[PATCH] writeback efficiency and QoS improvements

The radix-tree walk for writeback has a couple of problems:

a) It always scans a file from its first dirty page, so if someone
   is repeatedly dirtying the front part of a file, pages near the end
   may be starved of writeout.  (Well, not completely: the `kupdate'
   function will write an entire file once the file's dirty timestamp
   has expired).  

b) When the disk queues are huge (10000 requests), there can be a
   very large number of locked pages.  Scanning past these in writeback
   consumes quite some CPU time.

So in each address_space we record the index at which the last batch of
writeout terminated and start the next batch of writeback from that
point.

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index dc8c46fb4b69..bacf6bcbc7b7 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -324,6 +324,7 @@ struct address_space {
 	struct radix_tree_root	page_tree;	/* radix tree of all pages */
 	spinlock_t		tree_lock;	/* and spinlock protecting it */
 	unsigned long		nrpages;	/* number of total pages */
+	pgoff_t			writeback_index;/* writeback starts here */
 	struct address_space_operations *a_ops;	/* methods */
 	struct list_head	i_mmap;		/* list of private mappings */
 	struct list_head	i_mmap_shared;	/* list of shared mappings */