linux-kernel - Re: [PATCH 1/3] bdi: Use parent filesystem BDI for inodes not capable of writeback

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Date:	Tue, 27 Jul 2010 13:09:03 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/3] bdi: Use parent filesystem BDI for inodes not
 capable of writeback

> +static struct backing_dev_info *inode_to_bdi(struct inode *inode)
> +{
> +	struct backing_dev_info *bdi = inode->i_mapping->backing_dev_info;
> +
> +	/*
> +	 * This is a hack but it solves a problem with device inode
> +	 * for e.g. /dev/zero getting dirty (via touch or so) and confusing
> +	 * writeback code. In such cases we return the "parent" filesystem's
> +	 * bdi.
> +	 */
> +	if (bdi_cap_writeback_dirty(bdi))
> +		return bdi;
> +	return inode->i_sb->s_bdi;

When do we ever have a writeback-capable bdi that sits inside another
filesystem?  I think just always using inode->i_sb->s_bdi is the right
thing here.

And btw, having a BDI_CAP_NO_WRITEBACK instead of a BDI_CAP_WRITEBACK
is rather dumb, we'd better fix it up while we're at it.

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