linux-kernel - Re: [PATCH 2/7] writeback: switch to per-bdi threads for flushing data

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Message-Id: <1237210214.30224.3.camel@think.oraclecorp.com>
Date:	Mon, 16 Mar 2009 09:30:14 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, david@...morbit.com,
	npiggin@...e.de, linux-mm@...ck.org
Subject: Re: [PATCH 2/7] writeback: switch to per-bdi threads for flushing
 data

On Mon, 2009-03-16 at 06:22 -0400, Christoph Hellwig wrote:
> On Thu, Mar 12, 2009 at 03:33:43PM +0100, Jens Axboe wrote:
> > +static void bdi_kupdated(struct backing_dev_info *bdi)
> > +{
> > +	long nr_to_write;
> > +	struct writeback_control wbc = {
> > +		.bdi		= bdi,
> > +		.sync_mode	= WB_SYNC_NONE,
> > +		.nr_to_write	= 0,
> > +		.for_kupdate	= 1,
> > +		.range_cyclic	= 1,
> > +	};
> > +
> > +	sync_supers();
> 
> Not directly related to your patch, but can someone explain WTF
> sync_supers is doing here or in the old kupdated?  We're writing back
> dirty pages from the VM, and for some reason we try to also write back
> superblocks.   This doesn't really make any sense.

Some of our poor filesystem cousins don't write the super until kupdate
kicks them (see ext2_write_super).  kupdate has always been the periodic
FS thread of last resort.

-chris




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