linux-kernel - Re: [PATCH 08/10] mm: count writeback pages per BDI

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Message-Id: <20070421025525.042ed73a.akpm@linux-foundation.org>
Date:	Sat, 21 Apr 2007 02:55:25 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu, neilb@...e.de, dgc@....com,
	tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com
Subject: Re: [PATCH 08/10] mm: count writeback pages per BDI

On Fri, 20 Apr 2007 17:52:02 +0200 Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> Count per BDI writeback pages.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  include/linux/backing-dev.h |    1 +
>  mm/page-writeback.c         |   12 ++++++++++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/mm/page-writeback.c
> ===================================================================
> --- linux-2.6.orig/mm/page-writeback.c	2007-04-20 15:27:28.000000000 +0200
> +++ linux-2.6/mm/page-writeback.c	2007-04-20 15:28:10.000000000 +0200
> @@ -979,14 +979,18 @@ int test_clear_page_writeback(struct pag
>  	int ret;
>  
>  	if (mapping) {
> +		struct backing_dev_info *bdi = mapping->backing_dev_info;
>  		unsigned long flags;
>  
>  		write_lock_irqsave(&mapping->tree_lock, flags);
>  		ret = TestClearPageWriteback(page);
> -		if (ret)
> +		if (ret) {
>  			radix_tree_tag_clear(&mapping->page_tree,
>  						page_index(page),
>  						PAGECACHE_TAG_WRITEBACK);
> +			if (bdi_cap_writeback_dirty(bdi))
> +				__dec_bdi_stat(bdi, BDI_WRITEBACK);

Why do we test bdi_cap_writeback_dirty() here?

If we remove that test, we end up accumulating statistics for
non-writebackable backing devs, but does that matter?  Probably the common
case is writebackable backing-devs, so eliminating the test-n-branch might
be a net microgain.

-
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