mm,oom_reaper: do not attempt to reap a task twice

"mm, oom_reaper: do not attempt to reap a task twice" tried to give the
OOM reaper one more chance to retry using MMF_OOM_NOT_REAPABLE flag.
But the usefulness of the flag is rather limited and actually never
shown in practice.  If the flag is set, it means that the holder of
mm->mmap_sem cannot call up_write() due to presumably being blocked at
unkillable wait waiting for other thread's memory allocation.  But since
one of threads sharing that mm will queue that mm immediately via
task_will_free_mem() shortcut (otherwise, oom_badness() will select the
same mm again due to oom_score_adj value unchanged), retrying
MMF_OOM_NOT_REAPABLE mm is unlikely helpful.

Let's always set MMF_OOM_REAPED.

Link: http://lkml.kernel.org/r/1472119394-11342-3-git-send-email-mhocko@kernel.org
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -525,7 +525,6 @@ 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_REAPED		21	/* mm has been already reaped */
-#define MMF_OOM_NOT_REAPABLE	22	/* mm couldn't be reaped */
 
 #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK)
 

mm/oom_kill.c

@@ -578,20 +578,11 @@ static void oom_reap_task(struct task_struct *tsk)
 	if (attempts <= MAX_OOM_REAP_RETRIES)
 		goto done;
 
+	/* Ignore this mm because somebody can't call up_write(mmap_sem). */
+	set_bit(MMF_OOM_REAPED, &mm->flags);
+
 	pr_info("oom_reaper: unable to reap pid:%d (%s)\n",
 		task_pid_nr(tsk), tsk->comm);
-
-	/*
-	 * If we've already tried to reap this task in the past and
-	 * failed it probably doesn't make much sense to try yet again
-	 * so hide the mm from the oom killer so that it can move on
-	 * to another task with a different mm struct.
-	 */
-	if (test_and_set_bit(MMF_OOM_NOT_REAPABLE, &mm->flags)) {
-		pr_info("oom_reaper: giving up pid:%d (%s)\n",
-			task_pid_nr(tsk), tsk->comm);
-		set_bit(MMF_OOM_REAPED, &mm->flags);
-	}
 	debug_show_all_locks();
 
 done: