linux-kernel - Re: [PATCH v2] vmpressure: implement strict mode

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Date:	Fri, 28 Jun 2013 09:43:25 -0400
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	Anton Vorontsov <anton@...msg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, mhocko@...e.cz, kmpark@...radead.org,
	hyunhee.kim@...sung.com
Subject: Re: [PATCH v2] vmpressure: implement strict mode

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

Agreed this is a good solution.

> 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. Once (if ever) we remove the old
> behaviour, the apps trying the old-style lowercase levels will fail
> gracefully with EINVAL.

Why don't we just break it? There's no non-development kernel released
with this interface yet.
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