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

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Date:	Fri, 13 Apr 2007 11:14:20 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	William Lee Irwin III <wli@...omorphy.com>,
	Matt Mackall <mpm@...enic.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups

Andrew Morton wrote:
> On Fri, 13 Apr 2007 10:15:24 +1000 Nick Piggin <nickpiggin@...oo.com.au> wrote:

>>>>+               for (; i < 2 * chunk / KPMSIZE; i += 2, pfn++) {
>>>>+                       ppage = pfn_to_page(pfn);
>>>>+                       if (!ppage) {
>>>>+                               page[i] = 0;
>>>>+                               page[i + 1] = 0;
>>>>+                       } else {
>>>>+                               page[i] = ppage->flags;
>>>>+                               page[i + 1] = atomic_read(&ppage->_count);
>>>>+                       }
>>>>+               }
>>>
>>>
>>>Not a good idea to expose raw flags in this manner - it changes at the drop
>>>of a hat.  We'd need to also expose the kernel's PG_foo-to-bitnumber
>>>mapping to make this viable.
>>
>>I don't think it is viable because that makes the flags part of the
>>userspace ABI.
> 
> 
> 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.

Ooh, you wanted a _runtime_ mapping of flags, yeah then I guess that works.
Still seems like a basically hit and miss affair to just use flags. What if
you want to know the process mapping a page? With systemtap or something you
could walk the rmap structures. What if you want to look at pages along the
LRU list rather than per-pfn? What about connecting pages to inodes?

I thought this type of deep poking was the whole reason the probles thingies
were merged. I'm saddened that they're no good for this. I thought it would
be an ideal usage :(


>>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.

Yeah it is close, they need page_mapcount I think. But I was going to say
that satisfying an Oracle requirement is a good reason _not_ to merge it ;)
(I joke!)

-- 
SUSE Labs, Novell Inc.
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