linux-kernel - Re: [PATCH 0/2] kdump/mmap: Fix mmap of /proc/vmcore for s390

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Message-ID: <20130524170507.GG18218@redhat.com>
Date:	Fri, 24 May 2013 13:05:07 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
Cc:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	Jan Willeke <willeke@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH 0/2] kdump/mmap: Fix mmap of /proc/vmcore for s390

On Fri, May 24, 2013 at 06:46:53PM +0200, Michael Holzheu wrote:
> On Fri, 24 May 2013 11:28:49 -0400
> Vivek Goyal <vgoyal@...hat.com> wrote:
> 
> > On Fri, May 24, 2013 at 05:06:26PM +0200, Michael Holzheu wrote:
> 
> [snip] 
> 
> > As /proc/vmcore is the most used and useful interface, I prefer that
> > we swap memory and put that info in elf headers. For /dev/oldme, I
> > don't mind if we leave it as it is. If somebody really cares, then
> > I guess we need to write a new command line option which /dev/mem
> > can parse and which tells it about swaps so that /dev/oldmem can
> > map things correctly. (This is better than hardcoding things).
> 
> Besides of the potential /dev/oldmem issue, I still do not understand
> the option of doing the swap in the elf header. Looks like I missed
> here a fundamental design point of kdump :(
> 
> Is that done by specifying different virtual and physical addresses in
> the ELF header?

Nope. We keep the virtual to physical address mapping same. We just modify
the p_offset in PT_LOAD elf header to represent where actually the
memory is present physically. And when /proc/vmcore reads the data, it
reads it from p_offset.

IOW, p_offset and p_paddr will be different for swapped memory but 
should be same for memory which has not been swapped.

Thanks
Vivek
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