sched/numa: Check current->mm before allocating NUMA faults

task_numa_placement checks current->mm but after buffers for faults
have already been uselessly allocated. Move the check earlier.

[peterz@infradead.org: Identified the problem]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1381141781-10992-27-git-send-email-mgorman@suse.deSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -930,8 +930,6 @@ static void task_numa_placement(struct task_struct *p)
 	int seq, nid, max_nid = -1;
 	unsigned long max_faults = 0;
 
-	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;
@@ -998,6 +996,10 @@ void task_numa_fault(int last_nid, int node, int pages, bool migrated)
 	if (!numabalancing_enabled)
 		return;
 
+	/* for example, ksmd faulting in a user's mm */
+	if (!p->mm)
+		return;
+
 	/* For now, do not attempt to detect private/shared accesses */
 	priv = 1;