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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: Jan Kara <jack@suse.cz>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
This commit is contained in:
Wu Fengguang 2011-08-08 15:22:00 -06:00
parent d3bc1fef93
commit 2f800fbd77
2 changed files with 21 additions and 0 deletions
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2
include/linux/writeback.h
View File

@ -197,6 +197,8 @@ void writeback_set_ratelimit(void);
void tag_pages_for_writeback(struct address_space *mapping, void tag_pages_for_writeback(struct address_space *mapping,
pgoff_t start, pgoff_t end); pgoff_t start, pgoff_t end);
void account_page_redirty(struct page *page);
/* pdflush.c */ /* pdflush.c */
extern int nr_pdflush_threads; /* Global so it can be exported to sysctl extern int nr_pdflush_threads; /* Global so it can be exported to sysctl
read-only. */ read-only. */

19
mm/page-writeback.c
View File

@ -1824,6 +1824,24 @@ int __set_page_dirty_nobuffers(struct page *page)
} }
EXPORT_SYMBOL(__set_page_dirty_nobuffers); 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 * When a writepage implementation decides that it doesn't want to write this
* page for some reason, it should redirty the locked page via * page for some reason, it should redirty the locked page via
@ -1832,6 +1850,7 @@ EXPORT_SYMBOL(__set_page_dirty_nobuffers);
int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page) int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
{ {
wbc->pages_skipped++; wbc->pages_skipped++;
account_page_redirty(page);
return __set_page_dirty_nobuffers(page); return __set_page_dirty_nobuffers(page);
} }
EXPORT_SYMBOL(redirty_page_for_writepage); EXPORT_SYMBOL(redirty_page_for_writepage);