mm, oom: avoid looping when chosen thread detaches its mm

oom_kill_task() returns non-zero iff the chosen process does not have any
threads with an attached ->mm.

In such a case, it's better to just return to the page allocator and retry
the allocation because memory could have been freed in the interim and the
oom condition may no longer exist.  It's unnecessary to loop in the oom
killer and find another thread to kill.

This allows both oom_kill_task() and oom_kill_process() to be converted to
void functions.  If the oom condition persists, the oom killer will be
recalled.
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -434,14 +434,14 @@ static void dump_header(struct task_struct *p, gfp_t gfp_mask, int order,
 }
 
 #define K(x) ((x) << (PAGE_SHIFT-10))
-static int oom_kill_task(struct task_struct *p)
+static void oom_kill_task(struct task_struct *p)
 {
 	struct task_struct *q;
 	struct mm_struct *mm;
 
 	p = find_lock_task_mm(p);
 	if (!p)
-		return 1;
+		return;
 
 	/* mm cannot be safely dereferenced after task_unlock(p) */
 	mm = p->mm;
@@ -477,12 +477,10 @@ static int oom_kill_task(struct task_struct *p)
 
 	set_tsk_thread_flag(p, TIF_MEMDIE);
 	force_sig(SIGKILL, p);
-
-	return 0;
 }
 #undef K
 
-static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
+static void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 			     unsigned int points, unsigned long totalpages,
 			     struct mem_cgroup *memcg, nodemask_t *nodemask,
 			     const char *message)
@@ -501,7 +499,7 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	 */
 	if (p->flags & PF_EXITING) {
 		set_tsk_thread_flag(p, TIF_MEMDIE);
-		return 0;
+		return;
 	}
 
 	task_lock(p);
@@ -533,7 +531,7 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 		}
 	} while_each_thread(p, t);
 
-	return oom_kill_task(victim);
+	oom_kill_task(victim);
 }
 
 /*
@@ -580,15 +578,10 @@ void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask)
 	check_panic_on_oom(CONSTRAINT_MEMCG, gfp_mask, 0, NULL);
 	limit = mem_cgroup_get_limit(memcg) >> PAGE_SHIFT;
 	read_lock(&tasklist_lock);
-retry:
 	p = select_bad_process(&points, limit, memcg, NULL);
-	if (!p || PTR_ERR(p) == -1UL)
-		goto out;
-
-	if (oom_kill_process(p, gfp_mask, 0, points, limit, memcg, NULL,
-				"Memory cgroup out of memory"))
-		goto retry;
-out:
+	if (p && PTR_ERR(p) != -1UL)
+		oom_kill_process(p, gfp_mask, 0, points, limit, memcg, NULL,
+				 "Memory cgroup out of memory");
 	read_unlock(&tasklist_lock);
 }
 #endif
@@ -745,33 +738,24 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
 	if (sysctl_oom_kill_allocating_task &&
 	    !oom_unkillable_task(current, NULL, nodemask) &&
 	    current->mm) {
-		/*
-		 * oom_kill_process() needs tasklist_lock held.  If it returns
-		 * non-zero, current could not be killed so we must fallback to
-		 * the tasklist scan.
-		 */
-		if (!oom_kill_process(current, gfp_mask, order, 0, totalpages,
-				NULL, nodemask,
-				"Out of memory (oom_kill_allocating_task)"))
+		oom_kill_process(current, gfp_mask, order, 0, totalpages, NULL,
+				 nodemask,
+				 "Out of memory (oom_kill_allocating_task)");
 		goto out;
 	}
 
-retry:
 	p = select_bad_process(&points, totalpages, NULL, mpol_mask);
-	if (PTR_ERR(p) == -1UL)
-		goto out;
-
 	/* Found nothing?!?! Either we hang forever, or we panic. */
 	if (!p) {
 		dump_header(NULL, gfp_mask, order, NULL, mpol_mask);
 		read_unlock(&tasklist_lock);
 		panic("Out of memory and no killable processes...\n");
 	}
-
-	if (oom_kill_process(p, gfp_mask, order, points, totalpages, NULL,
-				nodemask, "Out of memory"))
-		goto retry;
+	if (PTR_ERR(p) != -1UL) {
+		oom_kill_process(p, gfp_mask, order, points, totalpages, NULL,
+				 nodemask, "Out of memory");
 		killed = 1;
+	}
 out:
 	read_unlock(&tasklist_lock);