linux-kernel - Re: [PATCH v3 3/5] kdump, documentation: describe craskernel CMA reservation

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Message-ID: <Z9Of+RYjpcDN7+ji@MiWiFi-R3L-srv>
Date: Fri, 14 Mar 2025 11:18:17 +0800
From: Baoquan He <bhe@...hat.com>
To: Jiri Bohac <jbohac@...e.cz>, David Hildenbrand <dhildenb@...hat.com>
Cc: akpm@...ux-foundation.org, Vivek Goyal <vgoyal@...hat.com>,
	Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.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, Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH v3 3/5] kdump, documentation: describe craskernel CMA
 reservation

Hi Jiri,

On 03/12/25 at 10:09pm, Jiri Bohac wrote:
......
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index fb8752b42ec8..895b974dc3bb 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -987,6 +987,28 @@
>  			0: to disable low allocation.
>  			It will be ignored when crashkernel=X,high is not used
>  			or memory reserved is below 4G.
> +	crashkernel=size[KMG],cma
> +			[KNL, X86] Reserve additional crash kernel memory from
> +			CMA. This reservation is usable by the first system's
> +			userspace memory and kernel movable allocations (memory
> +			balloon, zswap). Pages allocated from this memory range
> +			will not be included in the vmcore so this should not
> +			be used if dumping of userspace memory is intended and
> +			it has to be expected that some movable kernel pages
> +			may be missing from the dump.

Since David and Don expressed concern about the missing kernel pages
allocated from CMA area in v2, and you argued this is still useful for
VM system, I would like to invite David to help evaluate the whole
series if it's worth from the VM and MM point of view.

Thanks
Baoquan

> +
> +			A standard crashkernel reservation, as described above,
> +			is still needed to hold the crash kernel and initrd.
> +
> +			This option increases the risk of a kdump failure: DMA
> +			transfers configured by the first kernel may end up
> +			corrupting the second kernel's memory.
> +
> +			This reservation method is intended for systems that
> +			can't afford to sacrifice enough memory for standard
> +			crashkernel reservation and where less reliable and
> +			possibly incomplete kdump is preferable to no kdump at
> +			all.
>  
>  	cryptomgr.notests
>  			[KNL] Disable crypto self-tests
> 
> -- 
> Jiri Bohac <jbohac@...e.cz>
> SUSE Labs, Prague, Czechia
>