Re: [PATCH] writeback: avoid possible balance_dirty_pages() lockup on a light-load bdi

[Fengguang Wu <wfg@mail.ustc.edu.cn>]

On Wed, Oct 03, 2007 at 01:46:52PM +0100, richard kennedy wrote:
> On Tue, 2007-10-02 at 10:00 +0800, Fengguang Wu wrote:
> > ---
> >  mm/page-writeback.c |    5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > --- linux-2.6.22.orig/mm/page-writeback.c
> > +++ linux-2.6.22/mm/page-writeback.c
> > @@ -250,6 +250,11 @@ static void balance_dirty_pages(struct a
> >  			pages_written += write_chunk - wbc.nr_to_write;
> >  			if (pages_written >= write_chunk)
> >  				break;		/* We've done our duty */
> > +			if (list_empty(&mapping->host->i_sb->s_dirty) &&
> > +			    list_empty(&mapping->host->i_sb->s_io) &&
> > +			    nr_reclaimable + global_page_state(NR_WRITEBACK) <=
> > +				    dirty_thresh + (1 << (20-PAGE_CACHE_SHIFT)))
> > +				break;
> >  		}
> >  		congestion_wait(WRITE, HZ/10);
> >  	}
> 
> I've been testing 2.6.23-rc9 + this patch all morning but have just seen
> a lockup. As usual it happened  just after a large file copy finished
> and while nr_dirty is still large. I'm sorry to say I didn't have a
> serial console running so I don't have an other info. I will try again
> and see if I can capture some more data.
> 
> I did notice that at the beginning of my tests the dirty blocks are
> written back more quickly than usual
> 
> nr_dirty count after the copy finished and then 60 seconds later :-
> after copy	+60 seconds
> 73520		0
> 73533		0
> 68554		1
>  
> but after several iterations of my testcase & just before the lockup
> 68560		57165
> 71974		62896
>  
> which is about the same as a unpatched kernel.

Hi Richard,

Thank you for the testing.

However, my patch is kind of duplicate efforts. I was taking the
'do it if simple' attitude.  I can continue to improve it if you 
really want it. Otherwise I'd recommend you to test the coming
2.6.24-rc1 or backport the -mm writeback patches back to 2.6.23 and
test it there. Peter has did a good job on it.

Fengguang