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

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

Ah.. indeed so. There's also what looks like an infinite loop in there if nothing is migratable or if length is stupid large. The below should avoid the reported NULL deref as well as break out when we've reached the end of the address space.

---
 kernel/sched/fair.c |   23 ++++++++---------------
 1 files changed, 8 insertions(+), 15 deletions(-)

index c68b877..5a6d8f5 100644
@@ -939,29 +939,22 @@ void task_numa_work(struct callback_head *work)
 
 	down_read(&mm->mmap_sem);
 	vma = find_vma(mm, offset);
-again:
 	if (!vma) {
 		ACCESS_ONCE(mm->numa_scan_seq)++;
 		offset = 0;
 		vma = mm->mmap;
 	}
-	while (vma && !vma_migratable(vma)) {
-		vma = vma->vm_next;
-		if (!vma)
-			goto again;
-	}
-
-	offset = max(offset, vma->vm_start);
-	end = min(ALIGN(offset + length, HPAGE_SIZE), vma->vm_end);
-	length -= end - offset;
+	for (; vma && length > 0; vma = vma->vm_next) {
+		if (!vma_migratable(vma))
+			continue;
 
-	change_prot_none(vma, offset, end);
+		offset = max(offset, vma->vm_start);
+		end = min(ALIGN(offset + length, HPAGE_SIZE), vma->vm_end);
+		length -= end - offset;
 
-	offset = end;
+		change_prot_none(vma, offset, end);
 
-	if (length > 0) {
-		vma = vma->vm_next;
-		goto again;
+		offset = end;
 	}
 	mm->numa_scan_offset = offset;
 	up_read(&mm->mmap_sem);