[RFC V1 PATCH mm-hotfixes 1/3] mm: introduce and use {pgd,p4d}_populate_kernel()

Harry Yoo
2025-07-09T13:18:06Z

Intrdocue and use {pgd,p4d}_pouplate_kernel() in core MM code when
populating PGD and P4D entries corresponding to the kernel address
space. The main purpose of these helpers is to ensure synchronization of
the kernel portion of the top-level page tables whenever such an entry
is populated.

Until now, the kernel has relied on each architecture to handle
synchronization of top-level page tables in an ad-hoc manner.
For example, see commit 9b861528a801 ("x86-64, mem: Update all PGDs for
direct mapping and vmemmap mapping changes").

However, this approach has proven fragile, as it's easy to forget to
perform the necessary synchronization when introducing new changes.

To address this, introduce _kernel() varients of the page table
population helpers that invoke architecture-specific hooks to properly
synchronize the page tables.

For now, it only targets x86_64, so only PGD and P4D level helpers are
introduced. In theory, PUD and PMD level helpers can be added later if
needed by other architectures.

Currently it is no-op as no arch defines __HAVE_ARCH_SYNC_KERNEL_PGTABLES.

Cc: <stable@vger.kernel.org>
Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
include/asm-generic/pgalloc.h | 4 ++++
include/linux/pgalloc.h | 0
mm/kasan/init.c | 10 +++++-----
mm/percpu.c | 4 ++--
mm/sparse-vmemmap.c | 4 ++--
5 files changed, 13 insertions(+), 9 deletions(-)
create mode 100644 include/linux/pgalloc.h

diff --git a/include/asm-generic/pgalloc.h b/include/asm-generic/pgalloc.h
index 3c8ec3bfea44..6cac1ce64e01 100644
--- a/include/asm-generic/pgalloc.h
+++ b/include/asm-generic/pgalloc.h
@@ -295,6 +295,10 @@ static inline void pgd_free(struct mm_struct *mm, pgd_t *pgd)
__pgd_free(mm, pgd);
}
#endif
+#ifndef __HAVE_ARCH_SYNC_KERNEL_PGTABLE
+#define pgd_populate_kernel(addr, pgd, p4d) pgd_populate(&init_mm, pgd, p4d)
+#define p4d_populate_kernel(addr, p4d, pud) p4d_populate(&init_mm, p4d, pud)
+#endif

#endif /* CONFIG_MMU */

diff --git a/include/linux/pgalloc.h b/include/linux/pgalloc.h
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/mm/kasan/init.c b/mm/kasan/init.c
index ced6b29fcf76..43de820ee282 100644
--- a/mm/kasan/init.c
+++ b/mm/kasan/init.c
@@ -191,7 +191,7 @@ static int __ref zero_p4d_populate(pgd_t *pgd, unsigned long addr,
pud_t *pud;
pmd_t *pmd;

- p4d_populate(&init_mm, p4d,
+ p4d_populate_kernel(addr, p4d,
lm_alias(kasan_early_shadow_pud));
pud = pud_offset(p4d, addr);
pud_populate(&init_mm, pud,
@@ -212,7 +212,7 @@ static int __ref zero_p4d_populate(pgd_t *pgd, unsigned long addr,
} else {
p = early_alloc(PAGE_SIZE, NUMA_NO_NODE);
pud_init(p);
- p4d_populate(&init_mm, p4d, p);
+ p4d_populate_kernel(addr, p4d, p);
}
}
zero_pud_populate(p4d, addr, next);
@@ -251,10 +251,10 @@ int __ref kasan_populate_early_shadow(const void *shadow_start,
* puds,pmds, so pgd_populate(), pud_populate()
* is noops.
*/
- pgd_populate(&init_mm, pgd,
+ pgd_populate_kernel(addr, pgd,
lm_alias(kasan_early_shadow_p4d));
p4d = p4d_offset(pgd, addr);
- p4d_populate(&init_mm, p4d,
+ p4d_populate_kernel(addr, p4d,
lm_alias(kasan_early_shadow_pud));
pud = pud_offset(p4d, addr);
pud_populate(&init_mm, pud,
@@ -273,7 +273,7 @@ int __ref kasan_populate_early_shadow(const void *shadow_start,
if (!p)
return -ENOMEM;
} else {
- pgd_populate(&init_mm, pgd,
+ pgd_populate_kernel(addr, pgd,
early_alloc(PAGE_SIZE, NUMA_NO_NODE));
}
}
diff --git a/mm/percpu.c b/mm/percpu.c
index 782cc148b39c..57450a03c432 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -3134,13 +3134,13 @@ void __init __weak pcpu_populate_pte(unsigned long addr)

if (pgd_none(*pgd)) {
p4d = memblock_alloc_or_panic(P4D_TABLE_SIZE, P4D_TABLE_SIZE);
- pgd_populate(&init_mm, pgd, p4d);
+ pgd_populate_kernel(addr, pgd, p4d);
}

p4d = p4d_offset(pgd, addr);
if (p4d_none(*p4d)) {
pud = memblock_alloc_or_panic(PUD_TABLE_SIZE, PUD_TABLE_SIZE);
- p4d_populate(&init_mm, p4d, pud);
+ p4d_populate_kernel(addr, p4d, pud);
}

pud = pud_offset(p4d, addr);
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index fd2ab5118e13..e275310ac708 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -229,7 +229,7 @@ p4d_t * __meminit vmemmap_p4d_populate(pgd_t *pgd, unsigned long addr, int node)
if (!p)
return NULL;
pud_init(p);
- p4d_populate(&init_mm, p4d, p);
+ p4d_populate_kernel(addr, p4d, p);
}
return p4d;
}
@@ -241,7 +241,7 @@ pgd_t * __meminit vmemmap_pgd_populate(unsigned long addr, int node)
void *p = vmemmap_alloc_block_zero(PAGE_SIZE, node);
if (!p)
return NULL;
- pgd_populate(&init_mm, pgd, p);
+ pgd_populate_kernel(addr, pgd, p);
}
return pgd;
}
--
2.43.0