linux-kernel - [PATCH] writeback: skip balance_dirty

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Date:	Fri, 17 Dec 2010 19:21:11 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Hugh Dickins <hughd@...gle.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: [PATCH] writeback: skip balance_dirty_pages() for in-memory fs

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>
---
 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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