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

[Glauber Costa]

On 05/16/2013 12:20 PM, Oskar Andero wrote:
> 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
> 
No need to send it offline.
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