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

[Oskar Andero <oskar.andero@sonymobile.com>]

On 16:49 Wed 15 May     , Glauber Costa wrote:
> On 05/15/2013 06:47 PM, Oskar Andero wrote:
> > 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.
> > 
> If you are willing to wait a bit until it finally gets merged, please
> send it against my memcg.git in kernel.org (branch
> kmemcg-lru-shrinkers). I can carry your patch in our series.

Alright. I will apply PATCH 1/2 ontop of your kmemcg-lru-shrinker branch
and send it to you offline.

Thanks!

-Oskar

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