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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>