[PATCH 1/2] kexec: Print crash kernel reserved size from sysfs

[Rui Qi]

The --print-ckr-size option was added as a user-visible way for
kdump scripts to obtain the crash kernel reserved size. Its original
contract is to print the same value and unit as the kernel
kexec_crash_size sysfs attribute when Linux exports that attribute.

The current implementation derives the value from
get_crash_kernel_load_range(), but that helper describes where crash
kernel segments may be loaded. That is not the same as the total crash
kernel reservation.

The two meanings differ on systems with split crash kernel
reservations. A low reservation may be available to the crash kernel
after boot while the crash image itself must still be loaded into the
main/high reservation. Linux reports the total reserved crash kernel
size through /sys/kernel/kexec_crash_size, including split
reservations.

Prefer /sys/kernel/kexec_crash_size when available so
--print-ckr-size follows its original sysfs-aligned semantics. Keep the
existing load-range calculation as a fallback for older kernels.

Signed-off-by: Rui Qi <qirui.001 at bytedance.com>
---
 kexec/kexec.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/kexec/kexec.c b/kexec/kexec.c
index 58cab57a3ee3..99ec3ff3c373 100644
--- a/kexec/kexec.c
+++ b/kexec/kexec.c
@@ -60,6 +60,7 @@
 
 #define KEXEC_LOADED_PATH "/sys/kernel/kexec_loaded"
 #define KEXEC_CRASH_LOADED_PATH "/sys/kernel/kexec_crash_loaded"
+#define KEXEC_CRASH_SIZE_PATH "/sys/kernel/kexec_crash_size"
 
 unsigned long long mem_min = 0;
 unsigned long long mem_max = ULONG_MAX;
@@ -1495,14 +1496,30 @@ static inline unsigned long get_hotplug_kexec_flag(void)
 static void print_crashkernel_region_size(void)
 {
 	uint64_t start = 0, end = 0;
+	uint64_t size = 0;
+	FILE *fp;
 
-	if (is_crashkernel_mem_reserved() &&
-	    get_crash_kernel_load_range(&start, &end)) {
-		fprintf(stderr, "get_crash_kernel_load_range() failed.\n");
-		return;
+	fp = fopen(KEXEC_CRASH_SIZE_PATH, "r");
+	if (fp) {
+		if (fscanf(fp, "%" SCNu64, &size) == 1) {
+			fclose(fp);
+			printf("%" PRIu64 "\n", size);
+			return;
+		}
+		fclose(fp);
+	}
+
+	if (is_crashkernel_mem_reserved()) {
+		if (get_crash_kernel_load_range(&start, &end)) {
+			fprintf(stderr, "get_crash_kernel_load_range() failed.\n");
+			return;
+		}
+
+		if (start != end)
+			size = end - start + 1;
 	}
 
-	printf("%" PRIu64 "\n", (start != end) ? (end - start + 1) : 0UL);
+	printf("%" PRIu64 "\n", size);
 }
 
 int main(int argc, char *argv[])
-- 
2.52.0