Re: [PATCH v2] vmpressure: implement strict mode

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 27 Jun 2013 21:34:11 -0700 Anton Vorontsov <anton@enomsg.org> 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@enomsg.org> 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.


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