Re: [PATCH 4/8] vmscan: Do not writeback filesystem pages in direct reclaim

[Mel Gorman <mel@csn.ul.ie>]

On Wed, Jul 21, 2010 at 04:28:44PM +0200, Johannes Weiner wrote:
> On Wed, Jul 21, 2010 at 02:38:57PM +0100, Mel Gorman wrote:
> > Here is an updated version. Thanks very much
> > 
> > ==== CUT HERE ====
> > vmscan: Do not writeback filesystem pages in direct reclaim
> > 
> > When memory is under enough pressure, a process may enter direct
> > reclaim to free pages in the same manner kswapd does. If a dirty page is
> > encountered during the scan, this page is written to backing storage using
> > mapping->writepage. This can result in very deep call stacks, particularly
> > if the target storage or filesystem are complex. It has already been observed
> > on XFS that the stack overflows but the problem is not XFS-specific.
> > 
> > This patch prevents direct reclaim writing back filesystem pages by checking
> > if current is kswapd or the page is anonymous before writing back.  If the
> > dirty pages cannot be written back, they are placed back on the LRU lists
> > for either background writing by the BDI threads or kswapd. If in direct
> > lumpy reclaim and dirty pages are encountered, the process will stall for
> > the background flusher before trying to reclaim the pages again.
> > 
> > As the call-chain for writing anonymous pages is not expected to be deep
> > and they are not cleaned by flusher threads, anonymous pages are still
> > written back in direct reclaim.
> > 
> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > Acked-by: Rik van Riel <riel@redhat.com>
> 
> Cool!
> 
> Except for one last tiny thing...
> 
> > @@ -858,7 +872,7 @@ keep:
> >  
> >  	free_page_list(&free_pages);
> >  
> > -	list_splice(&ret_pages, page_list);
> 
> This will lose all retry pages forever, I think.
> 

Above this is

while (!list_empty(page_list)) {
	...
}

page_list should be empty and keep_locked is putting the pages on ret_pages
already so I think it's ok.

> > +	*nr_still_dirty = nr_dirty;
> >  	count_vm_events(PGACTIVATE, pgactivate);
> >  	return nr_reclaimed;
> >  }
> 
> Otherwise,
> Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
> 

Thanks!

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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