linux-kernel - Re: Crash during vmcore

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Message-ID: <4EC61B2A.7010000@redhat.com>
Date:	Fri, 18 Nov 2011 16:45:30 +0800
From:	Dave Young <dyoung@...hat.com>
To:	tim@...ecast.com
CC:	WANG Cong <xiyou.wangcong@...il.com>, Tejun Heo <tj@...nel.org>,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: Crash during vmcore_init

On 11/18/2011 04:43 PM, Dave Young wrote:

> On 11/18/2011 12:40 AM, Tim Hartrick wrote:
> 
>>
>> Dave, Tejun, Americo,
>>
>> Attached find three configs:
>>
>> Ubuntu 2.6.32-21-server - works
>> Ubuntu 2.6.38-8-server - fails
>> Ubuntu 3.3.1-030101-generic (stable) - fails
> 
> 
> Thanks, Tim
> 
>>
>> On Thu, 2011-11-17 at 15:21 +0800, Dave Young wrote:
>>> On 11/17/2011 01:22 PM, Tim Hartrick wrote:
>>>
>>>> Tejun, Dave,
>>>>
>>>> I will be happy to answer any questions about our environment or test
>>>> debug or other patches.  Just tell me what you need.
>>>
>>>
>>> Thank you. Can you share your kernel config?
>>>
>>>>
>>>> tim
>>>>
>>>> On Nov 16, 2011 8:44 PM, "Dave Young" <dyoung@...hat.com
>>>> <mailto:dyoung@...hat.com>> wrote:
>>>>
>>>>     On 11/17/2011 12:34 PM, Tejun Heo wrote:
>>>>
>>>>     > Hello,
>>>>     >
>>>>     > On Wed, Nov 16, 2011 at 7:30 PM, Dave Young <dyoung@...hat.com
>>>>     <mailto:dyoung@...hat.com>> wrote:
>>>>     >> This addr is converted to an invalid phys address,
>>>>     >
>>>>     > I'm a bit lost on the context here. Who's calling
>>>>     per_cpu_ptr_to_phys()?
>>>>
>>>>
>>>>     It's drivers/base/cpu.c : show_crash_notes()
>>>>
>>>>     >
>>>>     >> looking the code below:
>>>>     >>       if (in_first_chunk) {
>>>>     >>                if (!is_vmalloc_addr(addr))
>>>>     >>                        return __pa(addr);
>>>>     >>                else
>>>>     >>                        return page_to_phys(vmalloc_to_page(addr));
>>>>     >>        } else
>>>>     >>                return page_to_phys(pcpu_addr_to_page(addr));
>>>>     >>
>>>>     >> I dont understand per cpu allocation well, if addr is not in
>>>>     first chunk
>>>>     >> then it should be in vmalloc area?
>>>>     >
>>>>     > Yes, it is. First chunk can be embedded in the kernel linear address
>>>>     > space but from the second one, it's always set up from the top of the
>>>>     > vmalloc area with the same offset layout as the first chunk.
>>>>
>>>>
>>>>     in this case ffff880667c19ad0 fall out of vmalloc area and it's not in
>>>>     first chunk also.
> 
> 
> Tejun,
> 
> With config provided by Tim, I can reproduce this problem on a dell
> machine. I did some debug about this, found that fisrt_start <
> first_end, 


typo, I mean first_start > first_end

so there's no chance to check in for_each_possible_cpu(cpu)
> 
> why is the first_start/first_end wrong? pcpu_unit_offsets[] is not
> ordered? any idea?
> 
> I see below hack make the bug gone, it confirmed the addr is indeed in
> first chunk.
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index bf80e55..8f6eb58 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -984,26 +984,14 @@ phys_addr_t per_cpu_ptr_to_phys(void *addr)
>  {
>  	void __percpu *base = __addr_to_pcpu_ptr(pcpu_base_addr);
>  	bool in_first_chunk = false;
> -	unsigned long first_start, first_end;
>  	unsigned int cpu;
> 
> -	/*
> -	 * The following test on first_start/end isn't strictly
> -	 * necessary but will speed up lookups of addresses which
> -	 * aren't in the first chunk.
> -	 */
> -	first_start = pcpu_chunk_addr(pcpu_first_chunk, pcpu_first_unit_cpu, 0);
> -	first_end = pcpu_chunk_addr(pcpu_first_chunk, pcpu_last_unit_cpu,
> -				    pcpu_unit_pages);
> -	if ((unsigned long)addr >= first_start &&
> -	    (unsigned long)addr < first_end) {
> -		for_each_possible_cpu(cpu) {
> -			void *start = per_cpu_ptr(base, cpu);
> -
> -			if (addr >= start && addr < start + pcpu_unit_size) {
> -				in_first_chunk = true;
> -				break;
> -			}
> +	for_each_possible_cpu(cpu) {
> +		void *start = per_cpu_ptr(base, cpu);
> +
> +		if (addr >= start && addr < start + pcpu_unit_size) {
> +			in_first_chunk = true;
> +			break;
>  		}
>  	}
> 
>>>>
>>>>     >
>>>>     >> Tejun, do you have any idea about this?
>>>>     >
>>>>     > Can you please tell me how to reproduce the problem? I'll try to find
>>>>     > out what's going on.
>>>>
>>>>
>>>>     make sure kernel support CRASH DUMP, then cat
>>>>     /sys/devices/system/cpu/cpu[x]/crash_notes
>>>>
>>>>     Tim Hartrick <tim@...ecast.com <mailto:tim@...ecast.com>> reported
>>>>     the problem when test kdump.
>>>>     But I can not reproduce this. I think tim can help to test
>>>>
>>>>     >
>>>>     > Thanks.
>>>>     >
>>>>
>>>>
>>>>
>>>>     --
>>>>     Thanks
>>>>     Dave
>>>>
>>>
>>>
>>>
>>
> 
> 
> 



-- 
Thanks
Dave
--
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