oom: improve oom disable handling

Tetsuo has reported that sysrq triggered OOM killer will print a
misleading information when no tasks are selected:

  sysrq: SysRq : Manual OOM execution
  Out of memory: Kill process 4468 ((agetty)) score 0 or sacrifice child
  Killed process 4468 ((agetty)) total-vm:43704kB, anon-rss:1760kB, file-rss:0kB, shmem-rss:0kB
  sysrq: SysRq : Manual OOM execution
  Out of memory: Kill process 4469 (systemd-cgroups) score 0 or sacrifice child
  Killed process 4469 (systemd-cgroups) total-vm:10704kB, anon-rss:120kB, file-rss:0kB, shmem-rss:0kB
  sysrq: SysRq : Manual OOM execution
  sysrq: OOM request ignored because killer is disabled
  sysrq: SysRq : Manual OOM execution
  sysrq: OOM request ignored because killer is disabled
  sysrq: SysRq : Manual OOM execution
  sysrq: OOM request ignored because killer is disabled

The real reason is that there are no eligible tasks for the OOM killer
to select but since commit 7c5f64f8 ("mm: oom: deduplicate victim
selection code for memcg and global oom") the semantic of out_of_memory
has changed without updating moom_callback.

This patch updates moom_callback to tell that no task was eligible which
is the case for both oom killer disabled and no eligible tasks.  In
order to help distinguish first case from the second add printk to both
oom_killer_{enable,disable}.  This information is useful on its own
because it might help debugging potential memory allocation failures.

Fixes: 7c5f64f8 ("mm: oom: deduplicate victim selection code for memcg and global oom")
Link: http://lkml.kernel.org/r/20170404134705.6361-1-mhocko@kernel.orgSigned-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/tty/sysrq.c

@@ -372,7 +372,7 @@ static void moom_callback(struct work_struct *ignored)
 
 	mutex_lock(&oom_lock);
 	if (!out_of_memory(&oc))
-		pr_info("OOM request ignored because killer is disabled\n");
+		pr_info("OOM request ignored. No task eligible\n");
 	mutex_unlock(&oom_lock);
 }
 

mm/oom_kill.c

@@ -685,6 +685,7 @@ void exit_oom_victim(void)
 void oom_killer_enable(void)
 {
 	oom_killer_disabled = false;
+	pr_info("OOM killer enabled.\n");
 }
 
 /**
@@ -721,6 +722,7 @@ bool oom_killer_disable(signed long timeout)
 		oom_killer_enable();
 		return false;
 	}
+	pr_info("OOM killer disabled.\n");
 
 	return true;
 }