linux-kernel - Re: [patch] crashdump: fix undefined reference to `elfcorehdr

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Message-ID: <20080729003348.GG10434@verge.net.au>
Date:	Tue, 29 Jul 2008 10:33:50 +1000
From:	Simon Horman <horms@...ge.net.au>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Muli Ben-Yehuda <muli@...ibm.com>,
	Chandru <chandru@...ibm.com>, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Terry Loftin <terry.loftin@...com>,
	Tony Luck <tony.luck@...el.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-ia64@...r.kernel.org
Subject: Re: [patch] crashdump: fix undefined reference to `elfcorehdr_addr'

On Mon, Jul 28, 2008 at 09:31:10AM -0400, Vivek Goyal wrote:
> On Mon, Jul 28, 2008 at 11:51:19AM +1000, Simon Horman wrote:
> > [ Updated Vivek's email address to his vgoyal@...hat.com in CC list 
> >   Added Terry Loftin, Tony Luck, Erik Biedermann and linux-ia64 to CC list ]

[snip]

> > 1) Always parse the elfcorehdr kernel command line option
> >    and set elfcorehdr_addr accordingly - currently this is only
> >    done if CONFIG_PROC_VMCORE is set.
> > 
> >    This is nice as it won't need any modifications to kexec-tools
> >    nor any command line bloat.
> > 
> >    A minor difficulty is working out where to initialise elfcorehdr_addr.
> >    Sometimes in include/linux/crash_dump.h and sometimes in
> >    fs/proc/vmcore.c seems horrible to me.
> > 
> >    Another problem is that would be alive and well in
> >    code that really only uses it to check if kdump was activated or not
> >    - a minor naming issue.
> > 
> 
> Hi Simon,
> 
> There are some kernel bits (like iommu initialization patch), which need to
> take special action if they are booting after a kexec on panic (Generally we
> are referring it to booting into kdump kernel) and that's why the notion
> is_kdump_kernel().
> 
> To me, is_kdump_kernel() symbolizes whether I  am booting after kexec on
> panic and not just the fact if CONFIG_CRASH_DUMP is enabled or not in this
> kernel.
> 
> So I would think that lets not rename it to kernel_has_vmcore(), instead
> lets write few lines of comments before function is_kdump_kernel() to
> clarify its meaning.
> 
> Secondly, we are using elfcorehdr_addr to determine whether this kernel
> is booting after a panic so elfcorehdr_addr is not just limited to
> CONFIG_PROC_VMCORE now and we should probably pull it out of
> fs/proc/vmcore.c. How about declaring and initializing this variable in
> kernel/kexec.c under CONFIG_CRASH_DUMP and always parse elfcorehdr_addr
> irrespective of the setting of CONFIG_PROC_VMCORE?

Agreed. Like Eric I think that this is a reasonable solution given
the current state of things. I'll reply to your patches after looking
at them a bit more closely.

> > 2) Add a new kernel command line option, perhaps in_kdump
> > 
> >    This is bloat to get around elfcorehdr_addr initialisation and
> >    naming awkwardness above.
> > 
> > 3) Make select CONFIG_PROC_VMCORE when CONFIG_CRASH_DUMP is selected,
> >    or perhaps even just remove CONFIG_PROC_VMCORE and only use
> >    CONFIG_CRASH_DUMP instead. The effect would be the same either way.
> > 
> >    Pro: One less thing to be confused about
> > 
> >    Con: Bloat for people who want kdump without vmcore.
> >         I wonder what usage case that is.
> 
> Argument was people can use /dev/oldmem and not use /proc/vmcore. So far
> I don't know anybody who uses /dev/oldmem to capture dump and not
> /proc/vmcore.

What I was getting at is that frangly the three variables
CONFIG_KEXEC, CONFIG_CRASH_DUMP and CONFIG_PROC_VMCORE seem to
confuse people. I've seen them used incorrectly several times now.
So if there is a way to simplyfy things, even slightly, I think
that would be a good idea. But if there isn't, so be it.

-- 
Horms

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