From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [RFC] vfs/inode: For none-block-based filesystems default to
 sb->s_bdi
Date: Wed, 6 Oct 2010 04:55:17 -0400
Message-ID: <20101006085517.GB7295@lst.de>
References: <4CAA4B1D.1010904@panasas.com>
 <4CAA4EE5.2070308@panasas.com>
 <20101005083208.GA3514@quack.suse.cz>
 <4CAB2DE5.90801@panasas.com>
 <20101005150913.GB14516@quack.suse.cz>
 <4CAB4473.1070408@panasas.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Jan Kara <jack@suse.cz>, Christoph Hellwig <hch@infradead.org>,
	Al Viro <viro@ZenIV.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Trond Myklebust <Trond.Myklebust@netapp.com>,
	Benny Halevy <bhalevy@panasas.com>
To: Boaz Harrosh <bharrosh@panasas.com>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
Received: from canuck.infradead.org ([134.117.69.58]:46042 "EHLO
	canuck.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754245Ab0JFUdc (ORCPT
	<rfc822;linux-fsdevel@vger.kernel.org>);
	Wed, 6 Oct 2010 16:33:32 -0400
Content-Disposition: inline
In-Reply-To: <4CAB4473.1070408@panasas.com>
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>

On Tue, Oct 05, 2010 at 11:29:55AM -0400, Boaz Harrosh wrote:
> If you are right and inode->i_mapping.backing_dev_info will only be used for bdev
> inodes, then I prefer if it would be NULL by default and not pointing to
> &default_backing_dev_info. Will that be fixed as well?

It's been pointing to default_backing_dev_info since long before we got
the per-bdi writeback.  Once the writeback bits are're back to what we used
the BDI for before:

 - set VM readahead parameters
 - deal with per-backingdev congestion/unplugging
 - a couple of VM statistics

in general all these really should point to the same for a normal filesystem,
and at least point to the same in a group even for "strange" filesystems
that have multiple backing devices.  The BDI still is a rather strangely
defined beast, and so far all my attempts to make sense of it have failed.