linux-kernel - Re: [PATCH -mm 09/25] fix pagecache reclaim referenced bit check

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Date:	Fri, 6 Jun 2008 21:08:58 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, lee.schermerhorn@...com,
	kosaki.motohiro@...fujitsu.com, mbligh@...igh.org
Subject: Re: [PATCH -mm 09/25] fix pagecache reclaim referenced bit check

On Fri, 6 Jun 2008 18:04:51 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Fri, 06 Jun 2008 16:28:47 -0400
> Rik van Riel <riel@...hat.com> wrote:
> 
> > From: Rik van Riel <riel@...hat.com>
> > 
> > The -mm tree contains the patch
> > vmscan-give-referenced-active-and-unmapped-pages-a-second-trip-around-the-lru.patch
> > which gives referenced pagecache pages another trip around
> > the active list.  This seems to help keep frequently accessed
> > pagecache pages in memory.
> > 
> > However, it means that pagecache pages that get moved to the
> > inactive list do not have their referenced bit set, and a
> > reference to the page will not get it moved back to the active
> > list
> 
> Should that be "the next reference"?  Because two references _will_ cause
> the page to be activated?

Indeed, next reference would be a more accurate description.

> >.  This patch sets the referenced bit on pagecache pages
> > that get deactivated, so the next access to the page will promote
> > it back to the active list.
> > 
> > ---
> >  mm/vmscan.c |    4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > Index: linux-2.6.26-rc2-mm1/mm/vmscan.c
> > ===================================================================
> > --- linux-2.6.26-rc2-mm1.orig/mm/vmscan.c	2008-05-28 12:11:51.000000000 -0400
> > +++ linux-2.6.26-rc2-mm1/mm/vmscan.c	2008-05-28 12:14:34.000000000 -0400
> > @@ -1062,8 +1062,13 @@ static void shrink_active_list(unsigned 
> >  				list_add(&page->lru, &l_inactive);
> >  				pgmoved++;
> >  			}
> > -		} else
> > +		} else {
> >  			list_add(&page->lru, &l_inactive);
> > +			if (file && !page_mapped(page))
> > +				/* Bypass use-once, make the next access count.
> > +				 * See mark_page_accessed. */
> > +				SetPageReferenced(page);
> > +		}
> >  	}
> 
> Will this change also cause these pages to get a second trip around the
> inactive list?  Or do we at the end of the patch series end up
> reclaiming pagecache regardless of their PageReferenced()?  If so, it
> seems that we're making the pagecache pages a _lot_ stickier with this
> change and my one - an additional trip around the active list and an
> additional one around the inactive list.

At the end of the inactive list, we reclaim it regardless of
PageReferenced().  As for making the pagecache pages stickier,
we can balance that out by tweaking swappiness now that we
have page cache and swap/ram backed pages living on separate
sets of LRU lists.

The balancing becomes a lot easier than before.

> Changelog should spell all this out, I guess.

It's not the Changelog I'm worried about as much as the
people who are trying to read the code afterwards.

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