[PATCH v1 1/2] kho: check scratch vs CMA alignment at runtime

[Sourabh Jain]

KHO requires the scratch alignment to be at least CMA_MIN_ALIGNMENT_BYTES,
and this is currently enforced with a static_assert().

That assertion breaks the build on powerpc once CONFIG_KEXEC_HANDOVER is
enabled together with CONFIG_TRANSPARENT_HUGEPAGE or CONFIG_HUGETLB_PAGE:

    kernel/liveupdate/kexec_handover.c:49:39: error: expression in static assertion is not constant
       49 | static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
    ./include/linux/build_bug.h:80:56: note: in definition of macro '__static_assert'
       80 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
          |                                                        ^~~~
    kernel/liveupdate/kexec_handover.c:49:1: note: in expansion of macro 'static_assert'
       49 | static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);

CMA_MIN_ALIGNMENT_BYTES depends on pageblock_order, which is not a
compile-time constant with certain configs, such as
CONFIG_HUGETLB_PAGE_SIZE_VARIABLE on powerpc. The next patch in this
series enables KHO on powerpc, so the alignment must be checked at
runtime instead.

Replace the static_assert() with a runtime check at the start of
kho_reserve_scratch(), before any scratch memory is reserved. If the
scratch alignment is smaller than the CMA minimum alignment, warn once
and disable KHO rather than continue with a misaligned reservation.

Cc: Aditya Gupta <adityag@xxxxxxxxxxxxx>
Cc: Alexander Graf <graf@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Baoquan He <baoquan.he@xxxxxxxxx>
Cc: Christophe Leroy (CS GROUP) <chleroy@xxxxxxxxxx>
Cc: Hari Bathini <hbathini@xxxxxxxxxxxxx>
Cc: Madhavan Srinivasan <maddy@xxxxxxxxxxxxx>
Cc: Mahesh Salgaonkar <mahesh@xxxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
Cc: Pratyush Yadav <pratyush@xxxxxxxxxx>
Cc: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
Cc: Shivang Upadhyay <shivangu@xxxxxxxxxxxxx>
Cc: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
Signed-off-by: Sourabh Jain <sourabhjain@xxxxxxxxxxxxx>
---
 kernel/liveupdate/kexec_handover.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index 7c4d86daf86d..dc76dd6b74da 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -39,15 +39,7 @@
 #include "../kexec_internal.h"
 #include "kexec_handover_internal.h"
 
-/*
- * This is the minimal alignment required by deferred struct page init.
- * deferred_init_memmap_chunk frees memory to the buddy allocator, which looks
- * at the neighboring pages (up to MAX_PAGE_ORDER) to merge them.
- * If KHO scratch is not aligned to that value, buddy can access uninitialized
- * struct pages, which can cause a crash.
- */
 #define SCRATCH_ALIGNMENT_BYTES (PAGE_SIZE * MAX_ORDER_NR_PAGES)
-static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
 
 /* The magic token for preserved pages */
 #define KHO_PAGE_MAGIC 0x4b484f50U /* ASCII for 'KHOP' */
@@ -836,6 +828,21 @@ static void __init kho_reserve_scratch(void)
 	if (!kho_enable)
 		return;
 
+	/*
+	 * This is the minimal alignment required by deferred struct page init.
+	 * deferred_init_memmap_chunk frees memory to the buddy allocator, which looks
+	 * at the neighboring pages (up to MAX_PAGE_ORDER) to merge them.
+	 * If KHO scratch is not aligned to that value, buddy can access uninitialized
+	 * struct pages, which can cause a crash.
+	 */
+	if (WARN_ON_ONCE(SCRATCH_ALIGNMENT_BYTES < CMA_MIN_ALIGNMENT_BYTES)) {
+		pr_err("scratch alignment %lu < CMA minimum %lu, disabling\n",
+		       (unsigned long)SCRATCH_ALIGNMENT_BYTES,
+		       (unsigned long)CMA_MIN_ALIGNMENT_BYTES);
+		kho_enable = false;
+		return;
+	}
+
 	scratch_size_update();
 
 	/* FIXME: deal with node hot-plug/remove */
-- 
2.55.0