock(&zonelists_mutex);
 			build_all_zonelists(NULL);
+			mutex_unlock(&zonelists_mutex);
+		}
 	}
 out:
 	mutex_unlock(&zl_order_mutex);

@@ -2778,6 +2781,12 @@ static void setup_pageset(struct per_cpu_pageset *p, unsigned long batch);
 static DEFINE_PER_CPU(struct per_cpu_pageset, boot_pageset);
 static void setup_zone_pageset(struct zone *zone);
 
+/*
+ * Global mutex to protect against size modification of zonelists
+ * as well as to serialize pageset setup for the new populated zone.
+ */
+DEFINE_MUTEX(zonelists_mutex);
+
 /* return values int ....just for stop_machine() */
 static __init_refok int __build_all_zonelists(void *data)
 {

@@ -2821,6 +2830,10 @@ static __init_refok int __build_all_zonelists(void *data)
 	return 0;
 }
 
+/*
+ * Called with zonelists_mutex held always
+ * unless system_state == SYSTEM_BOOTING.
+ */
 void build_all_zonelists(void *data)
 {
 	set_zonelist_order();
Signature · 12 lines
1.5.6.1




-haicheng

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#6 lore raw

Looks good on its own....

Quoted 158 lines · Haicheng Li
Quoted 6 lines · Christoph Lameter
Quoted 1 line · Haicheng Li

+extern struct mutex zonelists_pageset_mutex;

The mutext is used for multiple serializations having to do with zones. "pageset" suggests its only for pagesets.

hmm yes, "pageset" sounds a little bit confusing.

Quoted 7 lines

So

zones_mutex?

or

zonelists_mutex?

I prefer zonelists_mutex.

Christoph, how about below patch? Thanks.

---
From 3ccfd04d6ae9127bf0f5472db0b266e7b3f158bd Mon Sep 17 00:00:00 2001
From: Haicheng Li <[email protected]>
Date: Wed, 19 May 2010 11:07:21 +0800
Subject: [PATCH] mem-hotplug: fix potential race while building zonelist for new populated zone

Add global mutex zonelists_mutex to fix the possible race:

CPU0 CPU1 CPU2
(1) zone->present_pages += online_pages;
(2) build_all_zonelists();
(3)
alloc_page();
(4) free_page();
(5) build_all_zonelists();
(6) __build_all_zonelists();
(7) zone->pageset = alloc_percpu();

In step (3,4), zone->pageset still points to boot_pageset, so bad things may happen if 2+ nodes are in this state. Even if only 1 node is accessing the boot_pageset, (3) may still consume too much memory to fail the memory allocations in step (7).

Besides, atomic operation ensures alloc_percpu() in step (7) will never fail since there is a new fresh memory block added in step(6).

Signed-off-by Haicheng Li <[email protected]>
Signed-off-by Wu Fengguang <[email protected]>
Reviewed-by Andi Kleen <[email protected]>
---
 include/linux/mmzone.h |    1 +
 mm/memory_hotplug.c    |   11 +++--------
 mm/page_alloc.c        |   15 ++++++++++++++-
 3 files changed, 18 insertions(+), 9 deletions(-)
include/linux/mmzone.h +1 -0
index dbbcd50..4d87baa 100644
@@ -641,6 +641,7 @@ typedef struct pglist_data {
 
 #include <linux/memory_hotplug.h>
 
+extern struct mutex zonelists_mutex;
 void get_zone_counts(unsigned long *active, unsigned long *inactive,
 			unsigned long *free);
 void build_all_zonelists(void *data);
mm/memory_hotplug.c +0 -5
index b564b6a..bc4a942 100644
@@ -389,11 +389,6 @@ int online_pages(unsigned long pfn, unsigned long
 nr_pages)
 	int nid;
 	int ret;
 	struct memory_notify arg;
-	/*
-	 * mutex to protect zone->pageset when it's still shared
-	 * in onlined_pages()
-	 */
-	static DEFINE_MUTEX(zone_pageset_mutex);
 
 	arg.start_pfn = pfn;
mm/memory_hotplug.c +2 -2
 	arg.nr_pages = nr_pages;

@@ -420,14 +415,14 @@ int online_pages(unsigned long pfn, unsigned long
 nr_pages)
 	 * This means the page allocator ignores this zone.
 	 * So, zonelist must be updated after online.
 	 */
-	mutex_lock(&zone_pageset_mutex);
+	mutex_lock(&zonelists_mutex);
 	if (!populated_zone(zone))
 		need_zonelists_rebuild = 1;
 
 	ret = walk_system_ram_range(pfn, nr_pages, &onlined_pages,
 		online_pages_range);
 	if (ret) {
-		mutex_unlock(&zone_pageset_mutex);
+		mutex_unlock(&zonelists_mutex);
 		printk(KERN_DEBUG "online_pages %lx at %lx failed\n",
 			nr_pages, pfn);
mm/memory_hotplug.c +1 -1
 		memory_notify(MEM_CANCEL_ONLINE, &arg);

@@ -441,7 +436,7 @@ int online_pages(unsigned long pfn, unsigned long
 nr_pages)
 	else
 		zone_pcp_update(zone);
 
-	mutex_unlock(&zone_pageset_mutex);
+	mutex_unlock(&zonelists_mutex);
 	setup_per_zone_wmarks();
 	calculate_zone_inactive_ratio(zone);
mm/memory_hotplug.c +0 -0
 	if (onlined_pages) {
mm/page_alloc.c +4 -1
index 72c1211..c8e6146 100644
@@ -2434,8 +2434,11 @@ int numa_zonelist_order_handler(ctl_table *table, int
 write,
 			strncpy((char*)table->data, saved_string,
 				NUMA_ZONELIST_ORDER_LEN);
 			user_zonelist_order = oldval;
-		} else if (oldval != user_zonelist_order)
+		} else if (oldval != user_zonelist_order) {
+			mutex_lock(&zonelists_mutex);
 			build_all_zonelists(NULL);
+			mutex_unlock(&zonelists_mutex);
+		}
 	}
 out:
mm/page_alloc.c +6 -0
 	mutex_unlock(&zl_order_mutex);

@@ -2778,6 +2781,12 @@ static void setup_pageset(struct per_cpu_pageset *p,
 unsigned long batch);
 static DEFINE_PER_CPU(struct per_cpu_pageset, boot_pageset);
 static void setup_zone_pageset(struct zone *zone);
 
+/*
+ * Global mutex to protect against size modification of zonelists
+ * as well as to serialize pageset setup for the new populated zone.
+ */
+DEFINE_MUTEX(zonelists_mutex);
+
 /* return values int ....just for stop_machine() */
 static __init_refok int __build_all_zonelists(void *data)

{

mm/page_alloc.c +4 -0
@@ -2821,6 +2830,10 @@ static __init_refok int __build_all_zonelists(void
 *data)
 	return 0;
 }
 
+/*
+ * Called with zonelists_mutex held always
+ * unless system_state == SYSTEM_BOOTING.
+ */
 void build_all_zonelists(void *data)
 {
Quoted 2 lines

set_zonelist_order();

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