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Date:   Thu, 16 May 2019 15:59:23 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vladimir Davydov <vdavydov.dev@...il.com>
Cc:     Jiri Slaby <jslaby@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        cgroups@...r.kernel.org,
        Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: [PATCH] memcg: make it work on sparse non-0-node systems

On Thu 09-05-19 15:25:26, Vladimir Davydov wrote:
> On Mon, Apr 29, 2019 at 12:59:39PM +0200, Jiri Slaby wrote:
> > We have a single node system with node 0 disabled:
> >   Scanning NUMA topology in Northbridge 24
> >   Number of physical nodes 2
> >   Skipping disabled node 0
> >   Node 1 MemBase 0000000000000000 Limit 00000000fbff0000
> >   NODE_DATA(1) allocated [mem 0xfbfda000-0xfbfeffff]
> > 
> > This causes crashes in memcg when system boots:
> >   BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
> >   #PF error: [normal kernel read fault]
> > ...
> >   RIP: 0010:list_lru_add+0x94/0x170
> > ...
> >   Call Trace:
> >    d_lru_add+0x44/0x50
> >    dput.part.34+0xfc/0x110
> >    __fput+0x108/0x230
> >    task_work_run+0x9f/0xc0
> >    exit_to_usermode_loop+0xf5/0x100
> > 
> > It is reproducible as far as 4.12. I did not try older kernels. You have
> > to have a new enough systemd, e.g. 241 (the reason is unknown -- was not
> > investigated). Cannot be reproduced with systemd 234.
> > 
> > The system crashes because the size of lru array is never updated in
> > memcg_update_all_list_lrus and the reads are past the zero-sized array,
> > causing dereferences of random memory.
> > 
> > The root cause are list_lru_memcg_aware checks in the list_lru code.
> > The test in list_lru_memcg_aware is broken: it assumes node 0 is always
> > present, but it is not true on some systems as can be seen above.
> > 
> > So fix this by checking the first online node instead of node 0.
> > 
> > Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Michal Hocko <mhocko@...nel.org>
> > Cc: Vladimir Davydov <vdavydov.dev@...il.com>
> > Cc: <cgroups@...r.kernel.org>
> > Cc: <linux-mm@...ck.org>
> > Cc: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> > ---
> >  mm/list_lru.c | 6 +-----
> >  1 file changed, 1 insertion(+), 5 deletions(-)
> > 
> > diff --git a/mm/list_lru.c b/mm/list_lru.c
> > index 0730bf8ff39f..7689910f1a91 100644
> > --- a/mm/list_lru.c
> > +++ b/mm/list_lru.c
> > @@ -37,11 +37,7 @@ static int lru_shrinker_id(struct list_lru *lru)
> >  
> >  static inline bool list_lru_memcg_aware(struct list_lru *lru)
> >  {
> > -	/*
> > -	 * This needs node 0 to be always present, even
> > -	 * in the systems supporting sparse numa ids.
> > -	 */
> > -	return !!lru->node[0].memcg_lrus;
> > +	return !!lru->node[first_online_node].memcg_lrus;
> >  }
> >  
> >  static inline struct list_lru_one *
> 
> Yep, I didn't expect node 0 could ever be unavailable, my bad.
> The patch looks fine to me:
> 
> Acked-by: Vladimir Davydov <vdavydov.dev@...il.com>
> 
> However, I tend to agree with Michal that (ab)using node[0].memcg_lrus
> to check if a list_lru is memcg aware looks confusing. I guess we could
> simply add a bool flag to list_lru instead. Something like this, may be:

Yes, this makes much more sense to me!

> 
> diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
> index aa5efd9351eb..d5ceb2839a2d 100644
> --- a/include/linux/list_lru.h
> +++ b/include/linux/list_lru.h
> @@ -54,6 +54,7 @@ struct list_lru {
>  #ifdef CONFIG_MEMCG_KMEM
>  	struct list_head	list;
>  	int			shrinker_id;
> +	bool			memcg_aware;
>  #endif
>  };
>  
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index 0730bf8ff39f..8e605e40a4c6 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -37,11 +37,7 @@ static int lru_shrinker_id(struct list_lru *lru)
>  
>  static inline bool list_lru_memcg_aware(struct list_lru *lru)
>  {
> -	/*
> -	 * This needs node 0 to be always present, even
> -	 * in the systems supporting sparse numa ids.
> -	 */
> -	return !!lru->node[0].memcg_lrus;
> +	return lru->memcg_aware;
>  }
>  
>  static inline struct list_lru_one *
> @@ -451,6 +447,7 @@ static int memcg_init_list_lru(struct list_lru *lru, bool memcg_aware)
>  {
>  	int i;
>  
> +	lru->memcg_aware = memcg_aware;
>  	if (!memcg_aware)
>  		return 0;
>  

-- 
Michal Hocko
SUSE Labs

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