Re: [PATCH] bdi_sync_writeback should WB_SYNC_NONE first

[Andrew Morton]

On Sun, 27 Sep 2009 18:44:32 +0200 Jens Axboe <jens.axboe@xxxxxxxxxx> wrote:

> On Sun, Sep 27 2009, Andrew Morton wrote:
> > On Fri, 25 Sep 2009 10:10:14 -0400 Chris Mason <chris.mason@xxxxxxxxxx> wrote:
> > 
> > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > > index 8e1e5e1..27f8e0e 100644
> > > --- a/fs/fs-writeback.c
> > > +++ b/fs/fs-writeback.c
> > > @@ -225,7 +225,7 @@ static void bdi_sync_writeback(struct backing_dev_info *bdi,
> > >  {
> > >  	struct wb_writeback_args args = {
> > >  		.sb		= sb,
> > > -		.sync_mode	= WB_SYNC_ALL,
> > > +		.sync_mode	= WB_SYNC_NONE,
> > >  		.nr_pages	= LONG_MAX,
> > >  		.range_cyclic	= 0,
> > >  	};
> > > @@ -236,6 +236,13 @@ static void bdi_sync_writeback(struct backing_dev_info *bdi,
> > >  
> > >  	bdi_queue_work(bdi, &work);
> > >  	bdi_wait_on_work_clear(&work);
> > > +
> > > +	args.sync_mode = WB_SYNC_ALL;
> > > +	args.nr_pages = LONG_MAX;
> > > +
> > > +	work.state = WS_USED | WS_ONSTACK;
> > > +	bdi_queue_work(bdi, &work);
> > > +	bdi_wait_on_work_clear(&work);
> > >  }
> > 
> > Those LONG_MAX's are a worry.  What prevents a very long
> > almost-livelock from occurring if userspace is concurrently dirtying
> > pagecache at a high rate?
> 
> Not sure whether Chris' system is back up again, but I discussed this
> with him on irc. Since the WB_SYNC_ALL writeback should be queued behind
> the WB_SYNC_NONE that the non-wait sync already issued, not sure why
> this patch makes a difference. It's definitely not the right approach.
> 

I wasn't referring to this patch actually.  The code as it stands in
Linus's tree right now attempts to write back up to 2^63 pages...  
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