[PATCH v2] memblock: don't allocate new reserve_mem regions on a KHO boot

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On a KHO boot, a reserve_mem region that can't be revived is allocated
from KHO_NOPRSRV memory, which includes the bootmem the next kernel
boots out of. With CONFIG_KEXEC_HANDOVER_DEBUG, preserving the region
then fails:

  WARNING: kernel/liveupdate/kexec_handover.c:1245 at kho_preserve_pages+0x168/0x180
  failed to prepare memblock FDT for KHO: -22

Without it, the next kernel puts its memmap on top of the region and
the ramoops records in it are lost.

There is no use case for adding regions across a live update, so fail
instead of allocating, and drop the TODO.

Fixes: f99230780211 ("memblock: add KHO support for reserve_mem")
Signed-off-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
---
Changes in v2:
- fail on a KHO boot instead of allocating outside bootmem, as agreed
  with Pratyush [1]. this drops v1's patch 1.
- not an RFC anymore.
- based on v7.4-rc1.

v1: https://lore.kernel.org/all/20261021211058.83483-3-azpijr@gmail.com/

[1] https://lore.kernel.org/all/2vxzfqxglrg6.fsf@kernel.org/

 mm/memblock.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/memblock.c b/mm/memblock.c
index a450092a80fb..e325e0542c9a 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -2827,7 +2827,16 @@ static int __init reserve_mem(char *p)
 	if (reserve_mem_kho_revive(name, size, align))
 		return 1;
 
-	/* TODO: Allocation must be outside of KHO_NOPRSRV region */
+	/*
+	 * On a KHO boot memblock only allocates from KHO_NOPRSRV memory here,
+	 * which includes the bootmem the next kernel boots out of. Only regions
+	 * handed over by the previous kernel are supported.
+	 */
+	if (is_kho_boot()) {
+		pr_err("reserve_mem: can't allocate \"%s\" on a KHO boot\n", name);
+		return -EOPNOTSUPP;
+	}
+
 	start = memblock_phys_alloc(size, align);
 	if (!start) {
 		pr_err("reserve_mem: memblock allocation failed\n");

base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
-- 
2.56.0