writeback: fix time ordering of the per superblock dirty inode lists 4

When the kupdate function has tried to write back an expired inode it will
then check to see whether some of the inode's pages are still dirty.

This can happen when the filesystem decided to not write a page for some
reason.  But it does _not_ occur due to redirtyings: a redirtying will set
I_DIRTY_PAGES.

What we need to do here is to set I_DIRTY_PAGES to reflect reality and to then
put the inode onto the _head_ of s_dirty for consideration on the next kupdate
pass, in five seconds time.

Problem is, the code failed to modify the inode's timestamp when pushing the
inode onto thehead of s_dirty.

The patch:

If there are no other inodes on s_dirty then we leave the inode's timestamp
alone: it is already expired.

If there _are_ other inodes on s_dirty then we arrange for this inode to get
the same timestamp as the inode which is at the head of s_dirty, thus
preserving the s_dirty ordering.  But we only need to do this if this inode
purports to have been dirtied before the one at head-of-list.

Cc: Mike Waychison <mikew@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fs-writeback.c

@@ -164,6 +164,28 @@ static void redirty_tail(struct inode *inode)
 	list_move(&inode->i_list, &sb->s_dirty);
 }
 
+/*
+ * Redirty an inode, but mark it as the very next-to-be-written inode on its
+ * superblock's dirty-inode list.
+ * We need to preserve s_dirty's reverse-time-orderedness, so we cheat by
+ * setting this inode's dirtied_when to the same value as that of the inode
+ * which is presently head-of-list, if present head-of-list is newer than this
+ * inode. (head-of-list is the least-recently-dirtied inode: the oldest one).
+ */
+static void redirty_head(struct inode *inode)
+{
+	struct super_block *sb = inode->i_sb;
+
+	if (!list_empty(&sb->s_dirty)) {
+		struct inode *head_inode;
+
+		head_inode = list_entry(sb->s_dirty.prev, struct inode, i_list);
+		if (time_after(inode->dirtied_when, head_inode->dirtied_when))
+			inode->dirtied_when = head_inode->dirtied_when;
+	}
+	list_move_tail(&inode->i_list, &sb->s_dirty);
+}
+
 /*
  * Write a single inode's dirty pages and inode data out to disk.
  * If `wait' is set, wait on the writeout.
@@ -225,7 +247,7 @@ __sync_single_inode(struct inode *inode, struct writeback_control *wbc)
 				 * uncongested.
 				 */
 				inode->i_state |= I_DIRTY_PAGES;
-				list_move_tail(&inode->i_list, &sb->s_dirty);
+				redirty_head(inode);
 			} else {
 				/*
 				 * Otherwise fully redirty the inode so that