libceph: fallback for when there isn't a pool-specific choose_arg

There is now a fallback to a choose_arg index of -1 if there isn't
a pool-specific choose_arg set.  If you create a per-pool weight-set,
that works for that pool.  Otherwise we try the compat/default one.  If
that doesn't exist either, then we use the normal CRUSH weights.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>

include/linux/crush/crush.h

@@ -193,7 +193,7 @@ struct crush_choose_arg {
 struct crush_choose_arg_map {
 #ifdef __KERNEL__
 	struct rb_node node;
-	u64 choose_args_index;
+	s64 choose_args_index;
 #endif
 	struct crush_choose_arg *args; /*!< replacement for each bucket
                                             in the crushmap */

net/ceph/osdmap.c

@@ -2301,10 +2301,17 @@ static u32 raw_pg_to_pps(struct ceph_pg_pool_info *pi,
 	}
 }
 
+/*
+ * Magic value used for a "default" fallback choose_args, used if the
+ * crush_choose_arg_map passed to do_crush() does not exist.  If this
+ * also doesn't exist, fall back to canonical weights.
+ */
+#define CEPH_DEFAULT_CHOOSE_ARGS	-1
+
 static int do_crush(struct ceph_osdmap *map, int ruleno, int x,
 		    int *result, int result_max,
 		    const __u32 *weight, int weight_max,
-		    u64 choose_args_index)
+		    s64 choose_args_index)
 {
 	struct crush_choose_arg_map *arg_map;
 	int r;
@@ -2313,6 +2320,9 @@ static int do_crush(struct ceph_osdmap *map, int ruleno, int x,
 
 	arg_map = lookup_choose_arg_map(&map->crush->choose_args,
 					choose_args_index);
+	if (!arg_map)
+		arg_map = lookup_choose_arg_map(&map->crush->choose_args,
+						CEPH_DEFAULT_CHOOSE_ARGS);
 
 	mutex_lock(&map->crush_workspace_mutex);
 	r = crush_do_rule(map->crush, ruleno, x, result, result_max,