linux-kernel - Re: [PATCH v2] kdump: round up the total memory size to 128M for crashkernel reservation

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Date:   Mon, 27 Jun 2022 18:35:43 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Tao Liu <ltao@...hat.com>
Cc:     vgoyal@...hat.com, dyoung@...hat.com, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH v2] kdump: round up the total memory size to 128M for
 crashkernel reservation

On 06/27/22 at 03:44pm, Tao Liu wrote:
> The total memory size we get in kernel is usually slightly less than
> the actual memory size because BIOS/firmware will reserve some memory
> region. So it won't export all memory as usable.
> 
> E.g, on my x86_64 kvm guest with 1G memory, the total_mem value shows:
> UEFI boot with ovmf:   0x3faef000
> Legacy boot kvm guest: 0x3ff7ec00
> 
> When specifying crashkernel=1G-2G:128M, if we have a 1G memory machine,
> we get total size 1023M from firmware. Then it will not fall into
> 1G-2G, thus no memory reserved. User will never know this, it is hard
> to let user know the exact total value in kernel.
> 
> One way is to use dmi/smbios to get physical memory size, but it's not
> reliable as well. According to Prarit hardware vendors sometimes screw
> this up. Thus round up total size to 128M to work around this problem.
> 
> This patch is a resend of [1] and rebased onto v5.19-rc2, and the
> original credit goes to Dave Young <dyoung@...hat.com>.

This should be put into cover letter, or put together with change log.
Other than this, LGTM,

Acked-by: Baoquan He <bhe@...hat.com>

> 
> [1]: http://lists.infradead.org/pipermail/kexec/2018-April/020568.html
> 
> Signed-off-by: Tao Liu <ltao@...hat.com>
> ---
> v1 -> v2:
> Modified commit log based on Baoquan's advice.
> ---
>  kernel/crash_core.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 71122e01623c..b58b27cbdb61 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -9,6 +9,7 @@
>  #include <linux/init.h>
>  #include <linux/utsname.h>
>  #include <linux/vmalloc.h>
> +#include <linux/sizes.h>
>  
>  #include <asm/page.h>
>  #include <asm/sections.h>
> @@ -43,6 +44,15 @@ static int __init parse_crashkernel_mem(char *cmdline,
>  					unsigned long long *crash_base)
>  {
>  	char *cur = cmdline, *tmp;
> +	unsigned long long total_mem = system_ram;
> +
> +	/*
> +	 * Firmware sometimes reserves some memory regions for its own use,
> +	 * so the system memory size is less than the actual physical memory
> +	 * size. Work around this by rounding up the total size to 128M,
> +	 * which is enough for most test cases.
> +	 */
> +	total_mem = roundup(total_mem, SZ_128M);
>  
>  	/* for each entry of the comma-separated list */
>  	do {
> @@ -87,13 +97,13 @@ static int __init parse_crashkernel_mem(char *cmdline,
>  			return -EINVAL;
>  		}
>  		cur = tmp;
> -		if (size >= system_ram) {
> +		if (size >= total_mem) {
>  			pr_warn("crashkernel: invalid size\n");
>  			return -EINVAL;
>  		}
>  
>  		/* match ? */
> -		if (system_ram >= start && system_ram < end) {
> +		if (total_mem >= start && total_mem < end) {
>  			*crash_size = size;
>  			break;
>  		}
> -- 
> 2.33.1
>