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Subject[PATCH 4/7] writeback: fix dirtied pages accounting on redirty
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).

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/linux/writeback.h | 2 ++
mm/page-writeback.c | 19 +++++++++++++++++++
2 files changed, 21 insertions(+)
--- linux-next.orig/mm/page-writeback.c 2011-11-28 21:23:23.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-11-28 21:23:24.000000000 +0800
@@ -1806,6 +1806,24 @@ int __set_page_dirty_nobuffers(struct pa
EXPORT_SYMBOL(__set_page_dirty_nobuffers);

/*
+ * Call this whenever redirtying a page, to de-account the dirty counters
+ * (NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied), so that they match the written
+ * counters (NR_WRITTEN, BDI_WRITTEN) in long term. The mismatches will lead to
+ * systematic errors in balanced_dirty_ratelimit and the dirty pages position
+ * control.
+ */
+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
* redirty_page_for_writepage() and it should then unlock the page and return 0
@@ -1813,6 +1831,7 @@ 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 2011-11-28 21:23:20.000000000 +0800
+++ linux-next/include/linux/writeback.h 2011-11-28 21:23:24.000000000 +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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