Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss section

[Xunlei Pang <xpang@redhat.com>]

On 03/23/2017 at 04:48 AM, Michael Holzheu wrote:
> Am Wed, 22 Mar 2017 12:30:04 +0800
> schrieb Dave Young <dyoung@redhat.com>:
>
>> On 03/21/17 at 10:18pm, Eric W. Biederman wrote:
>>> Dave Young <dyoung@redhat.com> writes:
>>>
> [snip]
>
>>>> I think makedumpfile is using it, but I also vote to remove the
>>>> CRASHTIME. It is better not to do this while crashing and a makedumpfile
>>>> userspace patch is needed to drop the use of it.
>>>>
>>>>> As we are looking at reliability concerns removing CRASHTIME should make
>>>>> everything in vmcoreinfo a boot time constant.  Which should simplify
>>>>> everything considerably.
>>>> It is a nice improvement..
>>> We also need to take a close look at what s390 is doing with vmcoreinfo.
>>> As apparently it is reading it in a different kind of crashdump process.
>> Yes, need careful review from s390 and maybe ppc64 especially about
>> patch 2/3, better to have comments from IBM about s390 dump tool and ppc
>> fadump. Added more cc.
> On s390 we have at least an issue with patch 1/3. For stand-alone dump
> and also because we create the ELF header for kdump in the new
> kernel we save the pointer to the vmcoreinfo note in the old kernel on a
> defined memory address in our absolute zero lowcore.
>
> This is done in arch/s390/kernel/setup.c:
>
> static void __init setup_vmcoreinfo(void)
> {
>         mem_assign_absolute(S390_lowcore.vmcore_info, paddr_vmcoreinfo_note());
> }
>
> Since with patch 1/3 paddr_vmcoreinfo_note() returns NULL at this point in
> time we have a problem here.
>
> To solve this - I think - we could move the initialization to
> arch/s390/kernel/machine_kexec.c:
>
> void arch_crash_save_vmcoreinfo(void)
> {
>         VMCOREINFO_SYMBOL(lowcore_ptr);
>         VMCOREINFO_SYMBOL(high_memory);
>         VMCOREINFO_LENGTH(lowcore_ptr, NR_CPUS);
>         mem_assign_absolute(S390_lowcore.vmcore_info, paddr_vmcoreinfo_note());
> }
>
> Probably related to this is my observation that patch 3/3 leads to
> an empty VMCOREINFO note for kdump on s390. The note is there ...
>
> # readelf -n /var/crash/127.0.0.1-2017-03-22-21:14:39/vmcore | grep VMCORE
>   VMCOREINFO           0x0000068e       Unknown note type: (0x00000000)
>
> But it contains only zeros.

Yes, this is a good catch, I will do more tests.

Thanks,
Xunlei

>
> Unfortunately I have not yet understood the reason for this.
>
> Michael
>
>
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