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

[Daisuke Nishimura <d-nishimura@mtf.biglobe.ne.jp>]

On Fri, 19 Dec 2008 18:29:29 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> 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.
> 
make sense.

> Then..could you try this ? I can't do test right now, sorry.
Yes, this patch fixes the probrem.

Just a few comments.
> ==
> 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)) {
list_empty check isn't necessarily needed(mem_cgroup_del_lru does it).

> +		/*
> + 		 * 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.)
> +  		 */
I think mem_cgroup_del_lru causes NULL pointer dereference bug
in !pc->mem_cgroup case.

> +		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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