[PATCH] Properly init address_space->writeback_index

[Chris Mason <chris.mason@oracle.com>]

write_cache_pages uses i_mapping->writeback_index to pick up where it
left off the last time a given inode was found by pdflush or
balance_dirty_pages (or anyone else who sets wbc->range_cyclic)

alloc_inode should set it to a sane value so that writeback doesn't start
in the middle of a file.  It is somewhat difficult to notice the bug since
write_cache_pages will loop around to the start of the file and the
elevator helps hide the resulting seeks.

For whatever reason, Btrfs hits this often.  Unpatched, untarring 30 copies of
the linux kernel in series runs at 47MB/s on a single sata drive.  With
this fix, it jumps to 62MB/s.

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

diff -r 67160ae21a58 fs/inode.c
--- a/fs/inode.c	Wed Aug 06 19:26:20 2008 -0700
+++ b/fs/inode.c	Thu Aug 14 10:15:49 2008 -0400
@@ -166,6 +166,7 @@ static struct inode *alloc_inode(struct 
 		mapping_set_gfp_mask(mapping, GFP_HIGHUSER_PAGECACHE);
 		mapping->assoc_mapping = NULL;
 		mapping->backing_dev_info = &default_backing_dev_info;
+		mapping->writeback_index = 0;
 
 		/*
 		 * If the block_device provides a backing_dev_info for client