[PATCH 4/5] writeback: fix dirtied pages accounting on redirty

[Wu Fengguang]

De-account the accumulative dirty counters on page redirty.

Page redirties (very common in ext4) will introduce mismatch between
counters (a) and (b)

a) NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied
b) NR_WRITTEN, BDI_WRITTEN

This will introduce systematic errors in balanced_rate and result in
dirty page position errors (ie. the dirty pages are no longer balanced
around the global/bdi setpoints).

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 include/linux/writeback.h |    2 ++
 mm/page-writeback.c       |   12 ++++++++++++
 2 files changed, 14 insertions(+)

--- linux-next.orig/mm/page-writeback.c	2011-11-17 20:57:15.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-11-17 20:57:16.000000000 +0800
@@ -1792,6 +1792,17 @@ int __set_page_dirty_nobuffers(struct pa
 }
 EXPORT_SYMBOL(__set_page_dirty_nobuffers);
 
+void account_page_redirty(struct page *page)
+{
+	struct address_space *mapping = page->mapping;
+	if (mapping && mapping_cap_account_dirty(mapping)) {
+		current->nr_dirtied--;
+		dec_zone_page_state(page, NR_DIRTIED);
+		dec_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
+	}
+}
+EXPORT_SYMBOL(account_page_redirty);
+
 /*
  * When a writepage implementation decides that it doesn't want to write this
  * page for some reason, it should redirty the locked page via
@@ -1800,6 +1811,7 @@ EXPORT_SYMBOL(__set_page_dirty_nobuffers
 int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
 {
 	wbc->pages_skipped++;
+	account_page_redirty(page);
 	return __set_page_dirty_nobuffers(page);
 }
 EXPORT_SYMBOL(redirty_page_for_writepage);
--- linux-next.orig/include/linux/writeback.h	2011-11-17 20:57:12.000000000 +0800
+++ linux-next/include/linux/writeback.h	2011-11-17 20:57:16.000000000 +0800
@@ -197,6 +197,8 @@ void writeback_set_ratelimit(void);
 void tag_pages_for_writeback(struct address_space *mapping,
 			     pgoff_t start, pgoff_t end);
 
+void account_page_redirty(struct page *page);
+
 /* pdflush.c */
 extern int nr_pdflush_threads;	/* Global so it can be exported to sysctl
 				   read-only. */


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