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 */