linux-kernel - Re: [PATCH] vmcore: call remap_pfn_range() separately for respective partial pages

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Date:	Tue, 03 Dec 2013 10:18:16 +0900
From:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Atsushi Kumagai <kumagai-atsushi@....nes.nec.co.jp>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>
Subject: Re: [PATCH] vmcore: call remap_pfn_range() separately for respective
 partial pages

(2013/12/03 0:27), Vivek Goyal wrote:
> On Thu, Nov 28, 2013 at 05:48:02PM +0900, HATAYAMA Daisuke wrote:
>> Hello Vivek,
>>
>> Here is a patch set for mmap failure for /proc/vmcore.
>> Could you try to use this on the problematic system?
>>
>> This patch doesn't copy partial pages to the 2nd kernel, only prepares
>> vmcore objects for respective partial pages to invoke remap_pfn_range()
>> for individual partial pages.
>
> Hi Hatayama,
>
> Thanks for the patch. Ok, I see that partial pages will be put in a separate
> call to remap_oldmem_pfn_range() and this time it should succeed.
>
> I am wondering what do you think about your old approach of copying
> only relevant old memory to a new kernel page in new kernel. I kind
> of feel little uncomfortable with the idea of rounding down start
> and roudning up end to page size boundaries and then accessing the
> full page using oldmem interface. A safer approach might be to allocate
> page in new kernel, read *only* those bytes as reported by elf header
> and fill rest of the page with zeros.
>
> Thanks
> Vivek
>

Even if copying partial pages into the 2nd kernel, we need to use ioremap()
once on them, and I think the ioremap() is exactly similar to
remap_pfn_range() for a single page. There seems no difference on safeness
between them.

Also, current /proc/vmcore shows user-land tools a shape with holes not
filled with zeros both in case of read() and in case of mmap(). If we adapt
copying one without reading data in holes, shape of /proc/vmcore gets
changed again.

-- 
Thanks.
HATAYAMA, Daisuke

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