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

Sourabh Jain
2026-04-08T11:32:49Z

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.
>
> 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.
>
> 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).

Since we are mentioning that this follows same policy as
crashkernel=size,high
in certain cases I think it is good to mention crashkernel=size,low is
ignored here.

> +
> + 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;

false to char *? We don't even need to initialize.

> +
> + 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;

Not all architectures support high crashkernel reservation. So high and
low_size
can be NULL.

I think we should just skip parse_crashkernel_boundary() call if high is
NULL. - Sourabh Jain
> + }
> +
> + 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;