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

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Date:	Fri, 13 Apr 2007 12:50:20 +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 12:18:56 +1000 Nick Piggin <nickpiggin@...oo.com.au> wrote:
> 
> 
>>>I guess one could generate an answer to the static question with systemtap,
>>>by accumulating running counts across the application lifetime and then
>>>snapshotting them.  Sounds hard though.
>>
>>Can't you just traverse arbitrary kernel data structures at a given point
>>in time, exactly like the /proc/ call is doing?
> 
> 
> Do a full pagetable walk, with all the associated locking from within
> a systemtap script?  I'd be surprised.  Maybe if it's mostly hand-coded
> in C, perhaps.

It looks like you can traverse arbitrary data structures, yes.

It definitely seems like you can use some kernel functions, but the
ones I saw may just be systemtap facilities. But what is so surprising
about being able to call a kernel function when running in kernel
context? Perhaps there is some fundamental limitation of kprobes that
I don't understand.

>  Then you just end up with the same thing, don't you?

Well _you_ do, because that happens to be exactly what you want. Bill
ends up with something that displays page_mapcount instead. And I
end up with something that traverses LRU lists rather than pfns. And
none of it goes in /proc/ or linux-2.6/.

So it isn't really the same thing at all.

-- 
SUSE Labs, Novell Inc.
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