[PATCH 1/1] kho: fix KHO_TREE_MAX_DEPTH for non-4KB page sizes

[George Guo <dongtai.guo@linux.dev>]

From: George Guo <guodongtai@kylinos.cn>

KHO_TREE_MAX_DEPTH is calculated as:

  DIV_ROUND_UP(KHO_ORDER_0_LOG2 - KHO_BITMAP_SIZE_LOG2,
               KHO_TABLE_SIZE_LOG2) + 1

For systems with 16KB pages (e.g. LoongArch), this gives a depth of 4,
with the top-level shift at bit 39.  The order-0 bit sits at bit 50
(KHO_ORDER_0_LOG2 = 64 - PAGE_SHIFT = 50).  When inserting or reading
a key, the index extracted at the top level is:

  (1 << 50) >> 39 = 2048

2048 is exactly the table size (PAGE_SIZE / sizeof(phys_addr_t) = 2048
for 16KB pages), so it wraps to 0, aliasing the order bit to index 0
and losing it silently.

On the second kernel, kho_radix_decode_key() sees a key without the
order bit, calls fls64() on the wrong bit, computes a wrong order and
thus a garbage physical address. phys_to_page() of that address faults
in kho_preserved_memory_reserve(), causing a kernel panic early in boot.

Fix by adding +1 to the DIV_ROUND_UP numerator so the formula accounts
for the order bit itself, giving depth 5 for 16KB pages. The top-level
shift becomes 50, and (1 << 50) >> 50 = 1, which is nonzero and
unambiguous. For 4KB and 64KB page sizes the depth is unchanged.

Fixes: 3f2ad90060f6 ("kho: adopt radix tree for preserved memory tracking")

Tested-by: Kexin Liu <liukexin@kylinos.cn>
Signed-off-by: George Guo <guodongtai@kylinos.cn>
---
 include/linux/kho/abi/kexec_handover.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/kho/abi/kexec_handover.h b/include/linux/kho/abi/kexec_handover.h
index 7e847a2339b0..db9bda6dd310 100644
--- a/include/linux/kho/abi/kexec_handover.h
+++ b/include/linux/kho/abi/kexec_handover.h
@@ -274,7 +274,7 @@ enum kho_radix_consts {
 	 * and 1 bitmap level.
 	 */
 	KHO_TREE_MAX_DEPTH =
-		DIV_ROUND_UP(KHO_ORDER_0_LOG2 - KHO_BITMAP_SIZE_LOG2,
+		DIV_ROUND_UP(KHO_ORDER_0_LOG2 - KHO_BITMAP_SIZE_LOG2 + 1,
 			     KHO_TABLE_SIZE_LOG2) + 1,
 };
 
-- 
2.25.1