linux-kernel - Re: [RFC 1/2] deactive invalidated pages

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Date:	Tue, 23 Nov 2010 14:58:56 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Ben Gamari <bgamari@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [RFC 1/2] deactive invalidated pages

On Tue, Nov 23, 2010 at 09:55:49AM -0500, Ben Gamari wrote:
> On Tue, 23 Nov 2010 09:38:59 +0000, Mel Gorman <mel@....ul.ie> wrote:
> > > If it's mapped pagecache then the user was being a bit silly (or didn't
> > > know that some other process had mapped the file).  In which case we
> > > need to decide what to do - leave the page alone, deactivate it, or
> > > half-deactivate it as this patch does.
> > > 
> > 
> > What are the odds of an fadvise() user having used mincore() in advance
> > to determine if the page was in use by another process? I would guess
> > "low" so this half-deactivate gives a chance for the page to be promoted
> > again as well as a chance for the flusher threads to clean the page if
> > it really is to be reclaimed.
> > 
> Do we really want to make the user jump through such hoops as using
> mincore() just to get the kernel to handle use-once pages properly?

I would think "no" which is why I support half-deactivating pages so they won't
have to. The downside is that it's essentially a race window as another process
needs to reactivate the page before it gets reclaimed to avoid a major fault.

> I hope the answer is no. I know that fadvise isn't supposed to be a
> magic bullet, but it would be nice if more processes would use it to
> indicate their access patterns and the only way that will happen is if
> it is reasonably straightforward to use.
> 
> - Ben
> 

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