[PATCH -mm 3/3] mm/oom_kill: fix the wrong task->mm == mm checks in

[Oleg Nesterov <oleg@redhat.com>]

Both "child->mm == mm" and "p->mm != mm" checks in oom_kill_process()
are wrong. ->mm can be if 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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 mm/oom_kill.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 8e7bed2..8ecac2ef 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -483,6 +483,17 @@ void oom_killer_enable(void)
 	oom_killer_disabled = false;
 }
 
+static bool process_has_mm(struct task_struct *p, struct mm_struct *mm)
+{
+	struct task_struct *t;
+
+	for_each_thread(p, t)
+		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
@@ -530,7 +541,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 		list_for_each_entry(child, &t->children, sibling) {
 			unsigned int child_points;
 
-			if (child->mm == p->mm)
+			if (process_has_mm(child, p->mm))
 				continue;
 			/*
 			 * oom_badness() returns 0 if the thread is unkillable
@@ -588,7 +599,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 			continue;
 		if (same_thread_group(p, victim))
 			continue;
-		if (p->mm != mm)
+		if (!process_has_mm(p, mm))
 			continue;
 		if (p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
 			continue;
-- 
2.4.3