re: sched, numa, mm: Implement constant, per task Working Set Sampling (WSS) rate

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Peter Zijlstra,

The patch 3d049f8a5398: "sched, numa, mm: Implement constant, per
task Working Set Sampling (WSS) rate" from Oct 14, 2012, leads to the
following warning:
kernel/sched/fair.c:954 task_numa_work()
	 error: we previously assumed 'vma' could be null (see line 948)

   943          if (!vma) {
   944                  ACCESS_ONCE(mm->numa_scan_seq)++;
   945                  offset = 0;
   946                  vma = mm->mmap;
   947          }
   948          while (vma && !vma_migratable(vma)) {
                       ^^^
If this is NULL,
   949                  vma = vma->vm_next;
   950                  if (!vma)
   951                          goto again;
   952          }
   953  
   954          offset = max(offset, vma->vm_start);
                                     ^^^^^^^^^^^^^
then it leads to a NULL dereference here.

regards,
dan carpenter