linux-kernel - Re: /proc/vmcore mmap() failure issue

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Message-ID: <528B3585.5080606@jp.fujitsu.com>
Date:	Tue, 19 Nov 2013 18:55:17 +0900
From:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
To:	Atsushi Kumagai <kumagai-atsushi@....nes.nec.co.jp>
CC:	"vgoyal@...hat.com" <vgoyal@...hat.com>,
	"bhe@...hat.com" <bhe@...hat.com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ebiederm@...ssion.com" <ebiederm@...ssion.com>,
	"dyoung@...hat.com" <dyoung@...hat.com>,
	"chaowang@...hat.com" <chaowang@...hat.com>
Subject: Re: /proc/vmcore mmap() failure issue

(2013/11/18 9:51), Atsushi Kumagai wrote:
> (2013/11/15 23:26), Vivek Goyal wrote:
>> On Fri, Nov 15, 2013 at 06:41:52PM +0900, HATAYAMA Daisuke wrote:
>>
>> [..]
>>>> Given the fact that hpa does not like fixing it in kernel. We are left
>>>> with option of fixing it in following places.
>>>>
>>>> - Drop partial pages in kexec-tools
>>>> - Drop partial pages in makeudmpfile.
>>>> - Read partial pages using read() interface in makedumpfile
>>>> - Modify /proc/vmcore to copy partial pages in second kernel's memory.
>>>>
>>>> It is not clear to me that partial pages are really useful.  So I want
>>>> to avoid modifying /proc/vmcore to deal with partial pages and increase
>>>> complexity.
>>>>
>>>> So fixing makedumpfile (either option2 or option 3) seems least risky
>>>> to me. In fact I would say let us keep it simple and truncate partial
>>>> pages in makedumpfile to keep it simple. And look at option 3 once we
>>>> have a strong use case for partial pages.
>>>>
>>>> What do you think?
>>>>
>>>
>>> As you say, it's not clear that partial pages are really useful, but on
>>> the other hand, it seems to me not clear that they are really useless.
>>> I think we should get them as long as we have access to them.
>>>
>>> It seems best to me the option 3). Switching between read and mmap would
>>> be not so complex and also it's by far flexible in makedumpfile than in
>>> kernel.
>>
>> Ok, I am fine with option 3. It is more complicated option but safe
>> option.
> 
> It sounds reasonable also to me.
> 
>> Is there any chance that you could look into fixing this. I have no
>> experience writing code for makedumpfile.
> 
> I'll send a patch to fix this soon.
> 

Thanks.

BTW, now the following patch has been applied on top of makedumpfile in
kexec-tools package on fedora in order to avoid the issue.

https://lists.fedoraproject.org/pipermail/kexec/2013-November/000254.html

I remember prototype version of mmap patch implemented a kind of --no-mmap option
and we could use it to disable mmap() use and use read() instead, I think
which is useful when we face this kind of issue.

-- 
Thanks.
HATAYAMA, Daisuke

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