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

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Date:	Fri, 28 Jun 2013 09:57:22 -0700
From:	Anton Vorontsov <anton@...msg.org>
To:	Luiz Capitulino <lcapitulino@...hat.com>
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 Fri, Jun 28, 2013 at 10:00:27AM -0400, Luiz Capitulino wrote:
> On Thu, 27 Jun 2013 22:07:12 -0700
> Anton Vorontsov <anton@...msg.org> wrote:
> 
> > On Thu, Jun 27, 2013 at 09:34:11PM -0700, Anton Vorontsov wrote:
> > > ... 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.
> > 
> > Btw, assuming that more levels can be added, there will be a problem:
> > imagine that an app hooked up onto low, med, crit levels in "strict"
> > mode... then once we add a new level, the app will start missing the new
> > level events.
> 
> That's how it's expected to work, because on strict mode you're notified
> for the level you registered for. So apps registering for critical, will
> still be notified on critical just like before.

Suppose you introduce a new level, and the system hits this level. Before,
the app would receive at least some notification for the given memory load
(i.e. one of the old levels), with the new level introduced in the kernel,
the app will receive no events at all. This makes a serious behavioural
change in the app (read: it'll break it).

Anton
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