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

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Date:	Thu, 16 May 2013 14:27:52 +0200
From:	Oskar Andero <oskar.andero@...ymobile.com>
To:	Dave Chinner <dchinner@...hat.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 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.

> 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 */

> 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.

-Oskar
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