[PATCH 04/13] writeback: prevent duplicate balance_dirty_pages_ratelimited() calls

[Wu Fengguang <fengguang.wu@intel.com>]

Andrew,
References: <20101117035821.000579293@intel.com>
Content-Disposition: inline; filename=writeback-fix-duplicate-bdp-calls.patch

When dd in 512bytes, balance_dirty_pages_ratelimited() used to be called
8 times for the same page, even if the page is only dirtied once. Fix it
with a (slightly racy) PageDirty() test.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/filemap.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- linux-next.orig/mm/filemap.c	2010-11-16 22:20:08.000000000 +0800
+++ linux-next/mm/filemap.c	2010-11-16 22:45:05.000000000 +0800
@@ -2258,6 +2258,7 @@ static ssize_t generic_perform_write(str
 	long status = 0;
 	ssize_t written = 0;
 	unsigned int flags = 0;
+	unsigned int dirty;
 
 	/*
 	 * Copies from kernel address space cannot fail (NFSD is a big user).
@@ -2306,6 +2307,7 @@ again:
 		pagefault_enable();
 		flush_dcache_page(page);
 
+		dirty = PageDirty(page);
 		mark_page_accessed(page);
 		status = a_ops->write_end(file, mapping, pos, bytes, copied,
 						page, fsdata);
@@ -2332,7 +2334,8 @@ again:
 		pos += copied;
 		written += copied;
 
-		balance_dirty_pages_ratelimited(mapping);
+		if (!dirty)
+			balance_dirty_pages_ratelimited(mapping);
 
 	} while (iov_iter_count(i));