linux-kernel - Accessing user memory from NMI

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Message-ID: <19001.63708.989256.78908@cargo.ozlabs.ibm.com>
Date:	Thu, 18 Jun 2009 18:20:44 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	linux-kernel@...r.kernel.org, benh@...nel.crashing.org
Subject: Accessing user memory from NMI

What was the conclusion you guys came to about doing a user stack
backtrace in an NMI handler?  Are you going to access user memory
directly or are you going to use the __fast_get_user_pages approach?

Ben H and I were talking today about what we'd need in order to be
able to read user memory in a PMU interrupt handler.  It looks like we
could read user memory directly with a bit of care, on 64-bit at
least.  Because of the MMU hash table that would almost always work
provided the page has already been touched (which stack pages would
have been), but there is a small chance that the access might fail
even if the address has a valid PTE.  At that point we could fall back
to the __fast_get_user_pages method, but I'm not sure it's worth it.

Paul.
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