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

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Date:	Wed, 15 May 2013 16:47:04 +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 16:18 Wed 15 May     , Glauber Costa wrote:
> On 05/15/2013 06:10 PM, Oskar Andero wrote:
> > 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?
> > 
> linux-mm, linux-fsdevel
> 
> Subject is "kmemcg shrinkers", but only the second part is memcg related.
> 
> >> 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?
> > 
> 
> Then in the future we change it.
> This is not a user visible API, we are free to change it at any time,
> under any conditions. There is only value in supporting different error
> codes if we intend to do something different about it. Otherwise, it is
> just churn.
> 
> Moreover, -1 does not necessarily mean error. It means "stop shrinking".
> There are many non-error conditions in which it could happen.
> 

Sure, maybe errno.h is not the right way to go. So, why not add the #define
instead? E.g. STOP_SHRINKING or something better than -1.

> > 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.
> > 
> Except it never is. But since we are touching this code anyway, I see no
> problems in expanding the test. What I don't see the point for, is the
> other patch in your series in which you return error codes.
> 
> >> 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.
> > 
> I see no objections for PATCH #1 that expands the check, as a cautionary
> measure. But I will oppose returning error codes from shrinkers without
> a solid reason for doing so (meaning a use case in which we really
> threat one of the errors differently)

Sorry for being over-zealous about the return codes and I understand
that it is really a minor thing and possibly also a philosophical
question. My only "solid" reasons are unintuiveness and readability.
That is how I came across it in the first place.

If no-one backs me up on this I will drop the second patch and resend
the first patch without RFC prefix.

-Oskar

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