[PATCH 04/13] writeback: introduce super_block.s_more_io_wait

[Fengguang Wu]

Introduce super_block.s_more_io_wait to park inodes that for some reason cannot
be synced immediately. They will be revisited in the next s_io enqueue time(<=5s).

The new data flow after this patchset:

s_dirty --> s_io --> s_more_io/s_more_io_wait --+
             ^                                  |
	     |                                  |
	     +----------------------------------+

- to fill s_io:
		s_more_io +
		s_dirty(expired) +
		s_more_io_wait
				---> s_io
- to drain s_io:
		s_io -+--> clean inodes goto inode_in_use/inode_unused
		      |
		      +--> s_more_io
		      |
		      +--> s_more_io_wait

Obviously there're no ordering or starvation problems in the queues:
- s_dirty is now a strict FIFO queue
- inode.dirtied_when is only set when made dirty
- once exipired, the dirty inode will stay in s_*io* queues until made clean
- the dirty inodes in s_*io* will be revisted in order, hence small files won't
  be starved by big dirty files.

Cc: Michael Rubin <mrubin@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Fengguang Wu <wfg@xxxxxxxxxxxxxxxx>
---
 fs/fs-writeback.c  |   16 +++++++++++++---
 fs/super.c         |    1 +
 include/linux/fs.h |    1 +
 3 files changed, 15 insertions(+), 3 deletions(-)

--- linux-mm.orig/fs/fs-writeback.c
+++ linux-mm/fs/fs-writeback.c
@@ -172,6 +172,14 @@ static void requeue_io(struct inode *ino
 	list_move(&inode->i_list, &inode->i_sb->s_more_io);
 }
 
+/*
+ * The inode should be retried after _sleeping_ for a while.
+ */
+static void requeue_io_wait(struct inode *inode)
+{
+	list_move(&inode->i_list, &inode->i_sb->s_more_io_wait);
+}
+
 static void inode_sync_complete(struct inode *inode)
 {
 	/*
@@ -206,13 +214,15 @@ static void queue_io(struct super_block 
 {
 	list_splice_init(&sb->s_more_io, sb->s_io.prev);
 	move_expired_inodes(&sb->s_dirty, &sb->s_io, older_than_this);
+	list_splice_init(&sb->s_more_io_wait, sb->s_io.prev);
 }
 
 int sb_has_dirty_inodes(struct super_block *sb)
 {
-	return !list_empty(&sb->s_dirty) ||
-	       !list_empty(&sb->s_io) ||
-	       !list_empty(&sb->s_more_io);
+	return !list_empty(&sb->s_dirty)   ||
+	       !list_empty(&sb->s_io)      ||
+	       !list_empty(&sb->s_more_io) ||
+	       !list_empty(&sb->s_more_io_wait);
 }
 EXPORT_SYMBOL(sb_has_dirty_inodes);
 
--- linux-mm.orig/fs/super.c
+++ linux-mm/fs/super.c
@@ -64,6 +64,7 @@ static struct super_block *alloc_super(s
 		INIT_LIST_HEAD(&s->s_dirty);
 		INIT_LIST_HEAD(&s->s_io);
 		INIT_LIST_HEAD(&s->s_more_io);
+		INIT_LIST_HEAD(&s->s_more_io_wait);
 		INIT_LIST_HEAD(&s->s_files);
 		INIT_LIST_HEAD(&s->s_instances);
 		INIT_HLIST_HEAD(&s->s_anon);
--- linux-mm.orig/include/linux/fs.h
+++ linux-mm/include/linux/fs.h
@@ -1011,6 +1011,7 @@ struct super_block {
 	struct list_head	s_dirty;	/* dirty inodes */
 	struct list_head	s_io;		/* parked for writeback */
 	struct list_head	s_more_io;	/* parked for more writeback */
+	struct list_head	s_more_io_wait;	/* parked for sleep-then-retry */
 	struct hlist_head	s_anon;		/* anonymous dentries for (nfs) exporting */
 	struct list_head	s_files;
 

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