Subject: Re: [PATCH 2/7] Assign bdi in super_block
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <jens.axboe@oracle.com>, linux-kernel@vger.kernel.org,
       linux-fsdevel@vger.kernel.org, chris.mason@oracle.com,
       hch@infradead.org, tytso@mit.edu, akpm@linux-foundation.org
In-Reply-To: <20090914130228.GG24075@duck.suse.cz>
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	 <1252920994-11141-3-git-send-email-jens.axboe@oracle.com>
	 <20090914130228.GG24075@duck.suse.cz>
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Date: Mon, 14 Sep 2009 14:25:24 -0400
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On Mon, 2009-09-14 at 15:02 +0200, Jan Kara wrote:
> On Mon 14-09-09 11:36:29, Jens Axboe wrote:
> > We do this automatically in get_sb_bdev() from the set_bdev_super()
> > callback. Filesystems that have their own private backing_dev_info
> > must assign that in ->fill_super().
> > 
> > Note that ->s_bdi assignment is required for proper writeback!
> > 
> > Acked-by: Christoph Hellwig <hch@infradead.org>
> > Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
>   Hmm, looking at this again, I'm not sure this will work for NFS. It seems
> to set mapping->backing_dev_info to its private backing dev info for
> regular files while it leaves it intact for other inodes (e.g.
> directories). I'm not sure why it does so but it seems its inodes end up on
> two different BDI lists and thus they wouldn't be synced properly. Trond,
> do I read the code properly?
>   Also we definitely need to set *some* bdi in nfs_get_sb as otherwise sync
> won't work for it.

There hasn't really been a need for a bdi in NFS other than for the
regular file read and writeback code. The main reason for making it
private was to ensure that we could set a per-superblock readahead limit
that was a decent multiple of the server's preferred read block size.

Is there any reason why we couldn't set sb->s_bdi to point to that
private bdi?

Cheers
  Trond

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