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

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Message-ID: <20100930000641.GA27865@lst.de>
Date:	Thu, 30 Sep 2010 02:06:41 +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, linux-mtd@...ts.infradead.org,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: Dirtiable inode bdi default != sb bdi btrfs

On Thu, Sep 30, 2010 at 01:38:07AM +0200, Jan Kara wrote:
> > 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.
> Here I don't agree. If you don't have some kind of exception, sb->s_bdi
> for both "block" and "mtd_inodefs" filesystems points to
> noop_backing_dev_info and you get no writeback for that one. So it isn't
> just a performance issue but also a correctness one.

Indeed - for internal filesystems that require writeback the change
causes trouble if they haven't registered a s_bdi.  But for all user
visible filesystems that doesn't happen as we require s_bdi for
sync or even unmounts to work.
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