Re: [RFC][PATCH 2/7] writeback: fix queue_io() ordering

[Jan Kara <jack@suse.cz>]

On Thu 10-09-09 09:26:24, Wu Fengguang wrote:
> 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?
  Sorry, my fault. For kupdate, we splice the list only once s_io is empty
so that's not an issue. So the patch looks good.
  Acked-by: Jan Kara <jack@suse.cz>

> > > CC: Dave Chinner <david@fromorbit.com>
> > > Cc: Martin Bligh <mbligh@google.com>
> > > Cc: Michael Rubin <mrubin@google.com>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Signed-off-by: Fengguang Wu <wfg@mail.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@suse.cz>
> > SUSE Labs, CR
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR