linux-kernel - Re: [RFC PATCH] proc: do not include shmem and driver pages in /proc/meminfo::Cached

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Date:	Fri, 19 Feb 2016 13:13:07 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Konstantin Khlebnikov <koct9i@...il.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kernel-team@...com
Subject: Re: [RFC PATCH] proc: do not include shmem and driver pages in
 /proc/meminfo::Cached

On Fri, 19 Feb 2016 09:40:45 +0300 Konstantin Khlebnikov <koct9i@...il.com> wrote:

> >> What are your thoughts on this?
> >
> > My thoughts are NAK.  A misleading stat is not so bad as a
> > misleading stat whose meaning we change in some random kernel.
> >
> > By all means improve Documentation/filesystems/proc.txt on Cached.
> > By all means promote Active(file)+Inactive(file)-Buffers as often a
> > better measure (though Buffers itself is obscure to me - is it intended
> > usually to approximate resident FS metadata?).  By all means work on
> > /proc/meminfo-v2 (though that may entail dispiritingly long discussions).
> >
> > We have to assume that Cached has been useful to some people, and that
> > they've learnt to subtract Shmem from it, if slow or no swap concerns them.
> >
> > Added Konstantin to Cc: he's had valuable experience of people learning
> > to adapt to the numbers that we put out.
> >
> 
> I think everything will ok. Subtraction of shmem isn't widespread practice,
> more like secret knowledge. This wasn't documented and people who use
> this should be aware that this might stop working at any time. So, ACK.

It worries me as well - we're deliberately altering the behaviour of
existing userspace code.  Not all of those alterations will be welcome!

We could add a shiny new field into meminfo and train people to migrate
to that.  But that would just be a sum of already-available fields.  In
an ideal world we could solve all of this with documentation and
cluebatting (and some apologizing!).