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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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  [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

#1236629 — [PATCH -mm v2 3/3] 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.

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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#1236764

On Wed, 30 Sep 2015, Oleg Nesterov wrote:

> 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>

Acked-by: David Rientjes <rientjes@google.com>
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#1237403

On Wed 30-09-15 20:24:11, Oleg Nesterov wrote:
> 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>

Acked-by: Michal Hocko <mhocko@suse.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

-- 
Michal Hocko
SUSE Labs
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#1237789

On Wed, 30 Sep 2015 20:24:11 +0200 Oleg Nesterov <oleg@redhat.com> wrote:

> 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.
> 
> ...
>
> +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;
> +}

Guys, please don't write write-only code.  This function is deeply
unobvious and I don't think a typical reader will have a hope of
understanding why things are this way.

I had a lame attempt:

--- a/mm/oom_kill.c~mm-oom_kill-fix-the-wrong-task-mm-==-mm-checks-in-oom_kill_process-fix
+++ a/mm/oom_kill.c
@@ -483,6 +483,12 @@ 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;
_

Which makes me wonder if "process_shared_mm" or even
"process_used_to_share_mm" would be better names...

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