linux-kernel - Re: kdump: No udev events for memory hotplug?

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Date:	Thu, 27 Oct 2011 15:45:44 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	holzheu@...ux.vnet.ibm.com
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: kdump: No udev events for memory hotplug?

On Thu, Oct 27, 2011 at 12:08 AM, Michael Holzheu
<holzheu@...ux.vnet.ibm.com> wrote:
> Hello Vivek and Eric,
>

Hello,

> I noticed that on my system kernel 3.1 *no* udev events for memory
> hotplug are generated. Same on my RHEL6.1.
>
> # udevadm monitor
> # echo offline > /sys/devices/system/memory/memory4/state
>
> -> No event
>
> But we need the udev events in order to do a kdump reload for setting up
> the ELF loads correctly.
>
> In my /etc/udev/rules.d/98-kexec.rules there are rules for memory
> hotplug:
>
> SUBSYSTEM=="memory", ACTION=="add", PROGRAM="/etc/init.d/kdump restart"
> SUBSYSTEM=="memory", ACTION=="remove", PROGRAM="/etc/init.d/kdump
> restart"
>
> Perhaps/probably I am missing something?

As Heiko pointed out, probably the memory-hotplug subsystem doesn't implement
uevent, this is why you can't get uevent when you hot-remove the memory.

For kdump, the memory is reserved by "crashkernel=", it is determined during
boot (but you can shrink it after boot), so it is not aware of memory hotplug.
IOW, if you hot-remove the memory where the crash kernel stays, the
'kdump restart'
will fail. But yes, restarting kdump can rebuild the memmap and ELF headers.

Thanks.
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