mm, oom: get rid of signal_struct::oom_victims

After "oom: keep mm of the killed task available" we can safely detect
an oom victim by checking task->signal->oom_mm so we do not need the
signal_struct counter anymore so let's get rid of it.

This alone wouldn't be sufficient for nommu archs because
exit_oom_victim doesn't hide the process from the oom killer anymore.
We can, however, mark the mm with a MMF flag in __mmput.  We can reuse
MMF_OOM_REAPED and rename it to a more generic MMF_OOM_SKIP.

Link: http://lkml.kernel.org/r/1472119394-11342-6-git-send-email-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
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/oom.h

@@ -58,6 +58,11 @@ static inline bool oom_task_origin(const struct task_struct *p)
 	return p->signal->oom_flag_origin;
 }
 
+static inline bool tsk_is_oom_victim(struct task_struct * tsk)
+{
+	return tsk->signal->oom_mm;
+}
+
 extern unsigned long oom_badness(struct task_struct *p,
 		struct mem_cgroup *memcg, const nodemask_t *nodemask,
 		unsigned long totalpages);

include/linux/sched.h

@@ -524,7 +524,7 @@ 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_SKIP		21	/* mm is of no interest for the OOM killer */
 
 #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK)
 
@@ -672,7 +672,6 @@ struct signal_struct {
 	atomic_t		sigcnt;
 	atomic_t		live;
 	int			nr_threads;
-	atomic_t oom_victims; /* # of TIF_MEDIE threads in this thread group */
 	struct list_head	thread_head;
 
 	wait_queue_head_t	wait_chldexit;	/* for wait4() */

kernel/fork.c

@@ -862,6 +862,7 @@ static inline void __mmput(struct mm_struct *mm)
 	}
 	if (mm->binfmt)
 		module_put(mm->binfmt->module);
+	set_bit(MMF_OOM_SKIP, &mm->flags);
 	mmdrop(mm);
 }
 

mm/oom_kill.c

@@ -186,7 +186,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 	 */
 	adj = (long)p->signal->oom_score_adj;
 	if (adj == OOM_SCORE_ADJ_MIN ||
-			test_bit(MMF_OOM_REAPED, &p->mm->flags) ||
+			test_bit(MMF_OOM_SKIP, &p->mm->flags) ||
 			in_vfork(p)) {
 		task_unlock(p);
 		return 0;
@@ -296,11 +296,11 @@ static int oom_evaluate_task(struct task_struct *task, void *arg)
 	/*
 	 * This task already has access to memory reserves and is being killed.
 	 * Don't allow any other task to have access to the reserves unless
-	 * the task has MMF_OOM_REAPED because chances that it would release
+	 * the task has MMF_OOM_SKIP because chances that it would release
 	 * any memory is quite low.
 	 */
-	if (!is_sysrq_oom(oc) && atomic_read(&task->signal->oom_victims)) {
-		if (test_bit(MMF_OOM_REAPED, &task->signal->oom_mm->flags))
+	if (!is_sysrq_oom(oc) && tsk_is_oom_victim(task)) {
+		if (test_bit(MMF_OOM_SKIP, &task->signal->oom_mm->flags))
 			goto next;
 		goto abort;
 	}
@@ -572,7 +572,7 @@ done:
 	 * Hide this mm from OOM killer because it has been either reaped or
 	 * somebody can't call up_write(mmap_sem).
 	 */
-	set_bit(MMF_OOM_REAPED, &mm->flags);
+	set_bit(MMF_OOM_SKIP, &mm->flags);
 
 	/* Drop a reference taken by wake_oom_reaper */
 	put_task_struct(tsk);
@@ -654,8 +654,6 @@ static void mark_oom_victim(struct task_struct *tsk)
 	if (test_and_set_tsk_thread_flag(tsk, TIF_MEMDIE))
 		return;
 
-	atomic_inc(&tsk->signal->oom_victims);
-
 	/* oom_mm is bound to the signal struct life time. */
 	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm))
 		atomic_inc(&tsk->signal->oom_mm->mm_count);
@@ -677,7 +675,6 @@ void exit_oom_victim(struct task_struct *tsk)
 {
 	if (!test_and_clear_tsk_thread_flag(tsk, TIF_MEMDIE))
 		return;
-	atomic_dec(&tsk->signal->oom_victims);
 
 	if (!atomic_dec_return(&oom_victims))
 		wake_up_all(&oom_victims_wait);
@@ -769,7 +766,7 @@ static bool task_will_free_mem(struct task_struct *task)
 	 * This task has already been drained by the oom reaper so there are
 	 * only small chances it will free some more
 	 */
-	if (test_bit(MMF_OOM_REAPED, &mm->flags))
+	if (test_bit(MMF_OOM_SKIP, &mm->flags))
 		return false;
 
 	if (atomic_read(&mm->mm_users) <= 1)
@@ -906,7 +903,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 			 * killer to guarantee OOM forward progress.
 			 */
 			can_oom_reap = false;
-			set_bit(MMF_OOM_REAPED, &mm->flags);
+			set_bit(MMF_OOM_SKIP, &mm->flags);
 			pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
 					task_pid_nr(victim), victim->comm,
 					task_pid_nr(p), p->comm);