Re: [PATCH] crash: Support high memory reservation for range syntax

[Sourabh Jain <sourabhjain@linux.ibm.com>]

Hello Youling,

On 04/04/26 13:11, Youling Tang wrote:
> From: Youling Tang <tangyouling@kylinos.cn>
>
> The crashkernel range syntax (range1:size1[,range2:size2,...]) allows
> automatic size selection based on system RAM, but it always reserves
> from low memory. When a large crashkernel is selected, this can
> consume most of the low memory, causing subsequent hardware
> hotplug or drivers requiring low memory to fail due to allocation
> failures.


Support for high crashkernel reservation has been added to
address the above problem.

However, high crashkernel reservation is not supported with
range-based crashkernel kernel command-line arguments.
For example: crashkernel=0M-1G:100M,1G-4G:160M,4G-8G:192M

Many users, including some distributions, use range-based
crashkernel configuration. So, adding support for high crashkernel
reservation with range-based configuration would be useful.

>
> Add a new optional conditional suffix ",>boundary" to the crashkernel
> range syntax. When the selected crashkernel size exceeds the specified
> boundary, the kernel will automatically apply the same reservation
> policy as "crashkernel=size,high" - preferring high memory first
> and reserving the default low memory area.

I think the approach to enable high crashkernel reservation
with range-based configuration makes the crashkernel kernel
argument more complex.

If the goal is to support high crashkernel reservation with
range-based kernel command-line arguments, how about:

crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset],high

instead of using >boundary?

>
> Syntax:
>      crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset],>boundary
>
> Example:
>      crashkernel=2G-16G:512M,16G-:1G,>512M
>
> This means:
>    - For 2G-16G RAM: reserve 512M normally
>    - For >16G RAM: reserve 1G with high memory preference (since 1G > 512M)
>
> For systems with >16G RAM, 1G is selected which exceeds 512M, so it
> will be reserved from high memory instead of consuming 1G of
> precious low memory.
>
> Signed-off-by: Youling Tang <tangyouling@kylinos.cn>
> ---
>   Documentation/admin-guide/kdump/kdump.rst     | 25 ++++++++-
>   .../admin-guide/kernel-parameters.txt         |  2 +-
>   kernel/crash_reserve.c                        | 56 ++++++++++++++++---
>   3 files changed, 73 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
> index 7587caadbae1..b5ae4556e9ca 100644
> --- a/Documentation/admin-guide/kdump/kdump.rst
> +++ b/Documentation/admin-guide/kdump/kdump.rst
> @@ -293,7 +293,28 @@ crashkernel syntax
>          2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
>          3) if the RAM size is larger than 2G, then reserve 128M
>   
> -3) crashkernel=size,high and crashkernel=size,low
> +3) range1:size1[,range2:size2,...][@offset],>boundary
> +   Optionally, the range list can be followed by a conditional suffix
> +   `,>boundary`. When the selected crashkernel size matches the
> +   condition, the kernel will reserve memory using the same policy as
> +   `crashkernel=size,high` (i.e. prefer high memory first and reserve the
> +   default low memory area).
> +
> +   The syntax is::
> +
> +        crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset],>boundary
> +        range=start-[end]
> +
> +   For example::
> +
> +        crashkernel=2G-16G:512M,16G-:1G,>512M
> +
> +   This would mean:
> +       1) if the RAM size is between 2G and 16G (exclusive), then reserve 512M.
> +       2) if the RAM size is larger than 16G, allocation will behave like
> +          `crashkernel=1G,high`.
> +
> +4) crashkernel=size,high and crashkernel=size,low
>   
>      If memory above 4G is preferred, crashkernel=size,high can be used to
>      fulfill that. With it, physical memory is allowed to be allocated from top,
> @@ -311,7 +332,7 @@ crashkernel syntax
>   
>               crashkernel=0,low
>   
> -4) crashkernel=size,cma
> +5) crashkernel=size,cma
>   
>   	Reserve additional crash kernel memory from CMA. This reservation is
>   	usable by the first system's userspace memory and kernel movable
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 03a550630644..b2e1892ab4d8 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1087,7 +1087,7 @@ Kernel parameters
>   			4G when '@offset' hasn't been specified.
>   			See Documentation/admin-guide/kdump/kdump.rst for further details.
>   
> -	crashkernel=range1:size1[,range2:size2,...][@offset]
> +	crashkernel=range1:size1[,range2:size2,...][@offset][,>boundary]
>   			[KNL] Same as above, but depends on the memory
>   			in the running system. The syntax of range is
>   			start-[end] where start and end are both
> diff --git a/kernel/crash_reserve.c b/kernel/crash_reserve.c
> index 62e60e0223cf..917738412390 100644
> --- a/kernel/crash_reserve.c
> +++ b/kernel/crash_reserve.c
> @@ -254,15 +254,47 @@ static __init char *get_last_crashkernel(char *cmdline,
>   	return ck_cmdline;
>   }
>   
> +/*
> + * This function parses command lines in the format
> + *
> + *   crashkernel=ramsize-range:size[,...][@offset],>boundary
> + */
> +static void __init parse_crashkernel_boundary(char *ck_cmdline,
> +					unsigned long long *boundary)
> +{
> +	char *cur = ck_cmdline, *next;
> +	char *first_gt = false;
> +
> +	first_gt = strchr(cur, '>');
> +	if (!first_gt)
> +		return;
> +
> +	cur = first_gt + 1;
> +	if (*cur == '\0' || *cur == ' ' || *cur == ',') {
> +		pr_warn("crashkernel: '>' specified without boundary size, ignoring\n");
> +		return;
> +	}
> +
> +	*boundary = memparse(cur, &next);
> +	if (cur == next) {
> +		pr_warn("crashkernel: invalid boundary size after '>'\n");
> +		return;
> +	}
> +}
> +
>   static int __init __parse_crashkernel(char *cmdline,
>   			     unsigned long long system_ram,
>   			     unsigned long long *crash_size,
>   			     unsigned long long *crash_base,
> -			     const char *suffix)
> +			     const char *suffix,
> +			     bool *high,
> +			     unsigned long long *low_size)
>   {
>   	char *first_colon, *first_space;
>   	char *ck_cmdline;
>   	char *name = "crashkernel=";
> +	unsigned long long boundary = 0;
> +	int ret;
>   
>   	BUG_ON(!crash_size || !crash_base);
>   	*crash_size = 0;
> @@ -283,10 +315,20 @@ static int __init __parse_crashkernel(char *cmdline,
>   	 */
>   	first_colon = strchr(ck_cmdline, ':');
>   	first_space = strchr(ck_cmdline, ' ');
> -	if (first_colon && (!first_space || first_colon < first_space))
> -		return parse_crashkernel_mem(ck_cmdline, system_ram,
> +	if (first_colon && (!first_space || first_colon < first_space)) {
> +		ret = parse_crashkernel_mem(ck_cmdline, system_ram,
>   				crash_size, crash_base);
>   
> +		/* Handle optional ',>boundary' condition for range ':' syntax only. */
> +		parse_crashkernel_boundary(ck_cmdline, &boundary);
> +		if (!ret && *crash_size > boundary) {
> +			*high = true;
> +			*low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
> +		}
> +
> +		return ret;
> +	}
> +
>   	return parse_crashkernel_simple(ck_cmdline, crash_size, crash_base);
>   }
>   
> @@ -310,7 +352,7 @@ int __init parse_crashkernel(char *cmdline,
>   
>   	/* crashkernel=X[@offset] */
>   	ret = __parse_crashkernel(cmdline, system_ram, crash_size,
> -				crash_base, NULL);
> +				crash_base, NULL, high, low_size);
>   #ifdef CONFIG_ARCH_HAS_GENERIC_CRASHKERNEL_RESERVATION
>   	/*
>   	 * If non-NULL 'high' passed in and no normal crashkernel
> @@ -318,7 +360,7 @@ int __init parse_crashkernel(char *cmdline,
>   	 */
>   	if (high && ret == -ENOENT) {
>   		ret = __parse_crashkernel(cmdline, 0, crash_size,
> -				crash_base, suffix_tbl[SUFFIX_HIGH]);
> +				crash_base, suffix_tbl[SUFFIX_HIGH], high, low_size);
>   		if (ret || !*crash_size)
>   			return -EINVAL;
>   
> @@ -327,7 +369,7 @@ int __init parse_crashkernel(char *cmdline,
>   		 * is not allowed.
>   		 */
>   		ret = __parse_crashkernel(cmdline, 0, low_size,
> -				crash_base, suffix_tbl[SUFFIX_LOW]);
> +				crash_base, suffix_tbl[SUFFIX_LOW], high, low_size);
>   		if (ret == -ENOENT) {
>   			*low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
>   			ret = 0;
> @@ -344,7 +386,7 @@ int __init parse_crashkernel(char *cmdline,
>   	 */
>   	if (cma_size)
>   		__parse_crashkernel(cmdline, 0, cma_size,
> -			&cma_base, suffix_tbl[SUFFIX_CMA]);
> +			&cma_base, suffix_tbl[SUFFIX_CMA], high, low_size);
>   #endif
>   	if (!*crash_size)
>   		ret = -EINVAL;