linux-kernel - Re: [patch] mm: remove sysctl to manually rescue unevictable pages

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Message-Id: <20110926161136.b4508ecb.akpm@google.com>
Date:	Mon, 26 Sep 2011 16:11:36 -0700
From:	Andrew Morton <akpm@...gle.com>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Kautuk Consul <consul.kautuk@...il.com>,
	Mel Gorman <mel@....ul.ie>,
	Minchan Kim <minchan.kim@...il.com>,
	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: remove sysctl to manually rescue unevictable pages

On Mon, 26 Sep 2011 13:29:45 +0200
Johannes Weiner <jweiner@...hat.com> wrote:

> On Sun, Sep 25, 2011 at 04:29:40PM +0530, Kautuk Consul wrote:
> > write_scan_unavictable_node checks the value req returned by
> > strict_strtoul and returns 1 if req is 0.
> > 
> > However, when strict_strtoul returns 0, it means successful conversion
> > of buf to unsigned long.
> > 
> > Due to this, the function was not proceeding to scan the zones for
> > unevictable pages even though we write a valid value to the 
> > scan_unevictable_pages sys file.
> 
> Given that there is not a real reason for this knob (anymore) and that
> it apparently never really worked since the day it was introduced, how
> about we just drop all that code instead?
> 

Yes, let's remove it if at all possible.

However, to be nice to people I do think we should emit a once-per-boot
printk when someone tries to use it, then remove it for real at least a
couple of kernel cycles later.  Just in case someone's script or tuning
app is trying to open that procfs file.

> 
> ---
> From: Johannes Weiner <jweiner@...hat.com>
> Subject: mm: remove sysctl to manually rescue unevictable pages
> 
> 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.
> 
> Signed-off-by: Johannes Weiner <jweiner@...hat.com>

The changelog failed to note that the sysctl doesn't actually *work*. 
This is a pretty strong argument for removing it ;)

Also, a reported-by:Kautuk would have been nice.

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