linux-kernel - Re: [PATCH 4/5] mm: vmscan: Immediately reclaim end-of-LRU dirty pages when writeback completes

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Date:	Wed, 13 Jul 2011 18:15:53 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	XFS <xfs@....sgi.com>, Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 4/5] mm: vmscan: Immediately reclaim end-of-LRU dirty
 pages when writeback completes

On Wed, Jul 13, 2011 at 06:40:40PM +0200, Johannes Weiner wrote:
> On Wed, Jul 13, 2011 at 03:31:26PM +0100, Mel Gorman wrote:
> > When direct reclaim encounters a dirty page, it gets recycled around
> > the LRU for another cycle. This patch marks the page PageReclaim using
> > deactivate_page() so that the page gets reclaimed almost immediately
> > after the page gets cleaned. This is to avoid reclaiming clean pages
> > that are younger than a dirty page encountered at the end of the LRU
> > that might have been something like a use-once page.
> > 
> > Signed-off-by: Mel Gorman <mgorman@...e.de>
> > ---
> >  include/linux/mmzone.h |    2 +-
> >  mm/vmscan.c            |   10 ++++++++--
> >  mm/vmstat.c            |    2 +-
> >  3 files changed, 10 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index c4508a2..bea7858 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -100,7 +100,7 @@ enum zone_stat_item {
> >  	NR_UNSTABLE_NFS,	/* NFS unstable pages */
> >  	NR_BOUNCE,
> >  	NR_VMSCAN_WRITE,
> > -	NR_VMSCAN_WRITE_SKIP,
> > +	NR_VMSCAN_INVALIDATE,
> >  	NR_VMSCAN_THROTTLED,
> >  	NR_WRITEBACK_TEMP,	/* Writeback using temporary buffers */
> >  	NR_ISOLATED_ANON,	/* Temporary isolated pages from anon lru */
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 9826086..8e00aee 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -834,8 +834,13 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >  			 */
> >  			if (page_is_file_cache(page) &&
> >  					(!current_is_kswapd() || priority >= DEF_PRIORITY - 2)) {
> > -				inc_zone_page_state(page, NR_VMSCAN_WRITE_SKIP);
> > -				goto keep_locked;
> > +				inc_zone_page_state(page, NR_VMSCAN_INVALIDATE);
> > +
> > +				/* Immediately reclaim when written back */
> > +				unlock_page(page);
> > +				deactivate_page(page);
> > +
> > +				goto keep_dirty;
> >  			}
> >  
> >  			if (references == PAGEREF_RECLAIM_CLEAN)
> > @@ -956,6 +961,7 @@ keep:
> >  		reset_reclaim_mode(sc);
> >  keep_lumpy:
> >  		list_add(&page->lru, &ret_pages);
> > +keep_dirty:
> >  		VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
> >  	}
> 
> I really like the idea behind this patch, but I think all those pages
> are lost as PageLRU is cleared on isolation and lru_deactivate_fn
> bails on them in turn.
> 
> If I'm not mistaken, the reference from the isolation is also leaked.

I think you're right. This patch was rushed and not thought through
properly.  The surprise it appeared to work at all. Will rework
it. Thanks.

-- 
Mel Gorman
SUSE Labs
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