[PATCH] sched: numa: ksm: fix oops in task_numa_placment()

[Hugh Dickins]

task_numa_placement() oopsed on NULL p->mm when task_numa_fault()
got called in the handling of break_ksm() for ksmd.  That might be a
peculiar case, which perhaps KSM could takes steps to avoid? but it's
more robust if task_numa_placement() allows for such a possibility.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 kernel/sched/fair.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- 3.7+git/kernel/sched/fair.c	2012-12-16 16:35:08.724441527 -0800
+++ linux/kernel/sched/fair.c	2012-12-18 21:37:24.727964195 -0800
@@ -793,8 +793,11 @@ unsigned int sysctl_numa_balancing_scan_
 
 static void task_numa_placement(struct task_struct *p)
 {
-	int seq = ACCESS_ONCE(p->mm->numa_scan_seq);
+	int seq;
 
+	if (!p->mm)	/* for example, ksmd faulting in a user's mm */
+		return;
+	seq = ACCESS_ONCE(p->mm->numa_scan_seq);
 	if (p->numa_scan_seq == seq)
 		return;
 	p->numa_scan_seq = seq;
--
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