Re: [PATCH v6] crash: export crashkernel CMA reservation to userspace

[Baoquan he <bhe@redhat.com>]

On 11/18/25 at 12:40pm, Sourabh Jain wrote:
> Add a sysfs entry /sys/kernel/kexec_crash_cma_ranges to expose all
> CMA crashkernel ranges.
> 
> 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
> 
> The reason for not including Crash CMA Ranges in /proc/iomem is to avoid
> conflicts. It has been observed that contiguous memory ranges are sometimes
> shown as two separate System RAM entries in /proc/iomem. If a CMA range
> overlaps two System RAM ranges, adding crashk_res to /proc/iomem can create
> a conflict. Reference [1] describes one such instance on the PowerPC
> architecture.
> 
> Link: https://lore.kernel.org/all/20251016142831.144515-1-sourabhjain@linux.ibm.com/ [1]
> 
> 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:
> 
> v4 -> v5:
>  https://lore.kernel.org/all/20251114152550.ac2dd5e23542f09c62defec7@linux-foundation.org/
>  - Splitted patch from the above patch series.
>  - Code to create kexec node under /sys/kernel is added, eariler it was
>    done in [02/05] of the above patch series.
> 
> v5 -> v6:
>  - Add Crash CMA Range sysfs interface under /sys/kernel
> 
> Note:
> This patch is dependent on the below patch:
> https://lore.kernel.org/all/20251117035153.1199665-1-sourabhjain@linux.ibm.com/
> 
> ---
>  .../ABI/testing/sysfs-kernel-kexec-kdump      | 10 +++++++++
>  kernel/ksysfs.c                               | 21 +++++++++++++++++++
>  2 files changed, 31 insertions(+)

Acked-by: Baoquan He <bhe@redhat.com>

> 
> diff --git a/Documentation/ABI/testing/sysfs-kernel-kexec-kdump b/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
> index 96b24565b68e..f6089e38de5f 100644
> --- a/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
> +++ b/Documentation/ABI/testing/sysfs-kernel-kexec-kdump
> @@ -41,3 +41,13 @@ 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/ksysfs.c b/kernel/ksysfs.c
> index eefb67d9883c..0ff2179bc603 100644
> --- a/kernel/ksysfs.c
> +++ b/kernel/ksysfs.c
> @@ -135,6 +135,24 @@ static ssize_t kexec_crash_loaded_show(struct kobject *kobj,
>  }
>  KERNEL_ATTR_RO(kexec_crash_loaded);
>  
> +#ifdef CONFIG_CRASH_RESERVE
> +static ssize_t kexec_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;
> +}
> +KERNEL_ATTR_RO(kexec_crash_cma_ranges);
> +#endif /* CONFIG_CRASH_RESERVE */
> +
>  static ssize_t kexec_crash_size_show(struct kobject *kobj,
>  				       struct kobj_attribute *attr, char *buf)
>  {
> @@ -260,6 +278,9 @@ static struct attribute * kernel_attrs[] = {
>  #ifdef CONFIG_CRASH_DUMP
>  	&kexec_crash_loaded_attr.attr,
>  	&kexec_crash_size_attr.attr,
> +#ifdef CONFIG_CRASH_RESERVE
> +	&kexec_crash_cma_ranges_attr.attr,
> +#endif
>  #endif
>  #endif
>  #ifdef CONFIG_VMCORE_INFO
> -- 
> 2.51.1
>