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

On Fri, Apr 13, 2007 at 10:03:56AM -0700, Andrew Morton wrote:
On Fri, 13 Apr 2007 11:24:36 -0500 Matt Mackall <mpm@xxxxxxxxxxx> 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?

No, and it probably should. Perhaps dirty as well, though I've kindof
lost the plot on how that works lately.

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.

Right.

--
Mathematics is the supreme nostalgia of our time.
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