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

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Message-ID: <4E83687D.1070507@am.sony.com>
Date:	Wed, 28 Sep 2011 11:33:33 -0700
From:	Frank Rowand <frank.rowand@...sony.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"Rowand, Frank" <Frank_Rowand@...yusa.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PREEMPT_RT_FULL: arm coredump fails for cpu >= 4

On 09/28/11 06:03, Peter Zijlstra wrote:
> 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?

Yep, that is the question.  I started fixing that yesterday, but realized
I was going about it the wrong way, so I sent a first version of the
patch that simply avoids the problem.

I'll be looking at whether I can fix it cleanly.

> 
> 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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