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

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

[Youling Tang]

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).
+
+   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;
-- 
2.43.0

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

[Sourabh Jain]

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;

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

[Youling Tang]

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@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@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;
>

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

[Sourabh Jain]

On 08/04/26 13:11, Youling Tang wrote:
> 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@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.

OK, in that case, we need an option like >boundary because
DEFAULT_CRASH_KERNEL_LOW_SIZE itself is 128 MB by default
(unless the architecture overrides it).

- Sourabh Jain

>
> 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@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;
>>