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

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

[Sourabh Jain]

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(+)

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

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

[Baoquan he]

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
>