linux-kernel - Re: Crash during vmcore

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Message-ID: <4EC61AB6.4090808@redhat.com>
Date:	Fri, 18 Nov 2011 16:43:34 +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 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, 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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