[PATCH v2] kho: do not reserve scratch memory in the kdump kernel

[Maxi Saparov]

From: Maxi Saparov <masaparov at coreweave.com>

If a kernel with CONFIG_KEXEC_HANDOVER_ENABLE_DEFAULT=y (or kho=on)
does not receive a handover FDT, it reserves KHO scratch memory in
kho_reserve_scratch().

The kdump kernel always takes this path. Since commit a6715d7ec472
("kho: skip KHO for crash kernel"), the kernel does not add KHO
metadata to the crash kimage. As a result, the kdump kernel never
receives a handover FDT and always reserves scratch memory.

The kdump kernel boots in the small crashkernel= memory reservation.
On our x86 hosts, the kernel and initramfs make approximately 176 MB
of early reservations. The scratch areas then use approximately
650 MB more of the crashkernel memory. When a large early allocation
fails, the kdump kernel panics:

  bio: can't create integrity buf pool

A kdump kernel has nothing to hand over: its only task is to dump the
memory of the old kernel and reboot. Fix is to disable KHO at the top of
kho_memory_init() so that the kdump kernel does not reserve scratch
memory.

Fixes: 3dc92c311498 ("kexec: add Kexec HandOver (KHO) generation helpers")
Cc: stable at vger.kernel.org
Signed-off-by: Maxi Saparov <masaparov at coreweave.com>
---
v2: Move the kdump check to the top of kho_memory_init() as an early return.
v1: https://lore.kernel.org/all/20260803193938.8778-1-maxi.saparov@gmail.com/

 kernel/liveupdate/kexec_handover.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index 6fad9152387a..97729dec6359 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -12,6 +12,7 @@
 
 #include <linux/cleanup.h>
 #include <linux/cma.h>
+#include <linux/crash_dump.h>
 #include <linux/kmemleak.h>
 #include <linux/count_zeros.h>
 #include <linux/kasan.h>
@@ -1631,6 +1632,12 @@ fs_initcall(kho_init);
 
 void __init kho_memory_init(void)
 {
+	if (is_kdump_kernel()) {
+		kho_enable = false;
+		pr_info("disabled in the kdump kernel\n");
+		return;
+	}
+
 	if (kho_in.scratch_phys) {
 		kho_scratch = phys_to_virt(kho_in.scratch_phys);
 

base-commit: 3a0b8fa2eb36afc88b62a95f33f0c77c71fa5ded
-- 
2.55.0