[PATCH v4] 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. A KHO boot with kho=off hands
nothing over, so it still allocates.

Add kho_is_enabled() and is_kho_boot() stubs so the memblock tests
still build.

Fixes: f99230780211 ("memblock: add KHO support for reserve_mem")
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
---
Changes in v4:
- only fail when KHO is enabled, a KHO boot with kho=off hands nothing
  over (Mukesh).
- add a kho_is_enabled() stub to the memblock tests.

Changes in v3:
- add an is_kho_boot() stub to the memblock tests (Mike).
- add Pasha's Reviewed-by.

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.

v3: https://lore.kernel.org/all/20261105213153.9572-1-azpijr@gmail.com/
v2: https://lore.kernel.org/all/20261104204235.9066-1-azpijr@gmail.com/
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                     | 12 +++++++++++-
 tools/testing/memblock/internal.h | 10 ++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/mm/memblock.c b/mm/memblock.c
index a450092a80fb..0f466fcfc719 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -2827,7 +2827,17 @@ 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, unless KHO is
+	 * disabled and nothing will be handed over.
+	 */
+	if (kho_is_enabled() && 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");
diff --git a/tools/testing/memblock/internal.h b/tools/testing/memblock/internal.h
index ba4d1c02fde5..a68124d76b10 100644
--- a/tools/testing/memblock/internal.h
+++ b/tools/testing/memblock/internal.h
@@ -51,4 +51,14 @@ static inline bool kho_bootmem_overlap(phys_addr_t phys, size_t size)
 	return false;
 }
 
+static inline bool kho_is_enabled(void)
+{
+	return false;
+}
+
+static inline bool is_kho_boot(void)
+{
+	return false;
+}
+
 #endif

base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
-- 
2.56.0