Commit dd12805ed1db7 in the linux-next kernel repository, titled

"XArray: Remove radix tree compatibility", changes the definition
of "radix_tree_root" back to be a struct.  However, the content of
the new structure differs from the original structure, so without
the patch, current linux-next kernels fail during initialization
with the error message "radix trees do not exist or have changed
their format".  Because the new "radix_tree_root" and "xarray"
structures have nearly the same layout, the existing functionality
for XArrays can be reused.
([email protected])

Author: Dave Anderson

bpf.c

@@ -221,7 +221,12 @@ bpf_init(struct bpf_info *bpf)
 			bpf->idr_type = IDR_ORIG;
 			do_old_idr(IDR_ORIG_INIT, 0, NULL);
 		} else if (STREQ(MEMBER_TYPE_NAME("idr", "idr_rt"), "radix_tree_root"))
-			bpf->idr_type = IDR_RADIX;
+			if (MEMBER_EXISTS("radix_tree_root", "rnode"))
+				bpf->idr_type = IDR_RADIX;
+			else if (MEMBER_EXISTS("radix_tree_root", "xa_head"))
+				bpf->idr_type = IDR_XARRAY;
+			else
+				error(FATAL, "cannot determine IDR list type\n");
 		else if (STREQ(MEMBER_TYPE_NAME("idr", "idr_rt"), "xarray"))
 			bpf->idr_type = IDR_XARRAY;
 		else

filesys.c

@@ -2217,7 +2217,9 @@ dump_inode_page_cache_info(ulong inode)
 
 	xarray = root_rnode = count = 0;
 	if (MEMBER_EXISTS("address_space", "i_pages") &&
-	    STREQ(MEMBER_TYPE_NAME("address_space", "i_pages"), "xarray"))
+	    (STREQ(MEMBER_TYPE_NAME("address_space", "i_pages"), "xarray") ||
+	    (STREQ(MEMBER_TYPE_NAME("address_space", "i_pages"), "radix_tree_root") &&
+	     MEMBER_EXISTS("radix_tree_root", "xa_head"))))
 		xarray = i_mapping + OFFSET(address_space_page_tree);
 	else 
 		root_rnode = i_mapping + OFFSET(address_space_page_tree);

ipcs.c

@@ -205,8 +205,12 @@ ipcs_init(void)
 	if (VALID_MEMBER(idr_idr_rt)) {
 		if (STREQ(MEMBER_TYPE_NAME("idr", "idr_rt"), "xarray"))
 			ipcs_table.init_flags |= IDR_XARRAY;
-		else
-			ipcs_table.init_flags |= IDR_RADIX;
+		else {
+			if (MEMBER_EXISTS("radix_tree_root", "rnode"))
+				ipcs_table.init_flags |= IDR_RADIX;
+			else if (MEMBER_EXISTS("radix_tree_root", "xa_head"))
+				ipcs_table.init_flags |= IDR_XARRAY;
+		}
 	} else
 		ipcs_table.init_flags |= IDR_ORIG;
 

kernel.c

@@ -534,9 +534,14 @@ kernel_init()
 
 	if (kernel_symbol_exists("irq_desc_tree")) {
 		get_symbol_type("irq_desc_tree", NULL, &req);
-		kt->flags2 |= STREQ(req.type_tag_name, "xarray") ?
-			IRQ_DESC_TREE_XARRAY : IRQ_DESC_TREE_RADIX;
-
+		if (STREQ(req.type_tag_name, "xarray")) {
+			kt->flags2 |= IRQ_DESC_TREE_XARRAY;
+		} else {
+			if (MEMBER_EXISTS("radix_tree_root", "xa_head"))
+				kt->flags2 |= IRQ_DESC_TREE_XARRAY;
+			else
+				kt->flags2 |= IRQ_DESC_TREE_RADIX;
+		}
 	}
 	STRUCT_SIZE_INIT(irq_data, "irq_data");
 	if (VALID_STRUCT(irq_data)) {

task.c

@@ -507,10 +507,17 @@ task_init(void)
 				OFFSET(pid_namespace_idr) + OFFSET(idr_idr_rt);
 			tt->flags |= PID_XARRAY;
 		} else if STREQ(MEMBER_TYPE_NAME("idr", "idr_rt"), "radix_tree_root") {
-			tt->refresh_task_table = refresh_radix_tree_task_table;
-			tt->pid_radix_tree = symbol_value("init_pid_ns") +
-				OFFSET(pid_namespace_idr) + OFFSET(idr_idr_rt);
-			tt->flags |= PID_RADIX_TREE;
+			if (MEMBER_EXISTS("radix_tree_root", "rnode")) {
+				tt->refresh_task_table = refresh_radix_tree_task_table;
+				tt->pid_radix_tree = symbol_value("init_pid_ns") +
+					OFFSET(pid_namespace_idr) + OFFSET(idr_idr_rt);
+				tt->flags |= PID_RADIX_TREE;
+			} else if (MEMBER_EXISTS("radix_tree_root", "xa_head")) {
+				tt->refresh_task_table = refresh_xarray_task_table;
+				tt->pid_xarray = symbol_value("init_pid_ns") +
+					OFFSET(pid_namespace_idr) + OFFSET(idr_idr_rt);
+				tt->flags |= PID_XARRAY;
+			}
 		} else 
 			error(FATAL, "unknown pid_namespace.idr type: %s\n",
 				MEMBER_TYPE_NAME("idr", "idr_rt"));

tools.c

@@ -4241,7 +4241,10 @@ cmd_tree()
 			}
 
 			if (STRNEQ(optarg, "ra"))
-				type_flag = RADIXTREE_REQUEST;
+				if (MEMBER_EXISTS("radix_tree_root", "xa_head"))
+					type_flag = XARRAY_REQUEST;
+				else
+					type_flag = RADIXTREE_REQUEST;
 			else if (STRNEQ(optarg, "rb"))
 				type_flag = RBTREE_REQUEST;
 			else if (STRNEQ(optarg, "x"))