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Commit 2f800fbd authored by Wu Fengguang's avatar Wu Fengguang
Browse files

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: default avatarJan Kara <jack@suse.cz>
Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: default avatarWu Fengguang <fengguang.wu@intel.com>
parent d3bc1fef

include/linux/writeback.h

+2 −0
Original line number Diff line number Diff line
@@ -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. */

mm/page-writeback.c

+19 −0
Original line number Diff line number Diff line
@@ -1824,6 +1824,24 @@ int __set_page_dirty_nobuffers(struct page *page) 
}

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
@@ -1832,6 +1850,7 @@ 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);