Now, when we remove memcg, we call force_empty().
This call drops all page_cgroup accounting in this mem_cgroup but doesn't
drop pages. So, some page caches can be remaind as "not accounted" memory
while they are alive. (because it's accounted only when add_to_page_cache())
If they are not used by other memcg, global LRU will drop them.

This patch tries to drop pages at removing memcg. Other memcg will
reload and re-account page caches. (but this will increase page-in
after rmdir().)

Consideration: should we recharge all pages to the parent at last ?
               But it's not precise logic.

Changelog v1->v2
 - renamed res_counter_empty().

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

---
 include/linux/res_counter.h |   11 +++++++++++
 mm/memcontrol.c             |   19 +++++++++++++++++++
 2 files changed, 30 insertions(+)

Index: mm-2.6.26-rc2-mm1/mm/memcontrol.c
===================================================================
--- mm-2.6.26-rc2-mm1.orig/mm/memcontrol.c
+++ mm-2.6.26-rc2-mm1/mm/memcontrol.c
@@ -791,6 +791,20 @@ int mem_cgroup_shrink_usage(struct mm_st
 	return 0;
 }
 
+
+static void mem_cgroup_drop_all_pages(struct mem_cgroup *mem)
+{
+	int progress;
+	while (!res_counter_empty(&mem->res)) {
+		progress = try_to_free_mem_cgroup_pages(mem,
+					GFP_HIGHUSER_MOVABLE);
+		if (!progress) /* we did as much as possible */
+			break;
+		cond_resched();
+	}
+	return;
+}
+
 /*
  * This routine traverse page_cgroup in given list and drop them all.
  * *And* this routine doesn't reclaim page itself, just removes page_cgroup.
@@ -848,7 +862,12 @@ static int mem_cgroup_force_empty(struct
 	if (mem_cgroup_subsys.disabled)
 		return 0;
 
+	if (atomic_read(&mem->css.cgroup->count) > 0)
+		goto out;
+
 	css_get(&mem->css);
+	/* drop pages as much as possible */
+	mem_cgroup_drop_all_pages(mem);
 	/*
 	 * page reclaim code (kswapd etc..) will move pages between
 	 * active_list <-> inactive_list while we don't take a lock.
Index: mm-2.6.26-rc2-mm1/include/linux/res_counter.h
===================================================================
--- mm-2.6.26-rc2-mm1.orig/include/linux/res_counter.h
+++ mm-2.6.26-rc2-mm1/include/linux/res_counter.h
@@ -153,4 +153,15 @@ static inline void res_counter_reset_fai
 	cnt->failcnt = 0;
 	spin_unlock_irqrestore(&cnt->lock, flags);
 }
+/* returns 0 if usage is 0. */
+static inline int res_counter_empty(struct res_counter *cnt)
+{
+	unsigned long flags;
+	int ret;
+
+	spin_lock_irqsave(&cnt->lock, flags);
+	ret = (cnt->usage == 0) ? 0 : 1;
+	spin_unlock_irqrestore(&cnt->lock, flags);
+	return ret;
+}
 #endif

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