Re: [PATCH v3 5/5] crash: export crashkernel CMA reservation to userspace

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

On 10/11/25 12:38, Baoquan he wrote:
> On 11/10/25 at 10:01am, Sourabh Jain wrote:
>> Add a sysfs entry /sys/kernel/kexec/crash_cma_ranges to expose all
>> CMA crashkernel ranges.
> I am not against this way. While wondering if it's more appropriate to
> export them into iomem_resource just like crashk_res and crashk_low_res
> doing.

Handling conflict is challenging. Hence we don't export crashk_res and
crashk_low_res to iomem on powerpc. Checkout [1]

And I think conflicts can occur regardless of the order in which System 
RAM and
Crash CMA ranges are added to iomem.

[1] 
https://lore.kernel.org/all/20251016142831.144515-1-sourabhjain@linux.ibm.com/

- Sourabh Jain

>
>> This allows userspace tools configuring kdump to determine how much
>> memory is reserved for crashkernel. If CMA is used, tools can warn
>> users when attempting to capture user pages with CMA reservation.
>>
>> The new sysfs hold the CMA ranges in below format:
>>
>> cat /sys/kernel/kexec/crash_cma_ranges
>> 100000000-10c7fffff
>>
>> Cc: Aditya Gupta <adityag@linux.ibm.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Baoquan he <bhe@redhat.com>
>> Cc: Dave Young <dyoung@redhat.com>
>> Cc: Hari Bathini <hbathini@linux.ibm.com>
>> Cc: Jiri Bohac <jbohac@suse.cz>
>> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
>> Cc: Mahesh J Salgaonkar <mahesh@linux.ibm.com>
>> Cc: Pingfan Liu <piliu@redhat.com>
>> Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>> Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
>> Cc: Vivek Goyal <vgoyal@redhat.com>
>> Cc: linuxppc-dev@lists.ozlabs.org
>> Cc: kexec@lists.infradead.org
>> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
>> ---
>> Changelog:
>>   - Add the missing hunk to export crash_cma_ranges sysfs
>>
>> ---
>>   .../ABI/testing/sysfs-kernel-kexec-kdump        | 10 ++++++++++
>>   kernel/kexec_core.c                             | 17 +++++++++++++++++
>>   2 files changed, 27 insertions(+)
>>
>> diff --git a/Documentation/ABI/testing/sysfs-kernel-kexec-kdump b/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
>> index 00c00f380fea..f59051b5d96d 100644
>> --- a/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
>> +++ b/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
>> @@ -49,3 +49,13 @@ Description:	read only
>>   		is used by the user space utility kexec to support updating the
>>   		in-kernel kdump image during hotplug operations.
>>   User:		Kexec tools
>> +
>> +What:		/sys/kernel/kexec/crash_cma_ranges
>> +Date:		Nov 2025
>> +Contact:	kexec@lists.infradead.org
>> +Description:	read only
>> +		Provides information about the memory ranges reserved from
>> +		the Contiguous Memory Allocator (CMA) area that are allocated
>> +		to the crash (kdump) kernel. It lists the start and end physical
>> +		addresses of CMA regions assigned for crashkernel use.
>> +User:		kdump service
>> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
>> index 7476a46de5d6..da6ff72b4669 100644
>> --- a/kernel/kexec_core.c
>> +++ b/kernel/kexec_core.c
>> @@ -1271,6 +1271,22 @@ static ssize_t crash_size_store(struct kobject *kobj,
>>   }
>>   static struct kobj_attribute crash_size_attr = __ATTR_RW(crash_size);
>>   
>> +static ssize_t crash_cma_ranges_show(struct kobject *kobj,
>> +				     struct kobj_attribute *attr, char *buf)
>> +{
>> +
>> +	ssize_t len = 0;
>> +	int i;
>> +
>> +	for (i = 0; i < crashk_cma_cnt; ++i) {
>> +		len += sysfs_emit_at(buf, len, "%08llx-%08llx\n",
>> +				     crashk_cma_ranges[i].start,
>> +				     crashk_cma_ranges[i].end);
>> +	}
>> +	return len;
>> +}
>> +static struct kobj_attribute crash_cma_ranges_attr = __ATTR_RO(crash_cma_ranges);
>> +
>>   #ifdef CONFIG_CRASH_HOTPLUG
>>   static ssize_t crash_elfcorehdr_size_show(struct kobject *kobj,
>>   			       struct kobj_attribute *attr, char *buf)
>> @@ -1289,6 +1305,7 @@ static struct attribute *kexec_attrs[] = {
>>   #ifdef CONFIG_CRASH_DUMP
>>   	&crash_loaded_attr.attr,
>>   	&crash_size_attr.attr,
>> +	&crash_cma_ranges_attr.attr,
>>   #ifdef CONFIG_CRASH_HOTPLUG
>>   	&crash_elfcorehdr_size_attr.attr,
>>   #endif
>> -- 
>> 2.51.1
>>