linux-kernel - Re: [patch] mm: disable user interface to manually rescue unevictable pages

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Date:	Tue, 4 Oct 2011 12:10:36 +0200
From:	Johannes Weiner <jweiner@...hat.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...gle.com>,
	Kautuk Consul <consul.kautuk@...il.com>,
	Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch] mm: disable user interface to manually rescue
 unevictable pages

On Wed, Sep 28, 2011 at 11:14:24AM +0900, Minchan Kim wrote:
> On Tue, Sep 27, 2011 at 09:27:14AM +0200, Johannes Weiner wrote:
> > At one point, anonymous pages were supposed to go on the unevictable
> > list when no swap space was configured, and the idea was to manually
> > rescue those pages after adding swap and making them evictable again.
> > But nowadays, swap-backed pages on the anon LRU list are not scanned
> > without available swap space anyway, so there is no point in moving
> > them to a separate list anymore.
> > 
> > The manual rescue could also be used in case pages were stranded on
> > the unevictable list due to race conditions.  But the code has been
> > around for a while now and newly discovered bugs should be properly
> > reported and dealt with instead of relying on such a manual fixup.
> > 
> > In addition to the lack of a usecase, the sysfs interface to rescue
> > pages from a specific NUMA node has been broken since its
> > introduction, so it's unlikely that anybody ever relied on that.
> > 
> > This patch removes the functionality behind the sysctl and the
> > node-interface and emits a one-time warning when somebody tries to
> > access either of them.
> > 
> > Signed-off-by: Johannes Weiner <jweiner@...hat.com>
> > Reported-by: Kautuk Consul <consul.kautuk@...il.com>
> Reviewed-by: Minchan Kim <minchan.kim@...il.com>

Thanks, Minchan.

> > @@ -3469,11 +3415,8 @@ int scan_unevictable_handler(struct ctl_table *table, int write,
> >  			   void __user *buffer,
> >  			   size_t *length, loff_t *ppos)
> >  {
> > +	warn_scan_unevictable_pages();
> >  	proc_doulongvec_minmax(table, write, buffer, length, ppos);
> > -
> > -	if (write && *(unsigned long *)table->data)
> > -		scan_all_zones_unevictable_pages();
> > -
> >  	scan_unevictable_pages = 0;
> 
> Nitpick:
> Could we remove this resetting with zero?

table->data = &scan_unevictable_pages, so I let it in such that the
generic sysctl stuff to process the input and clean up afterward is
complete.
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