linux-kernel - Replacement for page fault notifiers?

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Message-ID: <20080108210638.5c9e093a@daedalus.pq.iki.fi>
Date:	Tue, 8 Jan 2008 21:06:38 +0200
From:	Pekka Paalanen <pq@....fi>
To:	linux-kernel@...r.kernel.org
Cc:	jbeulich@...ell.com, proski@....org
Subject: Replacement for page fault notifiers?

Hello,

the commit "x86: optimize page faults like all other achitectures and
kill notifier cruft" by Christoph Hellwig removed the page fault
notifiers (e.g. register_page_fault_notifier()) completely in 2.6.24.
Mmio-trace [1] is using them for tracking memory mapped IO, and also
other people have mentioned they need it [2,3].

This is roughly how I understand mmio-trace is working. In a loadable
kernel module calls to __ioremap() are diverted to an mmio-trace
specific function. The function calls the real __ioremap() and marks
the returned pages as not present. Access to the mapped IO memory
causes a page fault. Mmio-trace's page fault handler marks the page
present, records the event, single-steps the faulting instruction, and
finally marks the page as not present again.

- Is there anything coming to replace register_page_fault_notifier()?
- Has someone found another way to accomplish the same without patching
the kernel?
- Should I start writing a patch to bring back the notifiers, based on
the commit that removed them?

People started to complain to me that mmio-trace does not compile for
2.6.24. At least the Nouveau project is a regular user of mmio-trace
for reverse-engineering, and the tool should be easy to use. Therefore
I would prefer not to force casual contributors to patch their kernels
as it might be too much for some. I am hoping that 2.6.25, if not
2.6.24, would contain a mechanism to accomplish what I need.

I am not a subscriber, so please keep me in CC. Also I CC'd the people
who wrote about the same issue in the past.

Thanks.

[1] http://nouveau.freedesktop.org/wiki/MmioTrace
[2] http://marc.info/?l=linux-kernel&m=119540769014745&w=2
[3] http://article.gmane.org/gmane.linux.kernel/609521

-- 
Pekka Paalanen
http://www.iki.fi/pq/
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