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   1 From 30def93565e5ba08676aa2b9083f253fc586dbed Mon Sep 17 00:00:00 2001
   2 From: Johannes Weiner <hannes@cmpxchg.org>
   3 Date: Thu, 29 Jul 2021 14:53:44 -0700
   4 Subject: mm: memcontrol: fix blocking rstat function called from atomic cgroup1 thresholding code
   5
   6 From: Johannes Weiner <hannes@cmpxchg.org>
   7
   8 commit 30def93565e5ba08676aa2b9083f253fc586dbed upstream.
   9
  10 Dan Carpenter reports:
  11
  12     The patch 2d146aa3aa84: "mm: memcontrol: switch to rstat" from Apr
  13     29, 2021, leads to the following static checker warning:
  14
  15             kernel/cgroup/rstat.c:200 cgroup_rstat_flush()
  16             warn: sleeping in atomic context
  17
  18     mm/memcontrol.c
  19       3572  static unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
  20       3573  {
  21       3574          unsigned long val;
  22       3575
  23       3576          if (mem_cgroup_is_root(memcg)) {
  24       3577                  cgroup_rstat_flush(memcg->css.cgroup);
  25                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  26
  27     This is from static analysis and potentially a false positive.  The
  28     problem is that mem_cgroup_usage() is called from __mem_cgroup_threshold()
  29     which holds an rcu_read_lock().  And the cgroup_rstat_flush() function
  30     can sleep.
  31
  32       3578                  val = memcg_page_state(memcg, NR_FILE_PAGES) +
  33       3579                          memcg_page_state(memcg, NR_ANON_MAPPED);
  34       3580                  if (swap)
  35       3581                          val += memcg_page_state(memcg, MEMCG_SWAP);
  36       3582          } else {
  37       3583                  if (!swap)
  38       3584                          val = page_counter_read(&memcg->memory);
  39       3585                  else
  40       3586                          val = page_counter_read(&memcg->memsw);
  41       3587          }
  42       3588          return val;
  43       3589  }
  44
  45 __mem_cgroup_threshold() indeed holds the rcu lock.  In addition, the
  46 thresholding code is invoked during stat changes, and those contexts
  47 have irqs disabled as well.  If the lock breaking occurs inside the
  48 flush function, it will result in a sleep from an atomic context.
  49
  50 Use the irqsafe flushing variant in mem_cgroup_usage() to fix this.
  51
  52 Link: https://lkml.kernel.org/r/20210726150019.251820-1-hannes@cmpxchg.org
  53 Fixes: 2d146aa3aa84 ("mm: memcontrol: switch to rstat")
  54 Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
  55 Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
  56 Acked-by: Chris Down <chris@chrisdown.name>
  57 Reviewed-by: Rik van Riel <riel@surriel.com>
  58 Acked-by: Michal Hocko <mhocko@suse.com>
  59 Reviewed-by: Shakeel Butt <shakeelb@google.com>
  60 Cc: <stable@vger.kernel.org>
  61 Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
  62 Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
  63 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  64 ---
  65  mm/memcontrol.c |    3 ++-
  66  1 file changed, 2 insertions(+), 1 deletion(-)
  67
  68 --- a/mm/memcontrol.c
  69 +++ b/mm/memcontrol.c
  70 @@ -3394,7 +3394,8 @@ static unsigned long mem_cgroup_usage(st
  71         unsigned long val;
  72
  73         if (mem_cgroup_is_root(memcg)) {
  74 -               cgroup_rstat_flush(memcg->css.cgroup);
  75 +               /* mem_cgroup_threshold() calls here from irqsafe context */
  76 +               cgroup_rstat_flush_irqsafe(memcg->css.cgroup);
  77                 val = memcg_page_state(memcg, NR_FILE_PAGES) +
  78                         memcg_page_state(memcg, NR_ANON_MAPPED);
  79                 if (swap)