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 -- 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>