linux-kernel - Re: [PATCH v5 0/5] kdump: crashkernel reservation from CMA

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Message-ID: <aKjI0wcPDXH5mYcL@dwarf.suse.cz>
Date: Fri, 22 Aug 2025 21:45:23 +0200
From: Jiri Bohac <jbohac@...e.cz>
To: Breno Leitao <leitao@...ian.org>
Cc: Baoquan He <bhe@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
	Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org,
	akpm@...ux-foundation.org, Philipp Rudo <prudo@...hat.com>,
	Donald Dutile <ddutile@...hat.com>, Pingfan Liu <piliu@...hat.com>,
	Tao Liu <ltao@...hat.com>, linux-kernel@...r.kernel.org,
	David Hildenbrand <dhildenb@...hat.com>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH v5 0/5] kdump: crashkernel reservation from CMA

On Thu, Aug 21, 2025 at 01:35:35AM -0700, Breno Leitao wrote:
> I have another question. I assume it’s not possible to allocate only the
> CMA crashkernel area for the kdump kernel, since we need to keep the
> loaded kernel in the crashkernel area while the system is running.
> Therefore, specifying crashkernel=X (without ',cma') is necessary.

exactly

> At the same time, since the crashdump environment will use CMA, the
> crashkernel area itself doesn’t need to be very large, as the CMA space
> will be allocated later.
> 
> With that in mind, how do I find what is the recommended size for the
> crashkernel area, assuming the CMA area will be more than sufficient at
> runtime?
>
> Does it need ot be much higher than the size of kdump kernel and initrd?

I don't have a good answer now - I have this on my to-do list
and I need to investigate this more to come up with a formula to
calculate the required size to include this in the kdump tool I
maintain for SUSE. 

When testing the kernel part I used 
crashkernel=100M crashkernel=XXX,cma on a machine that would
normally require something like crashkernel=430M.

-- 
Jiri Bohac <jbohac@...e.cz>
SUSE Labs, Prague, Czechia