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[PATCH -mm v2 3/3] mm/oom_kill: fix the wrong task->mm == mm checks in oom_kill_process()

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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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 mm/oom_kill.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index c189ee5..034d219 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -483,6 +483,18 @@ void oom_killer_enable(void)
 	oom_killer_disabled = false;
 }
 
+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
@@ -530,7 +542,7 @@ 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_shares_mm(child, p->mm))
 				continue;
 			/*
 			 * oom_badness() returns 0 if the thread is unkillable
@@ -584,7 +596,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 	 */
 	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;
-- 
2.4.3

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[PATCH -mm v2 3/3] mm/oom_kill: fix the wrong task->mm == mm  checks in oom_kill_process() Oleg Nesterov <oleg@redhat.com> - 2015-09-30 20:30 +0200
  Re: [PATCH -mm v2 3/3] mm/oom_kill: fix the wrong task->mm == mm  checks in oom_kill_process() David Rientjes <rientjes@google.com> - 2015-09-30 23:20 +0200
  Re: [PATCH -mm v2 3/3] mm/oom_kill: fix the wrong task->mm == mm  checks in oom_kill_process() Michal Hocko <mhocko@kernel.org> - 2015-10-01 15:00 +0200
  Re: [PATCH -mm v2 3/3] mm/oom_kill: fix the wrong task->mm == mm  checks in oom_kill_process() Andrew Morton <akpm@linux-foundation.org> - 2015-10-02 00:30 +0200

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