[PATCH 2/2] memory-hotplug: fix kswapd looping forever problem

[Minchan Kim]

When hotplug offlining happens on zone A, it starts to mark freed page
as MIGRATE_ISOLATE type in buddy for preventing further allocation.
(MIGRATE_ISOLATE is very irony type because it's apparently on buddy
but we can't allocate them).
When the memory shortage happens during hotplug offlining,
current task starts to reclaim, then wake up kswapd.
Kswapd checks watermark, then go sleep because current zone_watermark_ok_safe
doesn't consider MIGRATE_ISOLATE freed page count.
Current task continue to reclaim in direct reclaim path without kswapd's helping.
The problem is that zone->all_unreclaimable is set by only kswapd
so that current task would be looping forever like below.

__alloc_pages_slowpath
restart:
	wake_all_kswapd
rebalance:
	__alloc_pages_direct_reclaim
		do_try_to_free_pages
			if global_reclaim && !all_unreclaimable
				return 1; /* It means we did did_some_progress */
	skip __alloc_pages_may_oom
	should_alloc_retry
		goto rebalance;

If we apply KOSAKI's patch[1] which doesn't depends on kswapd
about setting zone->all_unreclaimable, we can solve this problem
by killing some task in direct reclaim path. But it doesn't wake up kswapd, still.
It could be a problem still if other subsystem needs GFP_ATOMIC request.
So kswapd should consider MIGRATE_ISOLATE when it calculate free pages
BEFORE going sleep.

This patch counts the number of MIGRATE_ISOLATE page block and
zone_watermark_ok_safe will consider it if the system has such blocks
(fortunately, it's very rare so no problem in POV overhead and kswapd is never
hotpath).

[1] http://lkml.org/lkml/2012/6/14/74

Suggested-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Cc: Aaditya Kumar <aaditya.kumar.30@xxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---

Aaditya, coul you confirm this patch solve your problem and 
make sure nr_migrate_isolate is zero after hotplug end?

Thanks!

 include/linux/mmzone.h |    8 ++++++++
 mm/page_alloc.c        |   36 ++++++++++++++++++++++++++++++++++++
 mm/page_isolation.c    |   43 +++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 85 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index bf3404e..290e186 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -474,6 +474,14 @@ struct zone {
 	 * rarely used fields:
 	 */
 	const char		*name;
+#ifdef CONFIG_MEMORY_ISOLATION
+	/*
+	 * the number of MIGRATE_ISOLATE pageblock
+	 * We need this for accurate free page counting.
+	 * It's protected by zone->lock
+	 */
+	atomic_t		nr_migrate_isolate;
+#endif
 } ____cacheline_internodealigned_in_smp;
 
 typedef enum {
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c175fa9..626f877 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -218,6 +218,11 @@ EXPORT_SYMBOL(nr_online_nodes);
 
 int page_group_by_mobility_disabled __read_mostly;
 
+/*
+ * NOTE:
+ * Don't use set_pageblock_migratetype(page, MIGRATE_ISOLATE) direclty.
+ * Instead, use {un}set_pageblock_isolate.
+ */
 void set_pageblock_migratetype(struct page *page, int migratetype)
 {
 	if (unlikely(page_group_by_mobility_disabled))
@@ -1614,6 +1619,28 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
 	return true;
 }
 
+static unsigned long migrate_isolate_pages(struct zone *zone)
+{
+	unsigned long nr_pages = 0;
+
+	if (unlikely(atomic_read(&zone->nr_migrate_isolate))) {
+		unsigned long flags;
+		int order;
+		spin_lock_irqsave(&zone->lock, flags);
+		for (order = 0; order < MAX_ORDER; order++) {
+			struct free_area *area = &zone->free_area[order];
+			long count = 0;
+			struct list_head *curr;
+
+			list_for_each(curr, &area->free_list[MIGRATE_ISOLATE])
+				count++;
+			nr_pages += (count << order);
+		}
+		spin_unlock_irqrestore(&zone->lock, flags);
+	}
+	return nr_pages;
+}
+
 bool zone_watermark_ok(struct zone *z, int order, unsigned long mark,
 		      int classzone_idx, int alloc_flags)
 {
@@ -1629,6 +1656,14 @@ bool zone_watermark_ok_safe(struct zone *z, int order, unsigned long mark,
 	if (z->percpu_drift_mark && free_pages < z->percpu_drift_mark)
 		free_pages = zone_page_state_snapshot(z, NR_FREE_PAGES);
 
+	/*
+	 * If the zone has MIGRATE_ISOLATE type free page,
+	 * we should consider it, too. Otherwise, kswapd can sleep forever.
+	 */
+	free_pages -= migrate_isolate_pages(z);
+	if (free_pages < 0)
+		free_pages = 0;
+
 	return __zone_watermark_ok(z, order, mark, classzone_idx, alloc_flags,
 								free_pages);
 }
@@ -4407,6 +4442,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
 		lruvec_init(&zone->lruvec, zone);
 		zap_zone_vm_stats(zone);
 		zone->flags = 0;
+		atomic_set(&zone->nr_migrate_isolate, 0);
 		if (!size)
 			continue;
 
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 1a9cb36..e95a792 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -8,6 +8,45 @@
 #include <linux/memory.h>
 #include "internal.h"
 
+static void set_pageblock_isolate(struct zone *zone, struct page *page)
+{
+	int old_migratetype;
+	assert_spin_locked(&zone->lock);
+
+	if (unlikely(page_group_by_mobility_disabled)) {
+		set_pageblock_flags_group(page, MIGRATE_UNMOVABLE,
+				PB_migrate, PB_migrate_end);
+		return;
+	}
+
+	old_migratetype = get_pageblock_migratetype(page);
+	set_pageblock_flags_group(page, MIGRATE_ISOLATE,
+			PB_migrate, PB_migrate_end);
+
+	if (old_migratetype != MIGRATE_ISOLATE)
+		atomic_inc(&zone->nr_migrate_isolate);
+}
+
+static void unset_pageblock_isolate(struct zone *zone, struct page *page,
+		unsigned long migratetype)
+{
+	assert_spin_locked(&zone->lock);
+
+	if (unlikely(page_group_by_mobility_disabled)) {
+		set_pageblock_flags_group(page, migratetype,
+				PB_migrate, PB_migrate_end);
+		return;
+	}
+
+	BUG_ON(get_pageblock_migratetype(page) != MIGRATE_ISOLATE);
+	BUG_ON(migratetype == MIGRATE_ISOLATE);
+
+	set_pageblock_flags_group(page, migratetype,
+			PB_migrate, PB_migrate_end);
+	atomic_dec(&zone->nr_migrate_isolate);
+	BUG_ON(atomic_read(&zone->nr_migrate_isolate) < 0);
+}
+
 int set_migratetype_isolate(struct page *page)
 {
 	struct zone *zone;
@@ -54,7 +93,7 @@ int set_migratetype_isolate(struct page *page)
 
 out:
 	if (!ret) {
-		set_pageblock_migratetype(page, MIGRATE_ISOLATE);
+		set_pageblock_isolate(zone, page);
 		move_freepages_block(zone, page, MIGRATE_ISOLATE);
 	}
 
@@ -72,8 +111,8 @@ void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 	spin_lock_irqsave(&zone->lock, flags);
 	if (get_pageblock_migratetype(page) != MIGRATE_ISOLATE)
 		goto out;
-	set_pageblock_migratetype(page, migratetype);
 	move_freepages_block(zone, page, migratetype);
+	unset_pageblock_isolate(zone, page, migratetype);
 out:
 	spin_unlock_irqrestore(&zone->lock, flags);
 }
-- 
1.7.9.5

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