linux-kernel - Re: [PATCH] PREEMPT_RT_FULL: arm coredump fails for cpu >= 4

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Date:	Wed, 28 Sep 2011 15:03:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	frank.rowand@...sony.com
Cc:	"tglx@...utronix.de" <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PREEMPT_RT_FULL: arm coredump fails for cpu >= 4

On Tue, 2011-09-27 at 20:02 -0700, Frank Rowand wrote:
> 
> ARM can not use SPLIT_PTLOCK_CPUS if PREEMPT_RT_FULL because
> vectors_user_mapping() creates a VM_ALWAYSDUMP mapping of the vector page,
> but no ptl->lock has been allocated for the page.  An attempt to coredump
> that page will result in a kernel NULL pointer dereference when
> follow_page() attempts to lock the page.


> 
> This patch is needed only if mm-shrink-the-page-frame-to-rt-size.patch is
> applied. 

Yeah, vile hackery that is.. why isn't pgtable_page_ctor() called on
those pages?

Not that I care too much about split_pte_lock on ARM, they're mostly all
tiny machines anyway so the gain is marginal, but it would be good to
find out why the pgtable constructor isn't called properly.
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