Define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to ensure
page tables are properly synchronized when calling
p*d_populate_kernel().

For 5-level paging, synchronization is performed via
pgd_populate_kernel(). In 4-level paging, pgd_populate() is a no-op,
so synchronization is instead performed at the P4D level via
p4d_populate_kernel().

This fixes intermittent boot failures on systems using 4-level paging
and a large amount of persistent memory:

  BUG: unable to handle page fault for address: ffffe70000000034
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP NOPTI
  RIP: 0010:__init_single_page+0x9/0x6d
  Call Trace:
   <TASK>
   __init_zone_device_page+0x17/0x5d
   memmap_init_zone_device+0x154/0x1bb
   pagemap_range+0x2e0/0x40f
   memremap_pages+0x10b/0x2f0
   devm_memremap_pages+0x1e/0x60
   dev_dax_probe+0xce/0x2ec [device_dax]
   dax_bus_probe+0x6d/0xc9
   [... snip ...]
   </TASK>

It also fixes a crash in vmemmap_set_pmd() caused by accessing vmemmap
before sync_global_pgds() [1]:

  BUG: unable to handle page fault for address: ffffeb3ff1200000
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: Oops: 0002 [#1] PREEMPT SMP NOPTI
  Tainted: [W]=WARN
  RIP: 0010:vmemmap_set_pmd+0xff/0x230
   <TASK>
   vmemmap_populate_hugepages+0x176/0x180
   vmemmap_populate+0x34/0x80
   __populate_section_memmap+0x41/0x90
   sparse_add_section+0x121/0x3e0
   __add_pages+0xba/0x150
   add_pages+0x1d/0x70
   memremap_pages+0x3dc/0x810
   devm_memremap_pages+0x1c/0x60
   xe_devm_add+0x8b/0x100 [xe]
   xe_tile_init_noalloc+0x6a/0x70 [xe]
   xe_device_probe+0x48c/0x740 [xe]
   [... snip ...]

Cc: <stable@vger.kernel.org>
Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
Closes: https://lore.kernel.org/linux-mm/20250311114420.240341-1-gwan-gyeong.mun@intel.com [1]
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>
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 arch/x86/include/asm/pgtable_64_types.h |  3 +++
 arch/x86/mm/init_64.c                   | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
index 4604f924d8b8..7eb61ef6a185 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -36,6 +36,9 @@ static inline bool pgtable_l5_enabled(void)
 #define pgtable_l5_enabled() cpu_feature_enabled(X86_FEATURE_LA57)
 #endif /* USE_EARLY_PGTABLE_L5 */
 
+#define ARCH_PAGE_TABLE_SYNC_MASK \
+	(pgtable_l5_enabled() ? PGTBL_PGD_MODIFIED : PGTBL_P4D_MODIFIED)
+
 extern unsigned int pgdir_shift;
 extern unsigned int ptrs_per_p4d;
 
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 76e33bd7c556..b9426fce5f3e 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -223,6 +223,24 @@ static void sync_global_pgds(unsigned long start, unsigned long end)
 		sync_global_pgds_l4(start, end);
 }
 
+/*
+ * Make kernel mappings visible in all page tables in the system.
+ * This is necessary except when the init task populates kernel mappings
+ * during the boot process. In that case, all processes originating from
+ * the init task copies the kernel mappings, so there is no issue.
+ * Otherwise, missing synchronization could lead to kernel crashes due
+ * to missing page table entries for certain kernel mappings.
+ *
+ * Synchronization is performed at the top level, which is the PGD in
+ * 5-level paging systems. But in 4-level paging systems, however,
+ * pgd_populate() is a no-op, so synchronization is done at the P4D level.
+ * sync_global_pgds() handles this difference between paging levels.
+ */
+void arch_sync_kernel_mappings(unsigned long start, unsigned long end)
+{
+	sync_global_pgds(start, end);
+}
+
 /*
  * NOTE: This function is marked __ref because it calls __init function
  * (alloc_bootmem_pages). It's safe to do it ONLY when after_bootmem == 0.
-- 
2.43.0

On 18.08.25 04:02, Harry Yoo wrote:
> Define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to ensure
> page tables are properly synchronized when calling
> p*d_populate_kernel().
> 
> For 5-level paging, synchronization is performed via
> pgd_populate_kernel(). In 4-level paging, pgd_populate() is a no-op,
> so synchronization is instead performed at the P4D level via
> p4d_populate_kernel().
> 
> This fixes intermittent boot failures on systems using 4-level paging
> and a large amount of persistent memory:
> 
>    BUG: unable to handle page fault for address: ffffe70000000034
>    #PF: supervisor write access in kernel mode
>    #PF: error_code(0x0002) - not-present page
>    PGD 0 P4D 0
>    Oops: 0002 [#1] SMP NOPTI
>    RIP: 0010:__init_single_page+0x9/0x6d
>    Call Trace:
>     <TASK>
>     __init_zone_device_page+0x17/0x5d
>     memmap_init_zone_device+0x154/0x1bb
>     pagemap_range+0x2e0/0x40f
>     memremap_pages+0x10b/0x2f0
>     devm_memremap_pages+0x1e/0x60
>     dev_dax_probe+0xce/0x2ec [device_dax]
>     dax_bus_probe+0x6d/0xc9
>     [... snip ...]
>     </TASK>
> 
> It also fixes a crash in vmemmap_set_pmd() caused by accessing vmemmap
> before sync_global_pgds() [1]:
> 
>    BUG: unable to handle page fault for address: ffffeb3ff1200000
>    #PF: supervisor write access in kernel mode
>    #PF: error_code(0x0002) - not-present page
>    PGD 0 P4D 0
>    Oops: Oops: 0002 [#1] PREEMPT SMP NOPTI
>    Tainted: [W]=WARN
>    RIP: 0010:vmemmap_set_pmd+0xff/0x230
>     <TASK>
>     vmemmap_populate_hugepages+0x176/0x180
>     vmemmap_populate+0x34/0x80
>     __populate_section_memmap+0x41/0x90
>     sparse_add_section+0x121/0x3e0
>     __add_pages+0xba/0x150
>     add_pages+0x1d/0x70
>     memremap_pages+0x3dc/0x810
>     devm_memremap_pages+0x1c/0x60
>     xe_devm_add+0x8b/0x100 [xe]
>     xe_tile_init_noalloc+0x6a/0x70 [xe]
>     xe_device_probe+0x48c/0x740 [xe]
>     [... snip ...]
> 
> Cc: <stable@vger.kernel.org>
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Closes: https://lore.kernel.org/linux-mm/20250311114420.240341-1-gwan-gyeong.mun@intel.com [1]
> 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>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb