linux-kernel - Re: [RFC][PATCH 2/7] writeback: fix queue

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Message-ID: <20090910012624.GA10957@localhost>
Date:	Thu, 10 Sep 2009 09:26:24 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	"jack@...e.cz" <jack@...e.cz>,
	Artem Bityutskiy <dedekind1@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC][PATCH 2/7] writeback: fix queue_io() ordering

On Wed, Sep 09, 2009 at 11:53:30PM +0800, Jan Kara wrote:
> On Wed 09-09-09 22:51:43, Wu Fengguang wrote:
> > This was not a bug, since b_io is empty for kupdate writeback.
> > The next patch will do requeue_io() for non-kupdate writeback,
> > so let's fix it.
>   But doesn't this patch also need your "anti-starvation" patch?

Honza, can you show me that patch?

> Looking into the code, we put inode to b_more_io when nr_to_write
> drops to zero and this way we'd just start writing it again
> in the next round...

I'm confused. It's OK to start it in next round. Starvation can
occur if we start it immediately in the next writeback_inodes()
invocation. How can that happen with this patch?

Thanks,
Fengguang

> 							Honza
> > 
> > CC: Dave Chinner <david@...morbit.com>
> > Cc: Martin Bligh <mbligh@...gle.com>
> > Cc: Michael Rubin <mrubin@...gle.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Signed-off-by: Fengguang Wu <wfg@...l.ustc.edu.cn>
> > ---
> >  fs/fs-writeback.c |    7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > --- linux.orig/fs/fs-writeback.c	2009-09-09 21:41:14.000000000 +0800
> > +++ linux/fs/fs-writeback.c	2009-09-09 21:45:15.000000000 +0800
> > @@ -313,11 +313,14 @@ static void move_expired_inodes(struct l
> >  }
> >  
> >  /*
> > - * Queue all expired dirty inodes for io, eldest first.
> > + * Queue all expired dirty inodes for io, eldest first:
> > + * (newly dirtied) => b_dirty inodes
> > + *                 => b_more_io inodes
> > + *                 => remaining inodes in b_io => (dequeue for sync)
> >   */
> >  static void queue_io(struct bdi_writeback *wb, unsigned long *older_than_this)
> >  {
> > -	list_splice_init(&wb->b_more_io, wb->b_io.prev);
> > +	list_splice_init(&wb->b_more_io, &wb->b_io);
> >  	move_expired_inodes(&wb->b_dirty, &wb->b_io, older_than_this);
> >  }
> >  
> > 
> > -- 
> > 
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
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