[PATCH 7/12] memcg add function to move account

[KAMEZAWA Hiroyuki]

This patch provides a function to move account information of a page between
mem_cgroups.

This moving of page_cgroup is done under
 - lru_lock of source/destination mem_cgroup is held.
 - lock_page_cgroup() is held.

Then, a routine which touches pc->mem_cgroup without lock_page_cgroup() should
confirm pc->mem_cgroup is still valid or not. Typlical code can be following.

(while page is not under lock_page())
	mem = pc->mem_cgroup;
	mz = page_cgroup_zoneinfo(pc)
	spin_lock_irqsave(&mz->lru_lock);
	if (pc->mem_cgroup == mem)
		...../* some list handling */
	spin_unlock_irq(&mz->lru_lock);

Or better way is
	lock_page_cgroup(pc);
	....
	unlock_page_cgroup(pc);

But you should confirm the nest of lock and avoid deadlock.
(trylock is better if it's ok.)

If you find page_cgroup from mem_cgroup's LRU under mz->lru_lock,
you don't have to worry about what pc->mem_cgroup points to.

Changelog: (v4) -> (v5)
  - check for lock_page() is removed.
  - rewrote description.

Changelog: (v2) -> (v4)
  - added lock_page_cgroup().
  - splitted out from new-force-empty patch.
  - added how-to-use text.
  - fixed race in __mem_cgroup_uncharge_common().

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

 mm/memcontrol.c |   84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 81 insertions(+), 3 deletions(-)

Index: mmotm-2.6.27-rc7+/mm/memcontrol.c
===================================================================
--- mmotm-2.6.27-rc7+.orig/mm/memcontrol.c
+++ mmotm-2.6.27-rc7+/mm/memcontrol.c
@@ -426,6 +426,7 @@ int task_in_mem_cgroup(struct task_struc
 void mem_cgroup_move_lists(struct page *page, enum lru_list lru)
 {
 	struct page_cgroup *pc;
+	struct mem_cgroup *mem;
 	struct mem_cgroup_per_zone *mz;
 	unsigned long flags;
 
@@ -444,9 +445,14 @@ void mem_cgroup_move_lists(struct page *
 
 	pc = page_get_page_cgroup(page);
 	if (pc) {
+		mem = pc->mem_cgroup;
 		mz = page_cgroup_zoneinfo(pc);
 		spin_lock_irqsave(&mz->lru_lock, flags);
-		__mem_cgroup_move_lists(pc, lru);
+		/*
+		 * check against the race with move_account.
+		 */
+		if (likely(mem == pc->mem_cgroup))
+			__mem_cgroup_move_lists(pc, lru);
 		spin_unlock_irqrestore(&mz->lru_lock, flags);
 	}
 	unlock_page_cgroup(page);
@@ -567,6 +573,70 @@ unsigned long mem_cgroup_isolate_pages(u
 	return nr_taken;
 }
 
+/**
+ * mem_cgroup_move_account - move account of the page
+ * @page ... the target page of being moved.
+ * @pc   ... page_cgroup of the page.
+ * @from ... mem_cgroup which the page is moved from.
+ * @to   ... mem_cgroup which the page is moved to.
+ *
+ * The caller must confirm following.
+ * 1. disable irq.
+ * 2. lru_lock of old mem_cgroup should be held.
+ * 3. pc is guaranteed to be valid and on mem_cgroup's LRU.
+ *
+ * Because we cannot call try_to_free_page() here, the caller must guarantee
+ * this moving of charge never fails. (if charge fails, this call fails.)
+ * Currently this is called only against root cgroup.
+ * which has no limitation of resource.
+ * Returns 0 at success, returns 1 at failure.
+ */
+int mem_cgroup_move_account(struct page *page, struct page_cgroup *pc,
+	struct mem_cgroup *from, struct mem_cgroup *to)
+{
+	struct mem_cgroup_per_zone *from_mz, *to_mz;
+	int nid, zid;
+	int ret = 1;
+
+	VM_BUG_ON(!irqs_disabled());
+
+	nid = page_to_nid(page);
+	zid = page_zonenum(page);
+	from_mz =  mem_cgroup_zoneinfo(from, nid, zid);
+	to_mz =  mem_cgroup_zoneinfo(to, nid, zid);
+
+	if (res_counter_charge(&to->res, PAGE_SIZE)) {
+		/* Now, we assume no_limit...no failure here. */
+		return ret;
+	}
+	if (!try_lock_page_cgroup(page)) {
+		res_counter_uncharge(&to->res, PAGE_SIZE);
+		return ret;
+	}
+
+	if (page_get_page_cgroup(page) != pc) {
+		res_counter_uncharge(&to->res, PAGE_SIZE);
+		goto out;
+	}
+
+	if (spin_trylock(&to_mz->lru_lock)) {
+		__mem_cgroup_remove_list(from_mz, pc);
+		css_put(&from->css);
+		res_counter_uncharge(&from->res, PAGE_SIZE);
+		pc->mem_cgroup = to;
+		css_get(&to->css);
+		__mem_cgroup_add_list(to_mz, pc);
+		ret = 0;
+		spin_unlock(&to_mz->lru_lock);
+	} else {
+		res_counter_uncharge(&to->res, PAGE_SIZE);
+	}
+out:
+	unlock_page_cgroup(page);
+
+	return ret;
+}
+
 /*
  * Charge the memory controller for page usage.
  * Return
@@ -754,16 +824,24 @@ __mem_cgroup_uncharge_common(struct page
 	if ((ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
 	    && ((PageCgroupCache(pc) || page_mapped(page))))
 		goto unlock;
-
+retry:
+	mem = pc->mem_cgroup;
 	mz = page_cgroup_zoneinfo(pc);
 	spin_lock_irqsave(&mz->lru_lock, flags);
+	if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED &&
+	    unlikely(mem != pc->mem_cgroup)) {
+		/* MAPPED account can be done without lock_page().
+		   Check race with mem_cgroup_move_account() */
+		spin_unlock_irqrestore(&mz->lru_lock, flags);
+		goto retry;
+	}
 	__mem_cgroup_remove_list(mz, pc);
 	spin_unlock_irqrestore(&mz->lru_lock, flags);
 
 	page_assign_page_cgroup(page, NULL);
 	unlock_page_cgroup(page);
 
-	mem = pc->mem_cgroup;
+
 	res_counter_uncharge(&mem->res, PAGE_SIZE);
 	css_put(&mem->css);
 

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