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Date:   Mon, 26 Jul 2021 16:08:54 +0100
From:   Chris Down <chris@chrisdown.name>
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: <YP7QBuc+5gAlL4g7@chrisdown.name>
References: <20210726150019.251820-1-hannes@cmpxchg.org>
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Johannes Weiner writes:
>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>

Thanks, looks good.

Acked-by: Chris Down <chris@chrisdown.name>

>---
> 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
>
>