Re: [PATCH] mm: memcontrol: fix blocking rstat function called from atomic cgroup1 thresholding code

[Shakeel Butt <shakeelb@google.com>]

On Mon, Jul 26, 2021 at 8:01 AM Johannes Weiner <hannes@cmpxchg.org> 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>

Reviewed-by: Shakeel Butt <shakeelb@google.com>

BTW what do you think of removing stat flushes from the read side
(kernel and userspace) completely after periodic flushing and async
flushing from update side? Basically with "memcg: infrastructure to
flush memcg stats".