Re: [bug][mmtom] memcg: MEM_CGROUP_ZSTAT underflow

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 19 Dec 2008 17:29:03 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> Hi.
> 
> Current(I'm testing 2008-12-16-15-50 with some patches, though) memcg have
> MEM_CGROUP_ZSTAT underflow problem.
> 
> How to reproduce:
> - make a directory, set mem.limit.
> - run some programs exceeding mem.limit.
> - make another directory, and all the tasks in old directory to new one.
> - New directory's "inactive_anon" in memory.stat underflows.
> 
> From my investigation:
> - This problem seems to happen only when swapping anonymous pages. It seems
>   not to happen about shmem.
> - After removing memcg-fix-swap-accounting-leak-v3.patch(and of course
>   memcg-fix-swap-accounting-leak-doc-fix.patch), this problem doesn't happen.
> 
> Thoughts?
> 

Thanks, then we need v4 ...but it just because my memcg-synchronized-lru.patch's
assumption about SwapCache was broken or not sane.

It assumes pc->page_cgroup is not changed after added to LRU, but now, it changes
because it can be dropped from SwapCache and new pc->mem_cgroup can be assigned.
Maybe mem_cgroup_lru_fixup() isn't enough, now.

Then..could you try this ? I can't do test right now, sorry.
==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

As memcg-fix-swap-accounting-leak-v3.patch pointed out, SwapCache
can be not SwapCache before commit.

In this case, 
	- the page is completely uncharged.
	- but still on Old LRU.
	- pc->mem_cgroup is changed before it's removed from LRU.

For avoiding race, remove page_cgroup from old LRU before we call commit.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |   21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

Index: mmotm-Dec-17/mm/memcontrol.c
===================================================================
--- mmotm-Dec-17.orig/mm/memcontrol.c
+++ mmotm-Dec-17/mm/memcontrol.c
@@ -1152,12 +1152,27 @@ int mem_cgroup_cache_charge_swapin(struc
 void mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr)
 {
 	struct page_cgroup *pc;
+	struct zone *zone;
 
 	if (mem_cgroup_disabled())
 		return;
 	if (!ptr)
 		return;
+
 	pc = lookup_page_cgroup(page);
+
+	zone = page_zone(page);
+	spin_lock(&zone->lru_lock);
+	if (!PageSwapCache(page) && !list_empty(&pc->lru)) {
+		/*
+ 		 * We need to forget old LRU before modifying pc->mem_cgroup.
+ 		 * This is necessary only when the page is already uncharged
+ 		 * by delete_from_swap_cache().
+ 		 * (Nothing happens when pc->mem_cgroup is NULL.)
+  		 */
+		mem_cgroup_del_lru(page);
+	}
+	spin_unlock(&zone->lru_lock);
 	__mem_cgroup_commit_charge(ptr, pc, MEM_CGROUP_CHARGE_TYPE_MAPPED);
 	/*
 	 * Now swap is on-memory. This means this page may be
@@ -1246,6 +1261,12 @@ __mem_cgroup_uncharge_common(struct page
 
 	mem_cgroup_charge_statistics(mem, pc, false);
 	ClearPageCgroupUsed(pc);
+	/*
+ 	 * Don't clear pc->mem_cgroup because del_from_lru() will see this.
+ 	 * The fully unchaged page is assumed to be freed after us, so it's
+ 	 * safe. When this page is reused before free, we have to be careful.
+ 	 * (In SwapCache case...it can happen.)
+  	 */
 
 	mz = page_cgroup_zoneinfo(pc);
 	unlock_page_cgroup(pc);







> 
> Thanks,
> Daisuke Nishimura.
> 
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