[PATCH] Early writeback initialisation

Patch from Anders Gustafsson <andersg@0x63.nu>

We're getting a division-by-zero in the writeback code during early rootfs
population, because writeback has not yet been initialised.

Fix that by performing an explicit initialisation rather than relying on
initcall ordering.

include/linux/writeback.h

@@ -79,6 +79,7 @@ extern int dirty_writeback_centisecs;
 extern int dirty_expire_centisecs;
 
 
+void page_writeback_init(void);
 void balance_dirty_pages(struct address_space *mapping);
 void balance_dirty_pages_ratelimited(struct address_space *mapping);
 int pdflush_operation(void (*fn)(unsigned long), unsigned long arg0);

init/main.c

@@ -35,6 +35,7 @@
 #include <linux/profile.h>
 #include <linux/rcupdate.h>
 #include <linux/moduleparam.h>
+#include <linux/writeback.h>
 
 #include <asm/io.h>
 #include <asm/bugs.h>
@@ -439,6 +440,8 @@ asmlinkage void __init start_kernel(void)
 	vfs_caches_init(num_physpages);
 	radix_tree_init();
 	signals_init();
+	/* rootfs populating might need page-writeback */
+	page_writeback_init();
 	populate_rootfs();
 #ifdef CONFIG_PROC_FS
 	proc_root_init();

mm/page-writeback.c

@@ -369,7 +369,7 @@ static struct notifier_block ratelimit_nb = {
  * dirty memory thresholds: allowing too much dirty highmem pins an excessive
  * number of buffer_heads.
  */
-static int __init page_writeback_init(void)
+void __init page_writeback_init(void)
 {
 	long buffer_pages = nr_free_buffer_pages();
 	long correction;
@@ -392,9 +392,7 @@ static int __init page_writeback_init(void)
 	add_timer(&wb_timer);
 	set_ratelimit();
 	register_cpu_notifier(&ratelimit_nb);
-	return 0;
 }
-module_init(page_writeback_init);
 
 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
 {