linux-kernel - Re: [RFC PATCH 0/2] return value from shrinkers

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Date:	Wed, 15 May 2013 16:10:57 +0200
From:	Oskar Andero <oskar.andero@...ymobile.com>
To:	Glauber Costa <glommer@...allels.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Hugh Dickins <hughd@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Lekanovic, Radovan" <Radovan.Lekanovic@...ymobile.com>,
	David Rientjes <rientjes@...gle.com>,
	Dave Chinner <david@...morbit.com>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [RFC PATCH 0/2] return value from shrinkers

On 17:03 Tue 14 May     , Glauber Costa wrote:
> On 05/13/2013 06:16 PM, Oskar Andero wrote:
> > Hi,
> > 
> > In a previous discussion on lkml it was noted that the shrinkers use the
> > magic value "-1" to signal that something went wrong.
> > 
> > This patch-set implements the suggestion of instead using errno.h values
> > to return something more meaningful.
> > 
> > The first patch simply changes the check from -1 to any negative value and
> > updates the comment accordingly.
> > 
> > The second patch updates the shrinkers to return an errno.h value instead
> > of -1. Since this one spans over many different areas I need input on what is
> > a meaningful return value. Right now I used -EBUSY on everything for consitency.
> > 
> > What do you say? Is this a good idea or does it make no sense at all?
> > 
> > Thanks!
> > 
> 
> Right now me and Dave are completely reworking the way shrinkers
> operate. I suggest, first of all, that you take a look at that cautiously.

Sounds good. Where can one find the code for that?

> On the specifics of what you are doing here, what would be the benefit
> of returning something other than -1 ? Is there anything we would do
> differently for a return value lesser than 1?

Firstly, what bugs me is the magic and unintuitiveness of using -1 rather than a
more descriptive error code. IMO, even a #define SHRINK_ERROR -1 in some header
file would be better.

Expanding the test to <0 will open up for more granular error checks,
like -EAGAIN, -EBUSY and so on. Currently, they would all be treated the same,
but maybe in the future we would like to handle them differently?

Finally, looking at the code:
                        if (shrink_ret == -1)
                                break;
                        if (shrink_ret < nr_before)
                                ret += nr_before - shrink_ret;

This piece of code will only function if shrink_ret is either greater than zero
or -1. If shrink_ret is -2 this will lead to undefined behaviour.

> So far, shrink_slab behaves the same, you are just expanding the test.
> If you really want to push this through, I would suggest coming up with
> a more concrete reason for why this is wanted.

I don't know how well this patch is aligned with your current rework, but
based on my comments above, I don't see a reason for not taking it.

-Oskar
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