[PATCH v3] mm: fix KASAN build error due to p*d_populate_kernel()

[Harry Yoo <harry.yoo@oracle.com>]

Address a linker error introduced by a patch currently in mm-hotfixes:
"mm: introduce and use {pgd,p4d}_populate_kernel" [1].

KASAN unconditionally references kasan_early_shadow_{p4d,pud}.
However, these global variables may not exist depending on the number of
page table levels. For example, if CONFIG_PGTABLE_LEVELS=3, both
variables do not exist. Although KASAN may refernce non-existent
variables, it didn't break builds because calls to {pgd,p4d}_populate()
are optimized away at compile time.

However, {pgd,p4d}_populate_kernel() is defined as a function regardless
of the number of page table levels, so the compiler may not optimize
them away. In this case, the following linker error occurs:

ld.lld: error: undefined symbol: kasan_early_shadow_p4d
>>> referenced by init.c:260 (/home/hyeyoo/mm-new/mm/kasan/init.c:260)
>>>               mm/kasan/init.o:(kasan_populate_early_shadow) in archive vmlinux.a
>>> referenced by init.c:260 (/home/hyeyoo/mm-new/mm/kasan/init.c:260)
>>>               mm/kasan/init.o:(kasan_populate_early_shadow) in archive vmlinux.a
>>> did you mean: kasan_early_shadow_pmd
>>> defined in: vmlinux.a(mm/kasan/init.o)

ld.lld: error: undefined symbol: kasan_early_shadow_pud
>>> referenced by init.c:263 (/home/hyeyoo/mm-new/mm/kasan/init.c:263)
>>>               mm/kasan/init.o:(kasan_populate_early_shadow) in archive vmlinux.a
>>> referenced by init.c:263 (/home/hyeyoo/mm-new/mm/kasan/init.c:263)
>>>               mm/kasan/init.o:(kasan_populate_early_shadow) in archive vmlinux.a
>>> referenced by init.c:200 (/home/hyeyoo/mm-new/mm/kasan/init.c:200)
>>>               mm/kasan/init.o:(zero_p4d_populate) in archive vmlinux.a
>>> referenced 1 more times

Therefore, to allow calls to {pgd,p4d}_populate_kernel() to be optimized
out at compile time, define {pgd,p4d}_populate_kernel() as macros.
This way, when pgd_populate() or p4d_populate() are simply empty macros,
the corresponding *_populate_kernel() functions can also be optimized
away.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508181636.0Rtk0T7x-lkp@intel.com
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/lkml/20250821160515.611d191e@canb.auug.org.au
Link: https://lore.kernel.org/linux-mm/20250818020206.4517-3-harry.yoo@oracle.com [1]
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---

This is intended to be fold-merged into the patch
"mm: introduce and use {pgd,p4d}_populate_kernel". 

v2 -> v3:
- Explained that this fixes a linker error of a patch in mm-hotfixes. 
- Added links to error reports (Closes:) and Reported-by:

 include/linux/pgalloc.h | 33 +++++++++++++++++++--------------
 1 file changed, 19 insertions(+), 14 deletions(-)

diff --git a/include/linux/pgalloc.h b/include/linux/pgalloc.h
index 290ab864320f..9174fa59bbc5 100644
--- a/include/linux/pgalloc.h
+++ b/include/linux/pgalloc.h
@@ -5,20 +5,25 @@
 #include <linux/pgtable.h>
 #include <asm/pgalloc.h>
 
-static inline void pgd_populate_kernel(unsigned long addr, pgd_t *pgd,
-				       p4d_t *p4d)
-{
-	pgd_populate(&init_mm, pgd, p4d);
-	if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_PGD_MODIFIED)
-		arch_sync_kernel_mappings(addr, addr);
-}
+/*
+ * {pgd,p4d}_populate_kernel() are defined as macros to allow
+ * compile-time optimization based on the configured page table levels.
+ * Without this, linking may fail because callers (e.g., KASAN) may rely
+ * on calls to these functions being optimized away when passing symbols
+ * that exist only for certain page table levels.
+ */
+#define pgd_populate_kernel(addr, pgd, p4d)				\
+	do {								\
+		pgd_populate(&init_mm, pgd, p4d);			\
+		if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_PGD_MODIFIED)	\
+			arch_sync_kernel_mappings(addr, addr);		\
+	} while (0)
 
-static inline void p4d_populate_kernel(unsigned long addr, p4d_t *p4d,
-				       pud_t *pud)
-{
-	p4d_populate(&init_mm, p4d, pud);
-	if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_P4D_MODIFIED)
-		arch_sync_kernel_mappings(addr, addr);
-}
+#define p4d_populate_kernel(addr, p4d, pud)				\
+	do {								\
+		p4d_populate(&init_mm, p4d, pud);			\
+		if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_P4D_MODIFIED)	\
+			arch_sync_kernel_mappings(addr, addr);		\
+	} while (0)
 
 #endif /* _LINUX_PGALLOC_H */
-- 
2.43.0