linux-kernel - Re: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups

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Message-Id: <20070413100356.62c6d9c2.akpm@linux-foundation.org>
Date:	Fri, 13 Apr 2007 10:03:56 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	William Lee Irwin III <wli@...omorphy.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups

On Fri, 13 Apr 2007 11:24:36 -0500 Matt Mackall <mpm@...enic.com> wrote:

> > It *will* be viable.  If the application wants to know if a page is dirty,
> > it looks up "PG_dirty" in /proc/pg_foo-to-bitnumber and uses PG_dirty's
> > numerical offset when inspecting fields in /proc/kpagemap.  If correctly
> > designed, such a monitoring application will be able to report upon page
> > flags which we haven't even thought up yet.
> 
> We can probably fit this in the existing (variable-sized) header.

hm, OK..

> > > I wonder what they are needed for.
> > 
> > Poking deeply into the kernel to provide information about kernel state. 
> > 
> > There are real-world needs for this, and the people who develop tools to
> > process this information will have decent kernel understanding and will
> > know that the file's contents may alter across kernel versions.  It sure
> > beats poking around in /dev/kmem.
> > 
> > I doubt if there's a sensible way in which we can prettify this interface
> > without losing information.  But we should aim to make it as robust as
> > possible agaisnt future kenrel changes, of course.
> > 
> > And we should satisfy ourselves that all the required information has been
> > made available.  The fact that it will satisfy the Oracle requirement is
> > encouraging.
> > 
> > Matt, these changes make the new field in /proc/pid/smaps redundant, don't
> > they?
> 
> Which new field?

Referenced:

> From /proc/kpagemap + /proc/*/pagemap, you can
> basically synthesize any statistic you want, including all the
> existing ones. For some data, /proc/pid/smaps (or /proc/meminfo) will
> be considerably more efficient.

You'd need to poke clear_refs beforehand to make the referenced bits useful.

Actually, we also need to run around the ptes and collect the pte-referenced
bits too.  I don't think your code copes with any of that?
 
> But in general, most of the statistics in smaps are basically useless
> for shared mappings, just like RSS. Problem is, we really don't know
> what statistics we want yet, or even if it can be distilled down to
> simple numbers anyway.

yup.  But that's the whole point, really: don't prejudge what info userspace
is trying to collect.
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