crashkernel only allowing smaller allocations as kernel version increases

From: Robert LeBlanc <robert@leblancnet.us>
To: "Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>
Subject: crashkernel only allowing smaller allocations as kernel version increases
Date: Thu, 8 Dec 2016 11:36:03 -0700	[thread overview]
Message-ID: <CAANLjFrWDNaoopgeh6V_tY9arPsREpMbtsLzURiMOg0rgRMW0g@mail.gmail.com> (raw)

I've been trying to capture a core to help in debuging a problem, but
it seems that as I go up in kernel versions, I can only allocate less
and less memory to crashkernel and with 4.9-rc8 I can't allocate
enough to prevent OOM (specifically on CentOS with iSCSI root). I'm
seeing the same reduction of allocatable memory on both Debian and
CentOS on the same hardware

As an example:
Debian
3.16 - 1 GB is OK
4.9-rc7 - ~512MB is OK, but gets "kernel version is not supported".

CentOS
3.10 - 1 GB is OK
4.8 - ~800MB is OK
4.9-rc8 - ~512MB is OK, but OOMs

CentOS 4.9 kernel was built with default config, Debian 4.9 was built
by coping the distro 3.16.0-4 config and then running make menuconfig.

Trying to specify memory amount above this causes kdump to report that
no memory has been reserved. The hardware has 512GB of RAM. Am I
missing some config option that limiting how much crashkernel can
reserve?

I also verified on my test desktop with 8GB of RAM that setting
crashkernel=1024M also fails to allocate memory with 4.9-rc8, but
works fine with 128M for instance.

Thanks,

----------------
Robert LeBlanc
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