On Thu, 27 Jun 2013 21:34:11 -0700 Anton Vorontsov <anton@xxxxxxxxxx> wrote: > On Thu, Jun 27, 2013 at 06:13:53PM -0700, Andrew Morton wrote: > > On Thu, 27 Jun 2013 17:58:53 -0700 Anton Vorontsov <anton@xxxxxxxxxx> wrote: > > > Current frequency is 1/(2MB). Suppose we ended up scanning the whole > > > memory on a 2GB host, this will give us 1024 hits. Doesn't feel too much* > > > to me... But for what it worth, I am against adding read() to the > > > interface -- just because we can avoid the unnecessary switch into the > > > kernel. > > > > What was it they said about premature optimization? > > > > I think I'd rather do nothing than add a mode hack (already!). > > > > The information Luiz wants is already available with the existing > > interface, so why not just use it until there is a real demonstrated > > problem? > > > > But all this does point at the fact that the chosen interface was not a > > good one. And it's happening so soon :( A far better interface would > > be to do away with this level filtering stuff in the kernel altogether. > > OK, I am convinced that modes might be not necessary, but I see no big > problem in current situation, we can add the strict mode and deprecate the > "filtering" -- basically we'll implement the idea of requiring that > userspace registers a separate fd for each level. > > As one of the ways to change the interface, we can do the strict mode by > writing levels in uppercase, and warn_once on lowercase levels, describing > that the old behaviour will go away. I do think the feature is too young to be bothered about back-compatibility things. We could put a little patch into 3.10 tomorrow which disables the vmpressure feature (just putting a few "return 0"s in there would suffice), then turn the feature back on in 3.11-rc1. Another option is to change the interface in 3.11 and say "sorry" if that causes anyone trouble. But that's obviously less desirable. > Once (if ever) we remove the old > behaviour, the apps trying the old-style lowercase levels will fail > gracefully with EINVAL. > > Or we can be honest and admit that we can't be perfect and just add an > explicit versioning to the interface. :) > > It might be unfortunate that we did not foresee this and have to change > things that soon, but we did change interfaces in the past for a lot of > sysfs and proc knobs, so it is not something new. Once the vmpressure > feature will get even wider usage exposure, we might realize that we need > to make even more changes... Hopefully not ;) But the interface should be designed with that possibility in mind, of course. -- 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>