[PATCH] crash: Support high memory reservation for range syntax
Youling Tang
youling.tang at linux.dev
Wed Apr 8 00:41:14 PDT 2026
Hi, Sourabh
On 4/8/26 12:31, Sourabh Jain wrote:
> Hello Youling,
>
> On 04/04/26 13:11, Youling Tang wrote:
>> From: Youling Tang <tangyouling at 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?
This approach defaults all reservations to high memory. For example,
0M-2G:100M on a 1GB machine reserves 100M + DEFAULT_CRASH_KERNEL_LOW_SIZE,
which wastes memory on small systems.
I prefer small reservations defaulting to low memory, while large
reservations start from high memory (with a default-sized segment
reserved in low memory). This provides better flexibility for
distributions to handle diverse system configurations.
Thanks,
Youling.
>
>>
>> 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 at 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;
>
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