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Date: Mon, 17 Mar 2014 10:55:28 +0100
From: Vegard Nossum <vegard.nossum@oracle.com>
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To: Michal Hocko <mhocko@suse.cz>, Xishi Qiu <qiuxishi@huawei.com>
CC: Andrew Morton <akpm@linux-foundation.org>,
        Pekka Enberg <penberg@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        David Rientjes <rientjes@google.com>,
        Vegard Nossum <vegard.nossum@gmail.com>, Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, Li Zefan <lizefan@huawei.com>
Subject: Re: kmemcheck: OS boot failed because NMI handlers access the memory
 tracked by kmemcheck
References: <5326BE25.9090201@huawei.com> <20140317095141.GA4777@dhcp22.suse.cz>
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On 03/17/2014 10:51 AM, Michal Hocko wrote:
> On Mon 17-03-14 17:19:33, Xishi Qiu wrote:
>> OS boot failed when set cmdline kmemcheck=1. The reason is that
>> NMI handlers will access the memory from kmalloc(), this will cause
>> page fault, because memory from kmalloc() is tracked by kmemcheck.
>>
>> watchdog_nmi_enable()
>> 	perf_event_create_kernel_counter()
>> 		perf_event_alloc()
>> 			event = kzalloc(sizeof(*event), GFP_KERNEL);
>
> Where is this path called from an NMI context?
>
> Your trace bellow points at something else and it doesn't seem to
> allocate any memory either. It looks more like x86_perf_event_update
> sees an invalid perf_event or something like that...
>

It's not important that the kzalloc() is called from NMI context, it's 
important that the memory that was allocated is touched (read/written) 
from NMI context.

I'm currently looking into the possibility of handling recursive faults 
in kmemcheck (using the approach outlined by peterz; see 
https://lkml.org/lkml/2014/2/26/141).


Vegard