Introduce and use {pgd,p4d}_populate_kernel() in core MM code when populating PGD and P4D entries for the kernel address space. These helpers ensure proper synchronization of page tables when updating the kernel portion of top-level page tables.
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 for following reasons:
1) It is easy to forget to perform the necessary page table synchronization when introducing new changes. For instance, commit 4917f55b4ef9 ("mm/sparse-vmemmap: improve memory savings for compound devmaps") overlooked the need to synchronize page tables for the vmemmap area.
2) It is also easy to overlook that the vmemmap and direct mapping areas must not be accessed before explicit page table synchronization. For example, commit 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")) caused crashes by accessing the vmemmap area before calling sync_global_pgds().
To address this, as suggested by Dave Hansen, introduce _kernel() variants of the page table population helpers, which invoke architecture-specific hooks to properly synchronize page tables. These are introduced in a new header file, include/linux/pgalloc.h, so they can be called from common code.
They reuse existing infrastructure for vmalloc and ioremap. Synchronization requirements are determined by ARCH_PAGE_TABLE_SYNC_MASK, and the actual synchronization is performed by arch_sync_kernel_mappings().
This change currently targets only x86_64, so only PGD and P4D level helpers are introduced. Currently, these helpers are no-ops since no architecture sets PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
In theory, PUD and PMD level helpers can be added later if needed by other architectures. For now, 32-bit architectures (x86-32 and arm) only handle PGTBL_PMD_MODIFIED, so p*d_populate_kernel() will never affect them unless we introduce a PMD level helper.
[harry.yoo@oracle.com: fix KASAN build error due to p*d_populate_kernel()] Link: https://lkml.kernel.org/r/20250822020727.202749-1-harry.yoo@oracle.com Link: https://lkml.kernel.org/r/20250818020206.4517-3-harry.yoo@oracle.com Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges") Suggested-by: Dave Hansen dave.hansen@linux.intel.com Acked-by: Kiryl Shutsemau kas@kernel.org Reviewed-by: Mike Rapoport (Microsoft) rppt@kernel.org Reviewed-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com Acked-by: David Hildenbrand david@redhat.com Cc: Alexander Potapenko glider@google.com Cc: Alistair Popple apopple@nvidia.com Cc: Andrey Konovalov andreyknvl@gmail.com Cc: Andrey Ryabinin ryabinin.a.a@gmail.com Cc: Andy Lutomirski luto@kernel.org Cc: Aneesh Kumar K.V aneesh.kumar@linux.ibm.com Cc: Anshuman Khandual anshuman.khandual@arm.com Cc: Ard Biesheuvel ardb@kernel.org Cc: Arnd Bergmann arnd@arndb.de Cc: bibo mao maobibo@loongson.cn Cc: Borislav Betkov bp@alien8.de Cc: Christoph Lameter (Ampere) cl@gentwo.org Cc: Dennis Zhou dennis@kernel.org Cc: Dev Jain dev.jain@arm.com Cc: Dmitriy Vyukov dvyukov@google.com Cc: Gwan-gyeong Mun gwan-gyeong.mun@intel.com Cc: Ingo Molnar mingo@redhat.com Cc: Jane Chu jane.chu@oracle.com Cc: Joao Martins joao.m.martins@oracle.com Cc: Joerg Roedel joro@8bytes.org Cc: John Hubbard jhubbard@nvidia.com Cc: Kevin Brodsky kevin.brodsky@arm.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Michal Hocko mhocko@suse.com Cc: Oscar Salvador osalvador@suse.de Cc: Peter Xu peterx@redhat.com Cc: Peter Zijlstra peterz@infradead.org Cc: Qi Zheng zhengqi.arch@bytedance.com Cc: Ryan Roberts ryan.roberts@arm.com Cc: Suren Baghdasaryan surenb@google.com Cc: Tejun Heo tj@kernel.org Cc: Thomas Gleinxer tglx@linutronix.de Cc: Thomas Huth thuth@redhat.com Cc: Uladzislau Rezki (Sony) urezki@gmail.com Cc: Vincenzo Frascino vincenzo.frascino@arm.com Cc: Vlastimil Babka vbabka@suse.cz Cc: Pedro Falcato pfalcato@suse.de Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org [ Adjust context ] Signed-off-by: Harry Yoo harry.yoo@oracle.com --- include/linux/pgalloc.h | 29 +++++++++++++++++++++++++++++ include/linux/pgtable.h | 13 +++++++------ mm/kasan/init.c | 12 ++++++------ mm/percpu.c | 6 +++--- mm/sparse-vmemmap.c | 6 +++--- 5 files changed, 48 insertions(+), 18 deletions(-) create mode 100644 include/linux/pgalloc.h
diff --git a/include/linux/pgalloc.h b/include/linux/pgalloc.h new file mode 100644 index 000000000000..9174fa59bbc5 --- /dev/null +++ b/include/linux/pgalloc.h @@ -0,0 +1,29 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _LINUX_PGALLOC_H +#define _LINUX_PGALLOC_H + +#include <linux/pgtable.h> +#include <asm/pgalloc.h> + +/* + * {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) + +#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 */ diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h index e42388b6998b..78a518129e8f 100644 --- a/include/linux/pgtable.h +++ b/include/linux/pgtable.h @@ -1467,8 +1467,8 @@ static inline int pmd_protnone(pmd_t pmd)
/* * Architectures can set this mask to a combination of PGTBL_P?D_MODIFIED values - * and let generic vmalloc and ioremap code know when arch_sync_kernel_mappings() - * needs to be called. + * and let generic vmalloc, ioremap and page table update code know when + * arch_sync_kernel_mappings() needs to be called. */ #ifndef ARCH_PAGE_TABLE_SYNC_MASK #define ARCH_PAGE_TABLE_SYNC_MASK 0 @@ -1601,10 +1601,11 @@ static inline bool arch_has_pfn_modify_check(void) /* * Page Table Modification bits for pgtbl_mod_mask. * - * These are used by the p?d_alloc_track*() set of functions an in the generic - * vmalloc/ioremap code to track at which page-table levels entries have been - * modified. Based on that the code can better decide when vmalloc and ioremap - * mapping changes need to be synchronized to other page-tables in the system. + * These are used by the p?d_alloc_track*() and p*d_populate_kernel() + * functions in the generic vmalloc, ioremap and page table update code + * to track at which page-table levels entries have been modified. + * Based on that the code can better decide when page table changes need + * to be synchronized to other page-tables in the system. */ #define __PGTBL_PGD_MODIFIED 0 #define __PGTBL_P4D_MODIFIED 1 diff --git a/mm/kasan/init.c b/mm/kasan/init.c index 89895f38f722..afecc04b486a 100644 --- a/mm/kasan/init.c +++ b/mm/kasan/init.c @@ -13,9 +13,9 @@ #include <linux/mm.h> #include <linux/pfn.h> #include <linux/slab.h> +#include <linux/pgalloc.h>
#include <asm/page.h> -#include <asm/pgalloc.h>
#include "kasan.h"
@@ -197,7 +197,7 @@ 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, @@ -218,7 +218,7 @@ 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); @@ -257,10 +257,10 @@ 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, @@ -279,7 +279,7 @@ 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 d287cebd58ca..38d5121c2b65 100644 --- a/mm/percpu.c +++ b/mm/percpu.c @@ -3157,7 +3157,7 @@ int __init pcpu_embed_first_chunk(size_t reserved_size, size_t dyn_size, #endif /* BUILD_EMBED_FIRST_CHUNK */
#ifdef BUILD_PAGE_FIRST_CHUNK -#include <asm/pgalloc.h> +#include <linux/pgalloc.h>
#ifndef P4D_TABLE_SIZE #define P4D_TABLE_SIZE PAGE_SIZE @@ -3185,7 +3185,7 @@ void __init __weak pcpu_populate_pte(unsigned long addr) p4d = memblock_alloc(P4D_TABLE_SIZE, P4D_TABLE_SIZE); if (!p4d) goto err_alloc; - pgd_populate(&init_mm, pgd, p4d); + pgd_populate_kernel(addr, pgd, p4d); }
p4d = p4d_offset(pgd, addr); @@ -3193,7 +3193,7 @@ void __init __weak pcpu_populate_pte(unsigned long addr) pud = memblock_alloc(PUD_TABLE_SIZE, PUD_TABLE_SIZE); if (!pud) goto err_alloc; - 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 a2cbe44c48e1..589d6a262b6d 100644 --- a/mm/sparse-vmemmap.c +++ b/mm/sparse-vmemmap.c @@ -27,9 +27,9 @@ #include <linux/spinlock.h> #include <linux/vmalloc.h> #include <linux/sched.h> +#include <linux/pgalloc.h>
#include <asm/dma.h> -#include <asm/pgalloc.h>
/* * Allocate a block of memory to be used to back the virtual memory map @@ -225,7 +225,7 @@ 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; } @@ -237,7 +237,7 @@ 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; }