[PATCH v4 5/5] crash: export crashkernel CMA reservation to userspace

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

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>
---
 .../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