linux-kernel - Re: [PATCH] mm: vmscan: handle any negative return value from scan

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Date:	Fri, 17 May 2013 16:33:40 +1000
From:	Dave Chinner <dchinner@...hat.com>
To:	Oskar Andero <oskar.andero@...ymobile.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"Lekanovic, Radovan" <Radovan.Lekanovic@...ymobile.com>,
	David Rientjes <rientjes@...gle.com>,
	Glauber Costa <glommer@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] mm: vmscan: handle any negative return value from
 scan_objects

On Thu, May 16, 2013 at 02:27:52PM +0200, Oskar Andero wrote:
> On 13:52 Thu 16 May     , Dave Chinner wrote:
> > On Thu, May 16, 2013 at 10:42:16AM +0200, Oskar Andero wrote:
> > > The shrinkers must return -1 to indicate that it is busy. Instead, treat
> > > any negative value as busy.
> > 
> > Why? The API defines return condition for aborting a scan and gives
> > a specific value for doing that. i.e. explain why should change the
> > API to over-specify the 'abort scan" return value like this.
> 
> As I pointed out earlier, looking in to the code (from master):
> 	if (shrink_ret == -1)
> 		break;
> 	if (shrink_ret < nr_before)
> 		ret += nr_before - shrink_ret;
> 
> This piece of code lacks a sanity check and will only function if shrink_ret
> is either greater than zero or exactly -1. If shrink_ret is e.g. -2 this will
> lead to undefined behaviour.

If a shrinker is returning -2 then the shrinker is broken and needs
fixing.

> > FWIW, using "any" negative number for "abort scan" is a bad API
> > design decision. It means that in future we can't introduce
> > different negative return values in the API if we have a new to.
> > i.e. each specific negative return value needs to have the potential
> > for defining a different behaviour. 
> 
> An alternative to my patch would be to add:
> if (shrink_ret < -1)
>    /* handle illegal return code in some way */

How? We have one valid negative return code. WTF are we supposed to
do if a shrinker is passing undefined return values? IOWs, the only
sane thing to do is:

	BUG_ON(shrink_ret < -1);

> > So if any change needs to be made, it is to change the -1 return
> > value to an enum and have the shrinkers return that enum when they
> > want an abort.
> 
> I am all for an enum, but I still believe we should handle the case where
> the shrinkers return something wicked.

Which bit of "broken shrinkers need to be fixed" don't you
understand? A BUG_ON() will make sure they get fixed - anything else
that allows broken shrinkers to continue functioning is a completely
unacceptible solution.

-Dave.
-- 
Dave Chinner
dchinner@...hat.com
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