Re: [PATCH 05/10] mm, oom: skip vforked tasks from being selected

On Wed 15-06-16 16:51:06, Oleg Nesterov wrote:

On 06/09, Michal Hocko wrote:

quoted

--- a/include/linux/sched.h
+++ b/include/linux/sched.h

@@ -1883,6 +1883,32 @@ extern int arch_task_struct_size __read_mostly;
 #define TNF_FAULT_LOCAL	0x08
 #define TNF_MIGRATE_FAIL 0x10
 
+static inline bool in_vfork(struct task_struct *tsk)
+{
+	bool ret;
+
+	/*
+	 * need RCU to access ->real_parent if CLONE_VM was used along with
+	 * CLONE_PARENT.
+	 *
+	 * We check real_parent->mm == tsk->mm because CLONE_VFORK does not
+	 * imply CLONE_VM
+	 *
+	 * CLONE_VFORK can be used with CLONE_PARENT/CLONE_THREAD and thus
+	 * ->real_parent is not necessarily the task doing vfork(), so in
+	 * theory we can't rely on task_lock() if we want to dereference it.
+	 *
+	 * And in this case we can't trust the real_parent->mm == tsk->mm
+	 * check, it can be false negative. But we do not care, if init or
+	 * another oom-unkillable task does this it should blame itself.
+	 */
+	rcu_read_lock();
+	ret = tsk->vfork_done && tsk->real_parent->mm == tsk->mm;
+	rcu_read_unlock();
+
+	return ret;
+}

ACK, but why sched.h ? It has a single caller in oom_kill.c.

It felt like generally reusable helper to get subtle details about the
vfork right.
-- 
Michal Hocko
SUSE Labs

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