mm/oom_kill: fix the wrong task->mm == mm checks in oom_kill_process()

Both "child->mm == mm" and "p->mm != mm" checks in oom_kill_process() are
wrong.  task->mm can be NULL if the task is the exited group leader.  This
means in particular that "kill sharing same memory" loop can miss a
process with a zombie leader which uses the same ->mm.

Note: the process_has_mm(child, p->mm) check is still not 100% correct,
p->mm can be NULL too.  This is minor, but probably deserves a fix or a
comment anyway.

[akpm@linux-foundation.org: document process_shares_mm() a bit]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Kyle Walker <kwalker@redhat.com>
Cc: Stanislav Kozina <skozina@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Change-Id: I88d95c6ea31359de6cb50834e6ddce87d3afd1d8

1 files changed, 20 insertions, 2 deletions

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index f193d59d3..7923f10df 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -496,6 +496,24 @@ void oom_killer_enable(void)
 	oom_killer_disabled = false;
 }
 
+/*
+ * task->mm can be NULL if the task is the exited group leader.  So to
+ * determine whether the task is using a particular mm, we examine all the
+ * task's threads: if one of those is using this mm then this task was also
+ * using it.
+ */
+static bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
+{
+	struct task_struct *t;
+
+	for_each_thread(p, t) {
+		struct mm_struct *t_mm = READ_ONCE(t->mm);
+		if (t_mm)
+			return t_mm == mm;
+	}
+	return false;
+}
+
 #define K(x) ((x) << (PAGE_SHIFT-10))
 /*
  * Must be called while holding a reference to p, which will be released upon
@@ -557,7 +575,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 		        return;
             }
 
-			if (child->mm == p->mm)
+			if (process_shares_mm(child, p->mm))
 				continue;
 
 			/* Make sure no objections to killing the child */
@@ -618,7 +636,7 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 	 */
 	rcu_read_lock();
 	for_each_process(p) {
-		if (p->mm != mm)
+		if (!process_shares_mm(p, mm))
 			continue;
 		if (same_thread_group(p, victim))
 			continue;