oom, oom_reaper: protect oom_reaper_list using simpler way

"oom, oom_reaper: disable oom_reaper for oom_kill_allocating_task" tried
to protect oom_reaper_list using MMF_OOM_KILLED flag.  But we can do it
by simply checking tsk->oom_reaper_list != NULL.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -512,8 +512,6 @@ static inline int get_dumpable(struct mm_struct *mm)
 #define MMF_HAS_UPROBES		19	/* has uprobes */
 #define MMF_RECALC_UPROBES	20	/* MMF_HAS_UPROBES can be wrong */
 
-#define MMF_OOM_KILLED		21	/* OOM killer has chosen this mm */
-
 #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK)
 
 struct sighand_struct {

mm/oom_kill.c

@@ -547,7 +547,7 @@ static int oom_reaper(void *unused)
 
 static void wake_oom_reaper(struct task_struct *tsk)
 {
-	if (!oom_reaper_th)
+	if (!oom_reaper_th || tsk->oom_reaper_list)
 		return;
 
 	get_task_struct(tsk);
@@ -681,7 +681,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 	unsigned int victim_points = 0;
 	static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
 					      DEFAULT_RATELIMIT_BURST);
-	bool can_oom_reap;
+	bool can_oom_reap = true;
 
 	/*
 	 * If the task is already exiting, don't alarm the sysadmin or kill
@@ -743,10 +743,6 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 	/* Get a reference to safely compare mm after task_unlock(victim) */
 	mm = victim->mm;
 	atomic_inc(&mm->mm_count);
-
-	/* Make sure we do not try to oom reap the mm multiple times */
-	can_oom_reap = !test_and_set_bit(MMF_OOM_KILLED, &mm->flags);
-
 	/*
 	 * We should send SIGKILL before setting TIF_MEMDIE in order to prevent
 	 * the OOM victim from depleting the memory reserves from the user