Btrfs: Add readahead to the online shrinker, and a mount -o alloc_start= for testing

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/super.c

@@ -63,7 +63,8 @@ static void btrfs_put_super (struct super_block * sb)
 }
 
 enum {
-	Opt_subvol, Opt_nodatasum, Opt_nodatacow, Opt_max_extent, Opt_err,
+	Opt_subvol, Opt_nodatasum, Opt_nodatacow, Opt_max_extent,
+	Opt_alloc_start, Opt_err,
 };
 
 static match_table_t tokens = {
@@ -71,6 +72,7 @@ static match_table_t tokens = {
 	{Opt_nodatasum, "nodatasum"},
 	{Opt_nodatacow, "nodatacow"},
 	{Opt_max_extent, "max_extent=%s"},
+	{Opt_alloc_start, "alloc_start=%s"},
 	{Opt_err, NULL}
 };
 
@@ -162,6 +164,18 @@ static int parse_options (char * options,
 				}
 			}
 			break;
+		case Opt_alloc_start:
+			if (info) {
+				char *num = match_strdup(&args[0]);
+				if (num) {
+					info->alloc_start =
+						btrfs_parse_size(num);
+					kfree(num);
+					printk("btrfs: allocations start at "
+					       "%Lu\n", info->alloc_start);
+				}
+			}
+			break;
 		default:
 			break;
 		}