linux-kernel - Re: [PATCH] writeback: skip balance_dirty

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Message-ID: <alpine.LSU.2.00.1012202127310.16112@tigran.mtv.corp.google.com>
Date:	Mon, 20 Dec 2010 21:59:46 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Christoph Hellwig <hch@....de>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Dave Chinner <david@...morbit.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] writeback: skip balance_dirty_pages() for in-memory fs

On Fri, 17 Dec 2010, Wu Fengguang wrote:

> This avoids unnecessary checks and dirty throttling on tmpfs/ramfs.
> 
> It also prevents
> 
> [  388.126563] BUG: unable to handle kernel NULL pointer dereference at 0000000000000050
> 
> in the balance_dirty_pages tracepoint, which will call
> 
> 	dev_name(mapping->backing_dev_info->dev)
> 
> but shmem_backing_dev_info.dev is NULL.
> 
> CC: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>

Whilst I do like this change, and I do think it's the right thing to do
(given that the bdi has explicitly opted out of what it then got into),
I've a sneaking feeling that something somewhere may show a regression
from it.  IIRC, there were circumstances in which it actually did
(inadvertently) end up throttling the tmpfs writing - if there were
too many dirty non-tmpfs pages around??

What am I saying?!  I think I'm asking you to look more closely at what
actually used to happen, and be more explicit about the behavior you're
stopping here - although the patch is mainly code optimization, there
is some functional change I think.  (You do mention throttling on
tmpfs/ramfs, but the way it worked out wasn't straightforward.)

I'd better not burble on for a third paragraph!

Hugh

> ---
>  mm/page-writeback.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> --- linux-next.orig/mm/page-writeback.c	2010-12-17 19:09:19.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2010-12-17 19:09:22.000000000 +0800
> @@ -899,6 +899,9 @@ void balance_dirty_pages_ratelimited_nr(
>  {
>  	struct backing_dev_info *bdi = mapping->backing_dev_info;
>  
> +	if (!bdi_cap_account_dirty(bdi))
> +		return;
> +
>  	current->nr_dirtied += nr_pages_dirtied;
>  
>  	if (unlikely(!current->nr_dirtied_pause))
--
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