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

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Date:	Wed, 13 Nov 2013 18:00:56 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	Kexec Mailing List <kexec@...ts.infradead.org>,
	Baoquan He <bhe@...hat.com>, WANG Chao <chaowang@...hat.com>,
	Dave Young <dyoung@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: /proc/vmcore mmap() failure issue

On Wed, Nov 13, 2013 at 02:44:45PM -0800, H. Peter Anvin wrote:
> On 11/13/2013 02:41 PM, Vivek Goyal wrote:
> > 
> > Hi Peter,
> > 
> > I noticed we seem to be trimming away partial pages in memblock.
> > 
> > memblock_x86_fill() {
> > 	/* throw away partial pages */
> >         memblock_trim_memory(PAGE_SIZE);
> > }
> > 
> > But not in e820 hence they show up in /proc/iomem.
> > 
> 
> Why does /proc/iomem matter?

Kexec-tools parse /proc/iomem and prepare PT_LOAD elf headers for all
the RAM regions which need to be dumped out, including the partial page ones.
Second kernel parses these headers and exports to user space in /proc/vmcore.
Makedumpfile tries to mmap() vmcore then tries to map the partial page pfn
too and we run into issues.

> 
> > How about something along the lines as below patch. This fixes my
> > /proc/vmcore mmap() issue.
> 
> I'm not sure if what you're seeing is something that is better handled
> in userspace.

I think it should be easy to truncate ELF headers in kexec-tools too when
we are preparing elf headers. I am not sure if it is right thing to do or
not. If some entry is showing up in /proc/iomem as RAM, then kexec-tools
need to believe that it is a possibility that kernel is using that pfn and
that pfn needs to be dumped out in vmcore. IMHO, kernel should fix this
issue.

Secondly, I am writing in-kernel kexec support too and I prepare ELF
headers there too. And I am facing the same problem there too. So if
we truncate partial pages in e820 both kexec-tools and in kernel kexec
implementation don't have to do anything.

Thanks
Vivek
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