linux-kernel - Re: Dirtiable inode bdi default != sb bdi btrfs

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Message-ID: <20100929141006.GB7439@lst.de>
Date:	Wed, 29 Sep 2010 16:10:06 +0200
From:	Christoph Hellwig <hch@....de>
To:	Jan Kara <jack@...e.cz>
Cc:	Christoph Hellwig <hch@....de>,
	Chris Mason <chris.mason@...cle.com>,
	Cesar Eduardo Barros <cesarb@...arb.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Jens Axboe <jaxboe@...ionio.com>,
	linux-btrfs@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, stable@...nel.org,
	Jens Axboe <axboe@...nel.dk>,
	Micha?? Piotrowski <mkkp4x4@...il.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	kernel@...ts.fedoraproject.org
Subject: Re: Dirtiable inode bdi default != sb bdi btrfs

On Wed, Sep 29, 2010 at 02:18:08PM +0200, Jan Kara wrote:
> On Wed 29-09-10 10:19:36, Christoph Hellwig wrote:
> > ---
> > From: Christoph Hellwig <hch@....de>
> > Subject: [PATCH] writeback: always use sb->s_bdi for writeback purposes
> > 
> ...
> > The one exception for now is the block device filesystem which really
> > wants different writeback contexts for it's different (internal) inodes
> > to handle the writeout more efficiently.  For now we do this with
> > a hack in fs-writeback.c because we're so late in the cycle, but in
> > the future I plan to replace this with a superblock method that allows
> > for multiple writeback contexts per filesystem.
>   Another exception I know about is mtd_inodefs filesystem
> (drivers/mtd/mtdchar.c).

No, it's not.  MTD only has three different backing_dev_info instances
which have different flags in the mapping-relevant portion of the
backing_dev. 

>   So at least here you'd need also add a similar exception for
> "mtd_inodefs".

No.  For one thing we don't need any exception for correctnes alone -
even the block device variant would work fine with the default case.
But for mtd specificly we don't need an exception for performance either
given that there are no per-device bdis in mtd.

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