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Message-ID: <51949788.1030706@parallels.com> Date: Thu, 16 May 2013 12:23:36 +0400 From: Glauber Costa <glommer@...allels.com> To: Oskar Andero <oskar.andero@...ymobile.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 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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