When doing 1KB sequential writes to the same page,
balance_dirty_pages_ratelimited_nr() should be called once instead of 4
times, the latter makes the dirtier tasks be throttled much too heavy.

Fix it with proper de-accounting on clear_page_dirty_for_io().

CC: Chris Mason <redacted>
Signed-off-by: Wu Fengguang <redacted>
---
 fs/btrfs/file.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- linux-next.orig/fs/btrfs/file.c	2011-08-29 19:14:32.000000000 +0800
+++ linux-next/fs/btrfs/file.c	2011-08-29 19:14:40.000000000 +0800

@@ -1138,7 +1138,8 @@ again:
 				     GFP_NOFS);
 	}
 	for (i = 0; i < num_pages; i++) {
-		clear_page_dirty_for_io(pages[i]);
+		if (clear_page_dirty_for_io(pages[i]))
+			account_page_redirty(pages[i]);
 		set_page_extent_mapped(pages[i]);
 		WARN_ON(!PageLocked(pages[i]));
 	}


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