Date:	Fri, 7 Feb 2014 11:43:21 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Tejun Heo <tj@...nel.org>,
	Filipe Brandenburger <filbranden@...gle.com>,
	Li Zefan <lizefan@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	Markus Blank-Burian <burian@...nster.de>,
	Shawn Bohrer <shawn.bohrer@...il.com>, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] cgroup: use an ordered workqueue for cgroup destruction

On Thu, Feb 06, 2014 at 03:56:01PM -0800, Hugh Dickins wrote:
> Sometimes the cleanup after memcg hierarchy testing gets stuck in
> mem_cgroup_reparent_charges(), unable to bring non-kmem usage down to 0.
> 
> There may turn out to be several causes, but a major cause is this: the
> workitem to offline parent can get run before workitem to offline child;
> parent's mem_cgroup_reparent_charges() circles around waiting for the
> child's pages to be reparented to its lrus, but it's holding cgroup_mutex
> which prevents the child from reaching its mem_cgroup_reparent_charges().
> 
> Just use an ordered workqueue for cgroup_destroy_wq.
> 
> Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup destruction")
> Suggested-by: Filipe Brandenburger <filbranden@...gle.com>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Cc: stable@...r.kernel.org # 3.10+

I think this is a good idea for now and -stable:
Acked-by: Johannes Weiner <hannes@...xchg.org>

Long-term, I think we may want to get rid of the reparenting in
css_offline() entirely and only do it in the naturally ordered
css_free() callback.  We only reparent in css_offline() because
swapout records pin the css and we don't want to hang on to
potentially gigabytes of unreclaimable (css_tryget() disabled) cache
indefinitely until the last swapout records disappear.

So I'm currently mucking around with the following patch, which drops
the css pin from swapout records and reparents them in css_free().
It's lightly tested and there might well be dragons but I don't see
any fundamental problems with it.

What do you think?

---
 include/linux/page_cgroup.h |   8 ++++
 mm/memcontrol.c             | 101 +++++++++++++-------------------------------
 mm/page_cgroup.c            |  41 ++++++++++++++++++
 3 files changed, 78 insertions(+), 72 deletions(-)

diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
index 777a524716db..3ededda8934f 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -111,6 +111,8 @@ extern unsigned short swap_cgroup_cmpxchg(swp_entry_t ent,
 					unsigned short old, unsigned short new);
 extern unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id);
 extern unsigned short lookup_swap_cgroup_id(swp_entry_t ent);
+extern unsigned long swap_cgroup_migrate(unsigned short old,
+					 unsigned short new);
 extern int swap_cgroup_swapon(int type, unsigned long max_pages);
 extern void swap_cgroup_swapoff(int type);
 #else
@@ -127,6 +129,12 @@ unsigned short lookup_swap_cgroup_id(swp_entry_t ent)
 	return 0;
 }
 
+static inline unsigned long swap_cgroup_migrate(unsigned short old,
+						unsigned short new)
+{
+	return 0;
+}
+
 static inline int
 swap_cgroup_swapon(int type, unsigned long max_pages)
 {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 53385cd4e6f0..e2a0d3986c74 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -892,11 +892,9 @@ static long mem_cgroup_read_stat(struct mem_cgroup *memcg,
 	return val;
 }
 
-static void mem_cgroup_swap_statistics(struct mem_cgroup *memcg,
-					 bool charge)
+static void mem_cgroup_swap_statistics(struct mem_cgroup *memcg, long nr_pages)
 {
-	int val = (charge) ? 1 : -1;
-	this_cpu_add(memcg->stat->count[MEM_CGROUP_STAT_SWAP], val);
+	this_cpu_add(memcg->stat->count[MEM_CGROUP_STAT_SWAP], nr_pages);
 }
 
 static unsigned long mem_cgroup_read_events(struct mem_cgroup *memcg,
@@ -4234,15 +4232,12 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype,
 	 */
 
 	unlock_page_cgroup(pc);
-	/*
-	 * even after unlock, we have memcg->res.usage here and this memcg
-	 * will never be freed, so it's safe to call css_get().
-	 */
+
 	memcg_check_events(memcg, page);
-	if (do_swap_account && ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT) {
-		mem_cgroup_swap_statistics(memcg, true);
-		css_get(&memcg->css);
-	}
+
+	if (do_swap_account && ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
+		mem_cgroup_swap_statistics(memcg, 1);
+
 	/*
 	 * Migration does not charge the res_counter for the
 	 * replacement page, so leave it alone when phasing out the
@@ -4351,10 +4346,6 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
 
 	memcg = __mem_cgroup_uncharge_common(page, ctype, false);
 
-	/*
-	 * record memcg information,  if swapout && memcg != NULL,
-	 * css_get() was called in uncharge().
-	 */
 	if (do_swap_account && swapout && memcg)
 		swap_cgroup_record(ent, mem_cgroup_id(memcg));
 }
@@ -4383,8 +4374,7 @@ void mem_cgroup_uncharge_swap(swp_entry_t ent)
 		 */
 		if (!mem_cgroup_is_root(memcg))
 			res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
-		mem_cgroup_swap_statistics(memcg, false);
-		css_put(&memcg->css);
+		mem_cgroup_swap_statistics(memcg, -1);
 	}
 	rcu_read_unlock();
 }
@@ -4412,20 +4402,8 @@ static int mem_cgroup_move_swap_account(swp_entry_t entry,
 	new_id = mem_cgroup_id(to);
 
 	if (swap_cgroup_cmpxchg(entry, old_id, new_id) == old_id) {
-		mem_cgroup_swap_statistics(from, false);
-		mem_cgroup_swap_statistics(to, true);
-		/*
-		 * This function is only called from task migration context now.
-		 * It postpones res_counter and refcount handling till the end
-		 * of task migration(mem_cgroup_clear_mc()) for performance
-		 * improvement. But we cannot postpone css_get(to)  because if
-		 * the process that has been moved to @to does swap-in, the
-		 * refcount of @to might be decreased to 0.
-		 *
-		 * We are in attach() phase, so the cgroup is guaranteed to be
-		 * alive, so we can just call css_get().
-		 */
-		css_get(&to->css);
+		mem_cgroup_swap_statistics(from, -1);
+		mem_cgroup_swap_statistics(to, 1);
 		return 0;
 	}
 	return -EINVAL;
@@ -6611,7 +6589,6 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
 	kmem_cgroup_css_offline(memcg);
 
 	mem_cgroup_invalidate_reclaim_iterators(memcg);
-	mem_cgroup_reparent_charges(memcg);
 	mem_cgroup_destroy_all_caches(memcg);
 	vmpressure_cleanup(&memcg->vmpressure);
 }
@@ -6619,41 +6596,26 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
 static void mem_cgroup_css_free(struct cgroup_subsys_state *css)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+	unsigned long swaps_moved;
+	struct mem_cgroup *parent;
+
 	/*
-	 * XXX: css_offline() would be where we should reparent all
-	 * memory to prepare the cgroup for destruction.  However,
-	 * memcg does not do css_tryget() and res_counter charging
-	 * under the same RCU lock region, which means that charging
-	 * could race with offlining.  Offlining only happens to
-	 * cgroups with no tasks in them but charges can show up
-	 * without any tasks from the swapin path when the target
-	 * memcg is looked up from the swapout record and not from the
-	 * current task as it usually is.  A race like this can leak
-	 * charges and put pages with stale cgroup pointers into
-	 * circulation:
-	 *
-	 * #0                        #1
-	 *                           lookup_swap_cgroup_id()
-	 *                           rcu_read_lock()
-	 *                           mem_cgroup_lookup()
-	 *                           css_tryget()
-	 *                           rcu_read_unlock()
-	 * disable css_tryget()
-	 * call_rcu()
-	 *   offline_css()
-	 *     reparent_charges()
-	 *                           res_counter_charge()
-	 *                           css_put()
-	 *                             css_free()
-	 *                           pc->mem_cgroup = dead memcg
-	 *                           add page to lru
-	 *
-	 * The bulk of the charges are still moved in offline_css() to
-	 * avoid pinning a lot of pages in case a long-term reference
-	 * like a swapout record is deferring the css_free() to long
-	 * after offlining.  But this makes sure we catch any charges
-	 * made after offlining:
+	 * Migrate all swap entries to the parent.  There are no more
+	 * references to the css, so no new swap out records can show
+	 * up.  Any concurrently faulting pages will either get this
+	 * offline cgroup and charge the faulting task, or get the
+	 * migrated parent id and charge the parent for the in-memory
+	 * page.  uncharge_swap() will balance the res_counter in the
+	 * parent either way, whether it still fixes this group's
+	 * res_counter is irrelevant at this point.
 	 */
+	parent = parent_mem_cgroup(memcg);
+	if (!parent)
+		parent = root_mem_cgroup;
+	swaps_moved = swap_cgroup_migrate(mem_cgroup_id(memcg),
+					  mem_cgroup_id(parent));
+	mem_cgroup_swap_statistics(parent, swaps_moved);
+
 	mem_cgroup_reparent_charges(memcg);
 
 	memcg_destroy_kmem(memcg);
@@ -6966,7 +6928,6 @@ static void __mem_cgroup_clear_mc(void)
 {
 	struct mem_cgroup *from = mc.from;
 	struct mem_cgroup *to = mc.to;
-	int i;
 
 	/* we must uncharge all the leftover precharges from mc.to */
 	if (mc.precharge) {
@@ -6981,16 +6942,13 @@ static void __mem_cgroup_clear_mc(void)
 		__mem_cgroup_cancel_charge(mc.from, mc.moved_charge);
 		mc.moved_charge = 0;
 	}
-	/* we must fixup refcnts and charges */
+	/* we must fixup charges */
 	if (mc.moved_swap) {
 		/* uncharge swap account from the old cgroup */
 		if (!mem_cgroup_is_root(mc.from))
 			res_counter_uncharge(&mc.from->memsw,
 						PAGE_SIZE * mc.moved_swap);
 
-		for (i = 0; i < mc.moved_swap; i++)
-			css_put(&mc.from->css);
-
 		if (!mem_cgroup_is_root(mc.to)) {
 			/*
 			 * we charged both to->res and to->memsw, so we should
@@ -6999,7 +6957,6 @@ static void __mem_cgroup_clear_mc(void)
 			res_counter_uncharge(&mc.to->res,
 						PAGE_SIZE * mc.moved_swap);
 		}
-		/* we've already done css_get(mc.to) */
 		mc.moved_swap = 0;
 	}
 	memcg_oom_recover(from);
diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index cfd162882c00..ca04feaae7fe 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -459,6 +459,47 @@ unsigned short lookup_swap_cgroup_id(swp_entry_t ent)
 	return lookup_swap_cgroup(ent, NULL)->id;
 }
 
+/**
+ * swap_cgroup_migrate - migrate all entries of one id to another
+ * @old: old id
+ * @new: new id
+ *
+ * Caller has to be able to deal with swapon/swapoff racing.
+ *
+ * Returns number of migrated entries.
+ */
+unsigned long swap_cgroup_migrate(unsigned short old, unsigned short new)
+{
+	unsigned long migrated = 0;
+	unsigned int type;
+
+	for (type = 0; type < MAX_SWAPFILES; type++) {
+		struct swap_cgroup_ctrl *ctrl;
+		unsigned long flags;
+		unsigned int page;
+
+		ctrl = &swap_cgroup_ctrl[type];
+		spin_lock_irqsave(&ctrl->lock, flags);
+		for (page = 0; page < ctrl->length; page++) {
+			struct swap_cgroup *base;
+			pgoff_t offset;
+
+			base = page_address(ctrl->map[page]);
+			for (offset = 0; offset < SC_PER_PAGE; offset++) {
+				struct swap_cgroup *sc;
+
+				sc = base + offset;
+				if (sc->id == old) {
+					sc->id = new;
+					migrated++;
+				}
+			}
+		}
+		spin_unlock_irqrestore(&ctrl->lock, flags);
+	}
+	return migrated;
+}
+
 int swap_cgroup_swapon(int type, unsigned long max_pages)
 {
 	void *array;
-- 
1.8.5.3

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