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Date:   Mon, 26 Jul 2021 22:32:13 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Dan Carpenter <dan.carpenter@oracle.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH] mm: memcontrol: fix blocking rstat function called from
 atomic cgroup1 thresholding code
Message-ID: <YP8bzZJjbpEvi4lM@dhcp22.suse.cz>
References: <20210726150019.251820-1-hannes@cmpxchg.org>
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On Mon 26-07-21 11:00:19, Johannes Weiner wrote:
> Dan Carpenter reports:
> 
>     The patch 2d146aa3aa84: "mm: memcontrol: switch to rstat" from Apr
>     29, 2021, leads to the following static checker warning:
> 
> 	    kernel/cgroup/rstat.c:200 cgroup_rstat_flush()
> 	    warn: sleeping in atomic context
> 
>     mm/memcontrol.c
>       3572  static unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
>       3573  {
>       3574          unsigned long val;
>       3575
>       3576          if (mem_cgroup_is_root(memcg)) {
>       3577                  cgroup_rstat_flush(memcg->css.cgroup);
> 			    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
>     This is from static analysis and potentially a false positive.  The
>     problem is that mem_cgroup_usage() is called from __mem_cgroup_threshold()
>     which holds an rcu_read_lock().  And the cgroup_rstat_flush() function
>     can sleep.
> 
>       3578                  val = memcg_page_state(memcg, NR_FILE_PAGES) +
>       3579                          memcg_page_state(memcg, NR_ANON_MAPPED);
>       3580                  if (swap)
>       3581                          val += memcg_page_state(memcg, MEMCG_SWAP);
>       3582          } else {
>       3583                  if (!swap)
>       3584                          val = page_counter_read(&memcg->memory);
>       3585                  else
>       3586                          val = page_counter_read(&memcg->memsw);
>       3587          }
>       3588          return val;
>       3589  }
> 
> __mem_cgroup_threshold() indeed holds the rcu lock. In addition, the
> thresholding code is invoked during stat changes, and those contexts
> have irqs disabled as well. If the lock breaking occurs inside the
> flush function, it will result in a sleep from an atomic context.
> 
> Use the irsafe flushing variant in mem_cgroup_usage() to fix this.
> 
> Fixes: 2d146aa3aa84 ("mm: memcontrol: switch to rstat")
> Cc: <stable@vger.kernel.org>
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
>  mm/memcontrol.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ae1f5d0cb581..eb8e87c4833f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3574,7 +3574,8 @@ static unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
>  	unsigned long val;
>  
>  	if (mem_cgroup_is_root(memcg)) {
> -		cgroup_rstat_flush(memcg->css.cgroup);
> +		/* mem_cgroup_threshold() calls here from irqsafe context */
> +		cgroup_rstat_flush_irqsafe(memcg->css.cgroup);
>  		val = memcg_page_state(memcg, NR_FILE_PAGES) +
>  			memcg_page_state(memcg, NR_ANON_MAPPED);
>  		if (swap)
> -- 
> 2.32.0

-- 
Michal Hocko
SUSE Labs