linux-kernel - Re: [PATCH 2/4] writeback: avoid duplicate balance_dirty_pages

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Date:	Thu, 14 Apr 2011 08:30:45 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dave Chinner <david@...morbit.com>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 2/4] writeback: avoid duplicate
 balance_dirty_pages_ratelimited() calls

On Thu, Apr 14, 2011 at 05:53:07AM +0800, Jan Kara wrote:
> On Wed 13-04-11 16:59:39, Wu Fengguang wrote:
> > When dd in 512bytes, balance_dirty_pages_ratelimited() could be called 8
> > times for the same page, but obviously the page is only dirtied once.
> > 
> > Fix it with a (slightly racy) PageDirty() test.
> > 
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > ---
> >  mm/filemap.c |    5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > --- linux-next.orig/mm/filemap.c	2011-04-13 16:46:01.000000000 +0800
> > +++ linux-next/mm/filemap.c	2011-04-13 16:47:26.000000000 +0800
> > @@ -2313,6 +2313,7 @@ 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).
> > @@ -2361,6 +2362,7 @@ again:
> >  		pagefault_enable();
> >  		flush_dcache_page(page);
> >  
> > +		dirty = PageDirty(page);
>   This isn't completely right as we sometimes dirty the page in
> ->write_begin() (see e.g. block_write_begin() when we allocate blocks under
> an already uptodate page) and in such cases we would not call
> balance_dirty_pages(). So I'm not sure we can really do this
> optimization (although it's sad)...

Good catch, thanks! I evaluated three possible options, the last one
looks most promising (however is a radical change).

- do radix_tree_tag_get() before calling ->write_begin()
  simple but heavy weight

- add balance_dirty_pages_ratelimited() in __block_write_begin()
  seems not easy, too

- accurately account the dirtied pages in account_page_dirtied() rather than
  in balance_dirty_pages_ratelimited_nr(). This diff on top of my patchset
  illustrates the idea, but will need to sort out cases like direct IO ...

--- linux-next.orig/mm/page-writeback.c	2011-04-14 07:50:09.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-04-14 07:52:35.000000000 +0800
@@ -1295,8 +1295,6 @@ void balance_dirty_pages_ratelimited_nr(
 	if (!bdi_cap_account_dirty(bdi))
 		return;
 
-	current->nr_dirtied += nr_pages_dirtied;
-
 	if (dirty_exceeded_recently(bdi, MAX_PAUSE)) {
 		unsigned long max = current->nr_dirtied +
 						(128 >> (PAGE_SHIFT - 10));
@@ -1752,6 +1750,7 @@ void account_page_dirtied(struct page *p
 		__inc_bdi_stat(mapping->backing_dev_info, BDI_DIRTIED);
 		task_dirty_inc(current);
 		task_io_account_write(PAGE_CACHE_SIZE);
+		current->nr_dirtied++;
 	}
 }
 EXPORT_SYMBOL(account_page_dirtied);

> >  		mark_page_accessed(page);
> >  		status = a_ops->write_end(file, mapping, pos, bytes, copied,
> >  						page, fsdata);
> > @@ -2387,7 +2389,8 @@ again:
> >  		pos += copied;
> >  		written += copied;
> >  
> > -		balance_dirty_pages_ratelimited(mapping);
> > +		if (!dirty)
> > +			balance_dirty_pages_ratelimited(mapping);
> >  
> >  	} while (iov_iter_count(i));
> 
> 								Honza
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
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