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

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Message-ID: <20130517080054.GH24072@caracas.corpusers.net>
Date:	Fri, 17 May 2013 10:00:54 +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 08:33 Fri 17 May     , Dave Chinner wrote:
> 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.

The point is: returning -2 is just as magical and meaningful as returning -1.

Usually, returning a negative means "failure" (Chapter 16 CodingStyle), not
a perfectly valid "abort scan" as in this piece of code.

> > > 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);

Yes, of course! BUG_ON() is the proper way to handle an illegal value.
Now we are getting somewhere!

> > > 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.

BUG_ON() is perfect IMO and if everyone is ok with that I will send
version 2 of my patch.

Now there is just the matter of returning hardcoded -1. Would an enum in
shrinker.h add any value? I have gotten different feedback on this - some
say yea, others nay.
I think I have motivated it enough in this thread, so I am not going to
repeat myself.

-Oskar
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