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

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Date:   Wed, 22 Mar 2017 17:45:28 +0530
From:   Hari Bathini <hbathini@...ux.vnet.ibm.com>
To:     xlpang@...hat.com, Dave Young <dyoung@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Baoquan He <bhe@...hat.com>,
        Atsushi Kumagai <ats-kumagai@...jp.nec.com>,
        Petr Tesarik <ptesarik@...e.cz>, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, holzheu@...ux.vnet.ibm.com,
        kexec@...ts.infradead.org
Subject: Re: [PATCH v3 1/3] kexec: Move vmcoreinfo out of the kernel's .bss
 section

Hi Xunlei,


On Wednesday 22 March 2017 03:04 PM, Xunlei Pang wrote:
> On 03/22/2017 at 12:30 PM, Dave Young wrote:
>> On 03/21/17 at 10:18pm, Eric W. Biederman wrote:
>>> Dave Young <dyoung@...hat.com> writes:
>>>
>>>> On 03/20/17 at 10:33pm, Eric W. Biederman wrote:
>>>>> Xunlei Pang <xlpang@...hat.com> writes:
>>>>>
>>>>>> As Eric said,
>>>>>> "what we need to do is move the variable vmcoreinfo_note out
>>>>>> of the kernel's .bss section.  And modify the code to regenerate
>>>>>> and keep this information in something like the control page.
>>>>>>
>>>>>> Definitely something like this needs a page all to itself, and ideally
>>>>>> far away from any other kernel data structures.  I clearly was not
>>>>>> watching closely the data someone decided to keep this silly thing
>>>>>> in the kernel's .bss section."
>>>>>>
>>>>>> This patch allocates extra pages for these vmcoreinfo_XXX variables,
>>>>>> one advantage is that it enhances some safety of vmcoreinfo, because
>>>>>> vmcoreinfo now is kept far away from other kernel data structures.
>>>>> Can you preceed this patch with a patch that removes CRASHTIME from
>>>>> vmcoreinfo?  If someone actually cares we can add a separate note that holds
>>>>> a 64bit crashtime in the per cpu notes.
>>>> 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.
>>>>
> By moving the CRASHTIME info to the cpu note of crashed cpu may be a good
> way. In kdump kernel, notes of vmcore elfhdr will be merged into one big note
> section, I don't know how makedumpfile or crash handle the big note section?
> If they process the note in some order, breakage will definitely happen...
>
> There is also a fadump may be affected.
>

Would like to keep a tab of such change as fadump builds cpu notes 
differently
and such change may have an impact on it considering it depends on the same
tools - crash, makedumpfile..

Thanks
Hari