[PATCH V4 mm-hotfixes 1/3] mm: move page table sync declarations to linux/pgtable.h

[PATCH V4 mm-hotfixes 1/3] mm: move page table sync declarations to linux/pgtable.h

[Harry Yoo]

Move ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to
linux/pgtable.h so that they can be used outside of vmalloc and ioremap.

Cc: <stable@vger.kernel.org>
Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 include/linux/pgtable.h | 16 ++++++++++++++++
 include/linux/vmalloc.h | 16 ----------------
 2 files changed, 16 insertions(+), 16 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 4c035637eeb7..ba699df6ef69 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1467,6 +1467,22 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned
 }
 #endif
 
+/*
+ * 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.
+ */
+#ifndef ARCH_PAGE_TABLE_SYNC_MASK
+#define ARCH_PAGE_TABLE_SYNC_MASK 0
+#endif
+
+/*
+ * There is no default implementation for arch_sync_kernel_mappings(). It is
+ * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
+ * is 0.
+ */
+void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
+
 #endif /* CONFIG_MMU */
 
 /*
diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index fdc9aeb74a44..2759dac6be44 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -219,22 +219,6 @@ extern int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
 int vmap_pages_range(unsigned long addr, unsigned long end, pgprot_t prot,
 		     struct page **pages, unsigned int page_shift);
 
-/*
- * 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.
- */
-#ifndef ARCH_PAGE_TABLE_SYNC_MASK
-#define ARCH_PAGE_TABLE_SYNC_MASK 0
-#endif
-
-/*
- * There is no default implementation for arch_sync_kernel_mappings(). It is
- * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
- * is 0.
- */
-void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
-
 /*
  *	Lowlevel-APIs (not for driver use!)
  */
-- 
2.43.0

[PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Harry Yoo]

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. In theory, PUD and PMD level helpers can be added
later if needed by other architectures.

Currently this is a no-op, since no architecture sets
PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.

Cc: <stable@vger.kernel.org>
Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 include/linux/pgalloc.h | 24 ++++++++++++++++++++++++
 include/linux/pgtable.h |  4 ++--
 mm/kasan/init.c         | 12 ++++++------
 mm/percpu.c             |  6 +++---
 mm/sparse-vmemmap.c     |  6 +++---
 5 files changed, 38 insertions(+), 14 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..290ab864320f
--- /dev/null
+++ b/include/linux/pgalloc.h
@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_PGALLOC_H
+#define _LINUX_PGALLOC_H
+
+#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);
+}
+
+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);
+}
+
+#endif /* _LINUX_PGALLOC_H */
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index ba699df6ef69..0cf5c6c3e483 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1469,8 +1469,8 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned
 
 /*
  * 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
diff --git a/mm/kasan/init.c b/mm/kasan/init.c
index ced6b29fcf76..8fce3370c84e 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"
 
@@ -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 d9cbaee92b60..a56f35dcc417 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -3108,7 +3108,7 @@ 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
@@ -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 41aa0493eb03..dbd8daccade2 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>
 #include <asm/tlbflush.h>
 
 #include "hugetlb_vmemmap.h"
@@ -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

[PATCH V4 mm-hotfixes 3/3] x86/mm/64: define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings()

[Harry Yoo]

Define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to ensure
page tables are properly synchronized when calling p*d_populate_kernel().
It is inteneded to synchronize page tables via pgd_pouplate_kernel() when
5-level paging is in use and via p4d_pouplate_kernel() when 4-level paging
is used.

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>
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 arch/x86/include/asm/pgtable_64_types.h | 3 +++
 arch/x86/mm/init_64.c                   | 5 +++++
 2 files changed, 8 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..a78b498c0dc3 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -223,6 +223,11 @@ static void sync_global_pgds(unsigned long start, unsigned long end)
 		sync_global_pgds_l4(start, end);
 }
 
+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

Re: [PATCH V4 mm-hotfixes 1/3] mm: move page table sync declarations to linux/pgtable.h

[Mike Rapoport]

On Mon, Aug 11, 2025 at 02:34:18PM +0900, Harry Yoo wrote:
> Move ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to
> linux/pgtable.h so that they can be used outside of vmalloc and ioremap.
> 
> Cc: <stable@vger.kernel.org>
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  include/linux/pgtable.h | 16 ++++++++++++++++
>  include/linux/vmalloc.h | 16 ----------------
>  2 files changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 4c035637eeb7..ba699df6ef69 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1467,6 +1467,22 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned
>  }
>  #endif
>  
> +/*
> + * 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()

If ARCH_PAGE_TABLE_SYNC_MASK can be used outside vmalloc(), the comment
needs an update, maybe

... and let the generic code that modifies kernel page tables

Other than that

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> + * needs to be called.
> + */
> +#ifndef ARCH_PAGE_TABLE_SYNC_MASK
> +#define ARCH_PAGE_TABLE_SYNC_MASK 0
> +#endif
> +
> +/*
> + * There is no default implementation for arch_sync_kernel_mappings(). It is
> + * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
> + * is 0.
> + */
> +void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
> +
>  #endif /* CONFIG_MMU */
>  
>  /*
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index fdc9aeb74a44..2759dac6be44 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -219,22 +219,6 @@ extern int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
>  int vmap_pages_range(unsigned long addr, unsigned long end, pgprot_t prot,
>  		     struct page **pages, unsigned int page_shift);
>  
> -/*
> - * 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.
> - */
> -#ifndef ARCH_PAGE_TABLE_SYNC_MASK
> -#define ARCH_PAGE_TABLE_SYNC_MASK 0
> -#endif
> -
> -/*
> - * There is no default implementation for arch_sync_kernel_mappings(). It is
> - * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
> - * is 0.
> - */
> -void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
> -
>  /*
>   *	Lowlevel-APIs (not for driver use!)
>   */
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Mike Rapoport]

On Mon, Aug 11, 2025 at 02:34:19PM +0900, Harry Yoo wrote:
> 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. In theory, PUD and PMD level helpers can be added
> later if needed by other architectures.
> 
> Currently this is a no-op, since no architecture sets
> PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
> 
> Cc: <stable@vger.kernel.org>
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  include/linux/pgalloc.h | 24 ++++++++++++++++++++++++
>  include/linux/pgtable.h |  4 ++--
>  mm/kasan/init.c         | 12 ++++++------
>  mm/percpu.c             |  6 +++---
>  mm/sparse-vmemmap.c     |  6 +++---
>  5 files changed, 38 insertions(+), 14 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..290ab864320f
> --- /dev/null
> +++ b/include/linux/pgalloc.h
> @@ -0,0 +1,24 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_PGALLOC_H
> +#define _LINUX_PGALLOC_H
> +
> +#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);
> +}
> +
> +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);
> +}
> +
> +#endif /* _LINUX_PGALLOC_H */
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index ba699df6ef69..0cf5c6c3e483 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1469,8 +1469,8 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned
>  
>  /*
>   * 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
> diff --git a/mm/kasan/init.c b/mm/kasan/init.c
> index ced6b29fcf76..8fce3370c84e 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"
>  
> @@ -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 d9cbaee92b60..a56f35dcc417 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -3108,7 +3108,7 @@ 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
> @@ -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 41aa0493eb03..dbd8daccade2 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>
>  #include <asm/tlbflush.h>
>  
>  #include "hugetlb_vmemmap.h"
> @@ -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
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH V4 mm-hotfixes 3/3] x86/mm/64: define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings()

[Mike Rapoport]

On Mon, Aug 11, 2025 at 02:34:20PM +0900, 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().
> It is inteneded to synchronize page tables via pgd_pouplate_kernel() when
> 5-level paging is in use and via p4d_pouplate_kernel() when 4-level paging
> is used.
> 
> 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>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  arch/x86/include/asm/pgtable_64_types.h | 3 +++
>  arch/x86/mm/init_64.c                   | 5 +++++
>  2 files changed, 8 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..a78b498c0dc3 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -223,6 +223,11 @@ static void sync_global_pgds(unsigned long start, unsigned long end)
>  		sync_global_pgds_l4(start, end);
>  }
>  
> +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
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH V4 mm-hotfixes 1/3] mm: move page table sync declarations to linux/pgtable.h

[Harry Yoo]

On Mon, Aug 11, 2025 at 11:05:51AM +0300, Mike Rapoport wrote:
> On Mon, Aug 11, 2025 at 02:34:18PM +0900, Harry Yoo wrote:
> > Move ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to
> > linux/pgtable.h so that they can be used outside of vmalloc and ioremap.
> > 
> > Cc: <stable@vger.kernel.org>
> > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> > ---
> >  include/linux/pgtable.h | 16 ++++++++++++++++
> >  include/linux/vmalloc.h | 16 ----------------
> >  2 files changed, 16 insertions(+), 16 deletions(-)
> > 
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index 4c035637eeb7..ba699df6ef69 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -1467,6 +1467,22 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned
> >  }
> >  #endif
> >  
> > +/*
> > + * 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()
> 
> If ARCH_PAGE_TABLE_SYNC_MASK can be used outside vmalloc(), the comment
> needs an update, maybe
> 
> ... and let the generic code that modifies kernel page tables

Right, and patch 2 updates the comment as it uses it outside vmalloc():

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index ba699df6ef69..0cf5c6c3e483 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1469,8 +1469,8 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned

 /*
  * 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

Or if you think "page table update code" is unclear, please let me know.

> Other than that
> 
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Thanks a lot for all the reviews, Mike!

-- 
Cheers,
Harry / Hyeonggon

Re: [PATCH V4 mm-hotfixes 1/3] mm: move page table sync declarations to linux/pgtable.h

[Mike Rapoport]

On Mon, Aug 11, 2025 at 05:36:53PM +0900, Harry Yoo wrote:
> On Mon, Aug 11, 2025 at 11:05:51AM +0300, Mike Rapoport wrote:
> > On Mon, Aug 11, 2025 at 02:34:18PM +0900, Harry Yoo wrote:
> > > Move ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to
> > > linux/pgtable.h so that they can be used outside of vmalloc and ioremap.
> > > 
> > > Cc: <stable@vger.kernel.org>
> > > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> > > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> > > ---
> > >  include/linux/pgtable.h | 16 ++++++++++++++++
> > >  include/linux/vmalloc.h | 16 ----------------
> > >  2 files changed, 16 insertions(+), 16 deletions(-)
> > > 
> > > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > > index 4c035637eeb7..ba699df6ef69 100644
> > > --- a/include/linux/pgtable.h
> > > +++ b/include/linux/pgtable.h
> > > @@ -1467,6 +1467,22 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned
> > >  }
> > >  #endif
> > >  
> > > +/*
> > > + * 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()
> > 
> > If ARCH_PAGE_TABLE_SYNC_MASK can be used outside vmalloc(), the comment
> > needs an update, maybe
> > 
> > ... and let the generic code that modifies kernel page tables
> 
> Right, and patch 2 updates the comment as it uses it outside vmalloc():
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index ba699df6ef69..0cf5c6c3e483 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1469,8 +1469,8 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned
> 
>  /*
>   * 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
> 
> Or if you think "page table update code" is unclear, please let me know.

It's fine :)
 
> > Other than that
> > 
> > Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> 
> Thanks a lot for all the reviews, Mike!
> 
> -- 
> Cheers,
> Harry / Hyeonggon

-- 
Sincerely yours,
Mike.

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Lorenzo Stoakes]

On Mon, Aug 11, 2025 at 02:34:19PM +0900, Harry Yoo wrote:
> 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. In theory, PUD and PMD level helpers can be added
> later if needed by other architectures.
>
> Currently this is a no-op, since no architecture sets
> PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
>
> Cc: <stable@vger.kernel.org>
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  include/linux/pgalloc.h | 24 ++++++++++++++++++++++++

Could we put this in the correct place in MAINTAINERS please? I think
MEMORY MANAGEMENT - CORE is correct, given the below file is there.

>  include/linux/pgtable.h |  4 ++--

Thanks!

Re: [PATCH V4 mm-hotfixes 1/3] mm: move page table sync declarations to linux/pgtable.h

[Uladzislau Rezki]

On Mon, Aug 11, 2025 at 11:05:51AM +0300, Mike Rapoport wrote:
> On Mon, Aug 11, 2025 at 02:34:18PM +0900, Harry Yoo wrote:
> > Move ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to
> > linux/pgtable.h so that they can be used outside of vmalloc and ioremap.
> > 
> > Cc: <stable@vger.kernel.org>
> > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> > ---
> >  include/linux/pgtable.h | 16 ++++++++++++++++
> >  include/linux/vmalloc.h | 16 ----------------
> >  2 files changed, 16 insertions(+), 16 deletions(-)
> > 
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index 4c035637eeb7..ba699df6ef69 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -1467,6 +1467,22 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned
> >  }
> >  #endif
> >  
> > +/*
> > + * 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()
> 
> If ARCH_PAGE_TABLE_SYNC_MASK can be used outside vmalloc(), the comment
> needs an update, maybe
> 
> ... and let the generic code that modifies kernel page tables
> 
> Other than that
> 
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> 
> > + * needs to be called.
> > + */
> > +#ifndef ARCH_PAGE_TABLE_SYNC_MASK
> > +#define ARCH_PAGE_TABLE_SYNC_MASK 0
> > +#endif
> > +
> > +/*
> > + * There is no default implementation for arch_sync_kernel_mappings(). It is
> > + * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
> > + * is 0.
> > + */
> > +void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
> > +
> >  #endif /* CONFIG_MMU */
> >  
> >  /*
> > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > index fdc9aeb74a44..2759dac6be44 100644
> > --- a/include/linux/vmalloc.h
> > +++ b/include/linux/vmalloc.h
> > @@ -219,22 +219,6 @@ extern int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
> >  int vmap_pages_range(unsigned long addr, unsigned long end, pgprot_t prot,
> >  		     struct page **pages, unsigned int page_shift);
> >  
> > -/*
> > - * 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.
> > - */
> > -#ifndef ARCH_PAGE_TABLE_SYNC_MASK
> > -#define ARCH_PAGE_TABLE_SYNC_MASK 0
> > -#endif
> > -
> > -/*
> > - * There is no default implementation for arch_sync_kernel_mappings(). It is
> > - * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
> > - * is 0.
> > - */
> > -void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
> > -
> >  /*
> >   *	Lowlevel-APIs (not for driver use!)
> >   */
> > -- 
> > 2.43.0
> > 
> 
LGTM,

Reviewed-by: "Uladzislau Rezki (Sony)" <urezki@gmail.com>

--
Uladzislau Rezki

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Harry Yoo]

On Mon, Aug 11, 2025 at 10:10:58AM +0100, Lorenzo Stoakes wrote:
> On Mon, Aug 11, 2025 at 02:34:19PM +0900, Harry Yoo wrote:
> > 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. In theory, PUD and PMD level helpers can be added
> > later if needed by other architectures.
> >
> > Currently this is a no-op, since no architecture sets
> > PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
> >
> > Cc: <stable@vger.kernel.org>
> > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> > Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> > ---
> >  include/linux/pgalloc.h | 24 ++++++++++++++++++++++++
> 
> Could we put this in the correct place in MAINTAINERS please?

Definitely yes!

Since this series will be backported to about five -stable kernels
(v5.13.x and later), I will add that as part of a follow-up series
that is not intended for backporting.

Does that sound okay?

> I think MEMORY MANAGEMENT - CORE is correct, given the below file is there.

Thanks for confirming that!

-- 
Cheers,
Harry / Hyeonggon

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Lorenzo Stoakes]

On Mon, Aug 11, 2025 at 07:36:46PM +0900, Harry Yoo wrote:
> > >  include/linux/pgalloc.h | 24 ++++++++++++++++++++++++
> >
> > Could we put this in the correct place in MAINTAINERS please?
>
> Definitely yes!
>
> Since this series will be backported to about five -stable kernels
> (v5.13.x and later), I will add that as part of a follow-up series
> that is not intended for backporting.
>
> Does that sound okay?

Yes that's fine thanks!

Re: [PATCH V4 mm-hotfixes 1/3] mm: move page table sync declarations to linux/pgtable.h

[Lorenzo Stoakes]

On Mon, Aug 11, 2025 at 02:34:18PM +0900, Harry Yoo wrote:
> Move ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to
> linux/pgtable.h so that they can be used outside of vmalloc and ioremap.
>
> Cc: <stable@vger.kernel.org>
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>

LGTM, obviously assuming you address Mike's comments about... comments :)

So:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  include/linux/pgtable.h | 16 ++++++++++++++++
>  include/linux/vmalloc.h | 16 ----------------
>  2 files changed, 16 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 4c035637eeb7..ba699df6ef69 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1467,6 +1467,22 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned
>  }
>  #endif
>
> +/*
> + * 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.
> + */
> +#ifndef ARCH_PAGE_TABLE_SYNC_MASK
> +#define ARCH_PAGE_TABLE_SYNC_MASK 0
> +#endif
> +
> +/*
> + * There is no default implementation for arch_sync_kernel_mappings(). It is
> + * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
> + * is 0.
> + */
> +void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
> +
>  #endif /* CONFIG_MMU */
>
>  /*
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index fdc9aeb74a44..2759dac6be44 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -219,22 +219,6 @@ extern int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
>  int vmap_pages_range(unsigned long addr, unsigned long end, pgprot_t prot,
>  		     struct page **pages, unsigned int page_shift);
>
> -/*
> - * 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.
> - */
> -#ifndef ARCH_PAGE_TABLE_SYNC_MASK
> -#define ARCH_PAGE_TABLE_SYNC_MASK 0
> -#endif
> -
> -/*
> - * There is no default implementation for arch_sync_kernel_mappings(). It is
> - * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
> - * is 0.
> - */
> -void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
> -
>  /*
>   *	Lowlevel-APIs (not for driver use!)
>   */
> --
> 2.43.0
>

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Lorenzo Stoakes]

On Mon, Aug 11, 2025 at 02:34:19PM +0900, Harry Yoo wrote:
> 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

Well, arm defines ARCH_PAGE_TABLE_SYNC_MASK in arch/arm/include/asm/page.h. But
it aliases this to PGTBL_PMD_MODIFIED so will remain unaffected :)

> helpers are introduced. In theory, PUD and PMD level helpers can be added
> later if needed by other architectures.
>
> Currently this is a no-op, since no architecture sets
> PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
>
> Cc: <stable@vger.kernel.org>
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  include/linux/pgalloc.h | 24 ++++++++++++++++++++++++
>  include/linux/pgtable.h |  4 ++--
>  mm/kasan/init.c         | 12 ++++++------
>  mm/percpu.c             |  6 +++---
>  mm/sparse-vmemmap.c     |  6 +++---
>  5 files changed, 38 insertions(+), 14 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..290ab864320f
> --- /dev/null
> +++ b/include/linux/pgalloc.h
> @@ -0,0 +1,24 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_PGALLOC_H
> +#define _LINUX_PGALLOC_H
> +
> +#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)

Hm, ARCH_PAGE_TABLE_SYNC_MASK is only defined for x86 2, 3 page level and arm. I see:

#ifndef ARCH_PAGE_TABLE_SYNC_MASK
#define ARCH_PAGE_TABLE_SYNC_MASK 0
#endif

In linux/vmalloc.h, but you're not importing that?

It sucks that that there is there, but maybe you need to #include
<linux/vmalloc.h> for this otherwise this could be broken on other arches?

You may be getting lucky with nested header includes that causes this to be
picked up somewhere for you, or having it only declared for arches that define
it, but we should probably make this explicit.

Also arch_sync_kernel_mappings() is defined in linux/vmalloc.h so seems
sensible.


> +		arch_sync_kernel_mappings(addr, addr);
> +}
> +
> +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);

It's kind of weird we don't have this defined as a function for many arches,
(weird as well that we declare it in... vmalloc.h but I guess one for follow up
cleanups that).

But I see from the comment:

/*
 * There is no default implementation for arch_sync_kernel_mappings(). It is
 * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
 * is 0.
 */

So this seems intended... :)

The rest of this seems sensible, nice cleanup!

> +}
> +
> +#endif /* _LINUX_PGALLOC_H */
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index ba699df6ef69..0cf5c6c3e483 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1469,8 +1469,8 @@ static inline void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned
>
>  /*
>   * 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
> diff --git a/mm/kasan/init.c b/mm/kasan/init.c
> index ced6b29fcf76..8fce3370c84e 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"
>
> @@ -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 d9cbaee92b60..a56f35dcc417 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -3108,7 +3108,7 @@ 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
> @@ -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 41aa0493eb03..dbd8daccade2 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>
>  #include <asm/tlbflush.h>
>
>  #include "hugetlb_vmemmap.h"
> @@ -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
>

Re: [PATCH V4 mm-hotfixes 3/3] x86/mm/64: define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings()

[Lorenzo Stoakes]

On Mon, Aug 11, 2025 at 02:34:20PM +0900, 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().
> It is inteneded to synchronize page tables via pgd_pouplate_kernel() when
> 5-level paging is in use and via p4d_pouplate_kernel() when 4-level paging
> is used.
>

I think it's worth mentioning here that pgd_populate() is a no-op in 4-level
systems, so the sychronisation must occur at the P4D level, just to make this
clear.

> 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>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>

Other than nitty comments, this looks good to me, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  arch/x86/include/asm/pgtable_64_types.h | 3 +++
>  arch/x86/mm/init_64.c                   | 5 +++++
>  2 files changed, 8 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..a78b498c0dc3 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -223,6 +223,11 @@ static void sync_global_pgds(unsigned long start, unsigned long end)
>  		sync_global_pgds_l4(start, end);
>  }
>

Worth a comment to say 'if 4-level, then we synchronise at P4D level by
convention, however the same sync_global_pgds() applies'?

> +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
>

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Harry Yoo]

On Mon, Aug 11, 2025 at 12:38:37PM +0100, Lorenzo Stoakes wrote:
> On Mon, Aug 11, 2025 at 02:34:19PM +0900, Harry Yoo wrote:
> > 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

Hi Lorenzo, thanks for looking at this!

> Well, arm defines ARCH_PAGE_TABLE_SYNC_MASK in arch/arm/include/asm/page.h. But
> it aliases this to PGTBL_PMD_MODIFIED so will remain unaffected :)

Oh, here I just intended to explain why I didn't implement
{pud,pmd}_populate_kernel().

> > helpers are introduced. In theory, PUD and PMD level helpers can be added
> > later if needed by other architectures.
> >
> > Currently this is a no-op, since no architecture sets
> > PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
> >
> > Cc: <stable@vger.kernel.org>
> > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> > Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> > ---
> >  include/linux/pgalloc.h | 24 ++++++++++++++++++++++++
> >  include/linux/pgtable.h |  4 ++--
> >  mm/kasan/init.c         | 12 ++++++------
> >  mm/percpu.c             |  6 +++---
> >  mm/sparse-vmemmap.c     |  6 +++---
> >  5 files changed, 38 insertions(+), 14 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..290ab864320f
> > --- /dev/null
> > +++ b/include/linux/pgalloc.h
> > @@ -0,0 +1,24 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _LINUX_PGALLOC_H
> > +#define _LINUX_PGALLOC_H
> > +
> > +#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)
> 
> Hm, ARCH_PAGE_TABLE_SYNC_MASK is only defined for x86 2, 3 page level and arm. I see:
> 
> #ifndef ARCH_PAGE_TABLE_SYNC_MASK
> #define ARCH_PAGE_TABLE_SYNC_MASK 0
> #endif
> 
> In linux/vmalloc.h, but you're not importing that?

Patch 1 moves it from linux/vmalloc.h to linux/pgtable.h,
and linux/pgalloc.h includes linux/pgtable.h.

> It sucks that that there is there, but maybe you need to #include
> <linux/vmalloc.h> for this otherwise this could be broken on other arches?
>
> You may be getting lucky with nested header includes that causes this to be
> picked up somewhere for you, or having it only declared for arches that define
> it, but we should probably make this explicit.

...so I don't think I'm missing necessary header includes even on
other architectures?

> Also arch_sync_kernel_mappings() is defined in linux/vmalloc.h so seems
> sensible.

Also moved to linux/pgtable.h.

> > +		arch_sync_kernel_mappings(addr, addr);
> > +}
> > +
> > +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);
> 
> It's kind of weird we don't have this defined as a function for many arches,

That's really a mystery :)

I have no idea why other architectures don't handle this.

(At least on 64 bit arches) In theory I think only a few architectures
(like arm64 where a kernel page table is shared between tasks) don't have
to implement this.

Probably because it's a bit niche bug to hit?
(vmemmap, direct mapping, vmalloc/vmap area can span multiple PGD ranges)
AND (populating some PGD entries is done after boot process (e.g. memory
hot-plug or vmalloc())).

> (weird as well that we declare it in... vmalloc.h but I guess one for follow up
> cleanups that).
> 
> But I see from the comment:
> 
> /*
>  * There is no default implementation for arch_sync_kernel_mappings(). It is
>  * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
>  * is 0.
>  */
> 
> So this seems intended... :)
 
> The rest of this seems sensible, nice cleanup!

Thanks for looking at!

-- 
Cheers,
Harry / Hyeonggon

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Lorenzo Stoakes]

On Mon, Aug 11, 2025 at 09:12:08PM +0900, Harry Yoo wrote:
> On Mon, Aug 11, 2025 at 12:38:37PM +0100, Lorenzo Stoakes wrote:
> > On Mon, Aug 11, 2025 at 02:34:19PM +0900, Harry Yoo wrote:
> > > 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
>
> Hi Lorenzo, thanks for looking at this!
>
> > Well, arm defines ARCH_PAGE_TABLE_SYNC_MASK in arch/arm/include/asm/page.h. But
> > it aliases this to PGTBL_PMD_MODIFIED so will remain unaffected :)
>
> Oh, here I just intended to explain why I didn't implement
> {pud,pmd}_populate_kernel().

I'd add that arm handles PGTBL_PMD_MODIFIED and therefore remains unaffected
just to be super clear.

>
> > > helpers are introduced. In theory, PUD and PMD level helpers can be added
> > > later if needed by other architectures.
> > >
> > > Currently this is a no-op, since no architecture sets
> > > PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
> > >
> > > Cc: <stable@vger.kernel.org>
> > > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> > > Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> > > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>

Given that I missed you fixed the vmalloc.h thing, this LGTM so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> > > ---
> > >  include/linux/pgalloc.h | 24 ++++++++++++++++++++++++
> > >  include/linux/pgtable.h |  4 ++--
> > >  mm/kasan/init.c         | 12 ++++++------
> > >  mm/percpu.c             |  6 +++---
> > >  mm/sparse-vmemmap.c     |  6 +++---
> > >  5 files changed, 38 insertions(+), 14 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..290ab864320f
> > > --- /dev/null
> > > +++ b/include/linux/pgalloc.h
> > > @@ -0,0 +1,24 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +#ifndef _LINUX_PGALLOC_H
> > > +#define _LINUX_PGALLOC_H
> > > +
> > > +#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)
> >
> > Hm, ARCH_PAGE_TABLE_SYNC_MASK is only defined for x86 2, 3 page level and arm. I see:
> >
> > #ifndef ARCH_PAGE_TABLE_SYNC_MASK
> > #define ARCH_PAGE_TABLE_SYNC_MASK 0
> > #endif
> >
> > In linux/vmalloc.h, but you're not importing that?
>
> Patch 1 moves it from linux/vmalloc.h to linux/pgtable.h,
> and linux/pgalloc.h includes linux/pgtable.h.
>
> > It sucks that that there is there, but maybe you need to #include
> > <linux/vmalloc.h> for this otherwise this could be broken on other arches?
> >
> > You may be getting lucky with nested header includes that causes this to be
> > picked up somewhere for you, or having it only declared for arches that define
> > it, but we should probably make this explicit.
>
> ...so I don't think I'm missing necessary header includes even on
> other architectures?
>
> > Also arch_sync_kernel_mappings() is defined in linux/vmalloc.h so seems
> > sensible.
>
> Also moved to linux/pgtable.h.

Ah yeah damn, I missed that you do that there, ok well that's fine then :)

>
> > > +		arch_sync_kernel_mappings(addr, addr);
> > > +}
> > > +
> > > +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);
> >
> > It's kind of weird we don't have this defined as a function for many arches,
>
> That's really a mystery :)
>
> I have no idea why other architectures don't handle this.
>
> (At least on 64 bit arches) In theory I think only a few architectures
> (like arm64 where a kernel page table is shared between tasks) don't have
> to implement this.
>
> Probably because it's a bit niche bug to hit?
> (vmemmap, direct mapping, vmalloc/vmap area can span multiple PGD ranges)
> AND (populating some PGD entries is done after boot process (e.g. memory
> hot-plug or vmalloc())).

No comment is more why we don't just do a standard:

#ifndef xxx
#define xxx (0)
#endif

Or something. Just odd.

>
> > (weird as well that we declare it in... vmalloc.h but I guess one for follow up
> > cleanups that).
> >
> > But I see from the comment:
> >
> > /*
> >  * There is no default implementation for arch_sync_kernel_mappings(). It is
> >  * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK
> >  * is 0.
> >  */
> >
> > So this seems intended... :)
>
> > The rest of this seems sensible, nice cleanup!
>
> Thanks for looking at!
>
> --
> Cheers,
> Harry / Hyeonggon
>

Re: [PATCH V4 mm-hotfixes 3/3] x86/mm/64: define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings()

[Harry Yoo]

On Mon, Aug 11, 2025 at 12:46:32PM +0100, Lorenzo Stoakes wrote:
> On Mon, Aug 11, 2025 at 02:34:20PM +0900, 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().
> > It is inteneded to synchronize page tables via pgd_pouplate_kernel() when
> > 5-level paging is in use and via p4d_pouplate_kernel() when 4-level paging
> > is used.
> >
> 
> I think it's worth mentioning here that pgd_populate() is a no-op in 4-level
> systems, so the sychronisation must occur at the P4D level, just to make this
> clear.

Yeah, that's indeed confusing and agree that it's worth mentioning.
Will do. The new one:

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>
> > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> 
> Other than nitty comments, this looks good to me, so:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Thanks!

> > ---
> >  arch/x86/include/asm/pgtable_64_types.h | 3 +++
> >  arch/x86/mm/init_64.c                   | 5 +++++
> >  2 files changed, 8 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..a78b498c0dc3 100644
> > --- a/arch/x86/mm/init_64.c
> > +++ b/arch/x86/mm/init_64.c
> > @@ -223,6 +223,11 @@ static void sync_global_pgds(unsigned long start, unsigned long end)
> >  		sync_global_pgds_l4(start, end);
> >  }
> >
> 
> Worth a comment to say 'if 4-level, then we synchronise at P4D level by
> convention, however the same sync_global_pgds() applies'?

Maybe:

/*
 * 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 P4D level instead.
 * sync_global_pgds() handles this difference between paging levels.
 */

-- 
Cheers,
Harry / Hyeonggon

> > +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
> >

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Harry Yoo]

On Mon, Aug 11, 2025 at 01:18:12PM +0100, Lorenzo Stoakes wrote:
> On Mon, Aug 11, 2025 at 09:12:08PM +0900, Harry Yoo wrote:
> > On Mon, Aug 11, 2025 at 12:38:37PM +0100, Lorenzo Stoakes wrote:
> > > On Mon, Aug 11, 2025 at 02:34:19PM +0900, Harry Yoo wrote:
> > > > 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
> >
> > Hi Lorenzo, thanks for looking at this!
> >
> > > Well, arm defines ARCH_PAGE_TABLE_SYNC_MASK in arch/arm/include/asm/page.h. But
> > > it aliases this to PGTBL_PMD_MODIFIED so will remain unaffected :)
> >
> > Oh, here I just intended to explain why I didn't implement
> > {pud,pmd}_populate_kernel().
> 
> I'd add that arm handles PGTBL_PMD_MODIFIED and therefore remains unaffected
> just to be super clear.

Will do:

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.

> > > > helpers are introduced. In theory, PUD and PMD level helpers can be added
> > > > later if needed by other architectures.
> > > >
> > > > Currently this is a no-op, since no architecture sets
> > > > PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
> > > >
> > > > Cc: <stable@vger.kernel.org>
> > > > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> > > > Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> > > > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> 
> Given that I missed you fixed the vmalloc.h thing, this LGTM so:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Thanks!

-- 
Cheers,
Harry / Hyeonggon

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Lorenzo Stoakes]

On Tue, Aug 12, 2025 at 06:53:49PM +0900, Harry Yoo wrote:
> > I'd add that arm handles PGTBL_PMD_MODIFIED and therefore remains unaffected
> > just to be super clear.
>
> Will do:
>
> 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.

Sounds good!

Cheers, Lorenzo

Re: [PATCH V4 mm-hotfixes 3/3] x86/mm/64: define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings()

[Lorenzo Stoakes]

On Tue, Aug 12, 2025 at 05:59:02PM +0900, Harry Yoo wrote:
> On Mon, Aug 11, 2025 at 12:46:32PM +0100, Lorenzo Stoakes wrote:
> > On Mon, Aug 11, 2025 at 02:34:20PM +0900, 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().
> > > It is inteneded to synchronize page tables via pgd_pouplate_kernel() when
> > > 5-level paging is in use and via p4d_pouplate_kernel() when 4-level paging
> > > is used.
> > >
> >
> > I think it's worth mentioning here that pgd_populate() is a no-op in 4-level
> > systems, so the sychronisation must occur at the P4D level, just to make this
> > clear.
>
> Yeah, that's indeed confusing and agree that it's worth mentioning.
> Will do. The new one:
>
> 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().

That's great thanks!

>
> > > 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>
> > > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> >
> > Other than nitty comments, this looks good to me, so:
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> Thanks!
>
> > > ---
> > >  arch/x86/include/asm/pgtable_64_types.h | 3 +++
> > >  arch/x86/mm/init_64.c                   | 5 +++++
> > >  2 files changed, 8 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..a78b498c0dc3 100644
> > > --- a/arch/x86/mm/init_64.c
> > > +++ b/arch/x86/mm/init_64.c
> > > @@ -223,6 +223,11 @@ static void sync_global_pgds(unsigned long start, unsigned long end)
> > >  		sync_global_pgds_l4(start, end);
> > >  }
> > >
> >
> > Worth a comment to say 'if 4-level, then we synchronise at P4D level by
> > convention, however the same sync_global_pgds() applies'?
>
> Maybe:
>
> /*
>  * 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 P4D level instead.
>  * sync_global_pgds() handles this difference between paging levels.
>  */
>

That's great also, thanks!

> --
> Cheers,
> Harry / Hyeonggon
>
> > > +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
> > >
>

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Christophe Leroy]

Le 11/08/2025 à 07:34, Harry Yoo a écrit :
> 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. In theory, PUD and PMD level helpers can be added
> later if needed by other architectures.

AFAIK pmd_populate_kernel() already exist on all architectures, and I'm 
not sure it does what you expect. Or am I missing something ?

Christophe

Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Harry Yoo]

On Mon, Aug 25, 2025 at 01:27:20PM +0200, Christophe Leroy wrote:
> 
> 
> Le 11/08/2025 à 07:34, Harry Yoo a écrit :
> > 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. In theory, PUD and PMD level helpers can be added
> > later if needed by other architectures.
> 
> AFAIK pmd_populate_kernel() already exist on all architectures, and I'm not
> sure it does what you expect. Or am I missing something ?

It does not do what I expect.

Yes, if someone is going to introduce a PMD level helper, existing
pmd_populate_kernel() should be renamed or removed.

To be honest I'm not really sure why we need both pmd_populate() and
pmd_populate_kernel(). It is introduced by historical commit
3a0b82c08a0e8668 ("adds simple support for atomically-mapped PTEs.
On highmem systems this enables the allocation of the pagetables in
highmem.") [1], but as there's no explanation or comment so I can only
speculate.

Key differences I recognize is 1) the type of the last parameter is
pgtable_t (which can be either struct page * or pte_t * depending on
architecture) in pmd_populate() and pte_t * in pmd_populate_kernel(),
and 2) some architectures treat user and kernel page tables differently.

Regarding 1), I think a reasonable experience is that pmd_populate()
should take struct page * in some architectures because
with CONFIG_HIGHPTE=y pte_t * might not be accessible, but kernel
page tables are not allocated from highmem even with CONFIG_HIGHPTE=y
so pmd_populate_kernel() can take pte_t *, and that can save a few
instructions.

And some architectures (that does not support HIGHPTE?) define pgtable_t
as pte_t * to support sub-page page tables (Commit 2f569afd9ced
("CONFIG_HIGHPTE vs. sub-page page tables.")).

Maybe things to clean up in the future:

1) Once CONFIG_HIGHPTE is completely dropped (is that ever going to
   happen?), pte_t * can be used instead of struct page *. 

2) Convert users of pmd_populate_kernel() to use pmd_populate().
   But some architectures treat user and kernel page tables differently
   and that will be handled in pmd_populate()  (depending on
   (mm == &init_mm))

[1] https://web.git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=3a0b82c08a0e86683783c30d7fec9d1b06c2fe20

-- 
Cheers,
Harry / Hyeonggon