mm/mempolicy.c:offset_il_node() document and clarify

This code was pretty obscure and was relying upon obscure side-effects
of next_node(-1, ...) and was relying upon NUMA_NO_NODE being equal to
-1.

Clean that all up and document the function's intent.
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Cc: Joonsoo Kim <js1304@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -1758,23 +1758,25 @@ unsigned int mempolicy_slab_node(void)
 	}
 }
 
-/* Do static interleaving for a VMA with known offset. */
+/*
+ * Do static interleaving for a VMA with known offset @n.  Returns the n'th
+ * node in pol->v.nodes (starting from n=0), wrapping around if n exceeds the
+ * number of present nodes.
+ */
 static unsigned offset_il_node(struct mempolicy *pol,
-		struct vm_area_struct *vma, unsigned long off)
+			       struct vm_area_struct *vma, unsigned long n)
 {
 	unsigned nnodes = nodes_weight(pol->v.nodes);
 	unsigned target;
-	int c;
-	int nid = NUMA_NO_NODE;
+	int i;
+	int nid;
 
 	if (!nnodes)
 		return numa_node_id();
-	target = (unsigned int)off % nnodes;
-	c = 0;
-	do {
+	target = (unsigned int)n % nnodes;
+	nid = first_node(pol->v.nodes);
+	for (i = 0; i < target; i++)
 		nid = next_node(nid, pol->v.nodes);
-		c++;
-	} while (c <= target);
 	return nid;
 }