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

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

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;
>>