New subject: [PATCH 6.1.y] mm: introduce and use {pgd,p4d}_populate_kernel()

6 Sep 2025

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to stable@vger.kernel.org.
To reproduce the conflict and resubmit, you may use the following commands:
git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x f2d2f9598ebb0158a3fe17cda0106d7752e654a2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to 'stable@vger.kernel.org' --in-reply-to '2025090606-overthrow-bagginess-c68f@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..
Possible dependencies:
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From f2d2f9598ebb0158a3fe17cda0106d7752e654a2 Mon Sep 17 00:00:00 2001
From: Harry Yoo harry.yoo@oracle.com
Date: Mon, 18 Aug 2025 11:02:05 +0900
Subject: [PATCH] mm: introduce and use {pgd,p4d}_populate_kernel()
Introduce and use {pgd,p4d}_populate_kernel() in core MM code when
populating PGD and P4D entries for the kernel address space.  These
helpers ensure proper synchronization of page tables when updating the
kernel portion of top-level page tables.
Until now, the kernel has relied on each architecture to handle
synchronization of top-level page tables in an ad-hoc manner.  For
example, see commit 9b861528a801 ("x86-64, mem: Update all PGDs for direct
mapping and vmemmap mapping changes").
However, this approach has proven fragile for following reasons:
1) It is easy to forget to perform the necessary page table
     synchronization when introducing new changes.
     For instance, commit 4917f55b4ef9 ("mm/sparse-vmemmap: improve memory
     savings for compound devmaps") overlooked the need to synchronize
     page tables for the vmemmap area.
2) It is also easy to overlook that the vmemmap and direct mapping areas
     must not be accessed before explicit page table synchronization.
     For example, commit 8d400913c231 ("x86/vmemmap: handle unpopulated
     sub-pmd ranges")) caused crashes by accessing the vmemmap area
     before calling sync_global_pgds().
To address this, as suggested by Dave Hansen, introduce _kernel() variants
of the page table population helpers, which invoke architecture-specific
hooks to properly synchronize page tables.  These are introduced in a new
header file, include/linux/pgalloc.h, so they can be called from common
code.
They reuse existing infrastructure for vmalloc and ioremap.
Synchronization requirements are determined by ARCH_PAGE_TABLE_SYNC_MASK,
and the actual synchronization is performed by
arch_sync_kernel_mappings().
This change currently targets only x86_64, so only PGD and P4D level
helpers are introduced.  Currently, these helpers are no-ops since no
architecture sets PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
In theory, PUD and PMD level helpers can be added later if needed by other
architectures.  For now, 32-bit architectures (x86-32 and arm) only handle
PGTBL_PMD_MODIFIED, so p*d_populate_kernel() will never affect them unless
we introduce a PMD level helper.
[harry.yoo@oracle.com: fix KASAN build error due to p*d_populate_kernel()]
  Link: https://lkml.kernel.org/r/20250822020727.202749-1-harry.yoo@oracle.com
Link: https://lkml.kernel.org/r/20250818020206.4517-3-harry.yoo@oracle.com
Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
Signed-off-by: Harry Yoo harry.yoo@oracle.com
Suggested-by: Dave Hansen dave.hansen@linux.intel.com
Acked-by: Kiryl Shutsemau kas@kernel.org
Reviewed-by: Mike Rapoport (Microsoft) rppt@kernel.org
Reviewed-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com
Acked-by: David Hildenbrand david@redhat.com
Cc: Alexander Potapenko glider@google.com
Cc: Alistair Popple apopple@nvidia.com
Cc: Andrey Konovalov andreyknvl@gmail.com
Cc: Andrey Ryabinin ryabinin.a.a@gmail.com
Cc: Andy Lutomirski luto@kernel.org
Cc: "Aneesh Kumar K.V" aneesh.kumar@linux.ibm.com
Cc: Anshuman Khandual anshuman.khandual@arm.com
Cc: Ard Biesheuvel ardb@kernel.org
Cc: Arnd Bergmann arnd@arndb.de
Cc: bibo mao maobibo@loongson.cn
Cc: Borislav Betkov bp@alien8.de
Cc: Christoph Lameter (Ampere) cl@gentwo.org
Cc: Dennis Zhou dennis@kernel.org
Cc: Dev Jain dev.jain@arm.com
Cc: Dmitriy Vyukov dvyukov@google.com
Cc: Gwan-gyeong Mun gwan-gyeong.mun@intel.com
Cc: Ingo Molnar mingo@redhat.com
Cc: Jane Chu jane.chu@oracle.com
Cc: Joao Martins joao.m.martins@oracle.com
Cc: Joerg Roedel joro@8bytes.org
Cc: John Hubbard jhubbard@nvidia.com
Cc: Kevin Brodsky kevin.brodsky@arm.com
Cc: Liam Howlett liam.howlett@oracle.com
Cc: Michal Hocko mhocko@suse.com
Cc: Oscar Salvador osalvador@suse.de
Cc: Peter Xu peterx@redhat.com
Cc: Peter Zijlstra peterz@infradead.org
Cc: Qi Zheng zhengqi.arch@bytedance.com
Cc: Ryan Roberts ryan.roberts@arm.com
Cc: Suren Baghdasaryan surenb@google.com
Cc: Tejun Heo tj@kernel.org
Cc: Thomas Gleinxer tglx@linutronix.de
Cc: Thomas Huth thuth@redhat.com
Cc: "Uladzislau Rezki (Sony)" urezki@gmail.com
Cc: Vincenzo Frascino vincenzo.frascino@arm.com
Cc: Vlastimil Babka vbabka@suse.cz
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Morton akpm@linux-foundation.org

diff --git a/include/linux/pgalloc.h b/include/linux/pgalloc.h
new file mode 100644
index 000000000000..9174fa59bbc5
--- /dev/null
+++ b/include/linux/pgalloc.h
@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_PGALLOC_H
+#define _LINUX_PGALLOC_H
+
+#include <linux/pgtable.h>
+#include <asm/pgalloc.h>
+
+/*
+ * {pgd,p4d}_populate_kernel() are defined as macros to allow
+ * compile-time optimization based on the configured page table levels.
+ * Without this, linking may fail because callers (e.g., KASAN) may rely
+ * on calls to these functions being optimized away when passing symbols
+ * that exist only for certain page table levels.
+ */
+#define pgd_populate_kernel(addr, pgd, p4d)				\
+	do {								\
+		pgd_populate(&init_mm, pgd, p4d);			\
+		if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_PGD_MODIFIED)	\
+			arch_sync_kernel_mappings(addr, addr);		\
+	} while (0)
+
+#define p4d_populate_kernel(addr, p4d, pud)				\
+	do {								\
+		p4d_populate(&init_mm, p4d, pud);			\
+		if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_P4D_MODIFIED)	\
+			arch_sync_kernel_mappings(addr, addr);		\
+	} while (0)
+
+#endif /* _LINUX_PGALLOC_H */
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index ba699df6ef69..2b80fd456c8b 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
@@ -1954,10 +1954,11 @@ static inline bool arch_has_pfn_modify_check(void)
 /*
  * Page Table Modification bits for pgtbl_mod_mask.
  *
- * These are used by the p?d_alloc_track*() set of functions an in the generic
- * vmalloc/ioremap code to track at which page-table levels entries have been
- * modified. Based on that the code can better decide when vmalloc and ioremap
- * mapping changes need to be synchronized to other page-tables in the system.
+ * These are used by the p?d_alloc_track*() and p*d_populate_kernel()
+ * functions in the generic vmalloc, ioremap and page table update code
+ * to track at which page-table levels entries have been modified.
+ * Based on that the code can better decide when page table changes need
+ * to be synchronized to other page-tables in the system.
  */
 #define		__PGTBL_PGD_MODIFIED	0
 #define		__PGTBL_P4D_MODIFIED	1
diff --git a/mm/kasan/init.c b/mm/kasan/init.c
index 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;
 }

    

FAILED: patch "[PATCH] mm: introduce and use {pgd,p4d}_populate_kernel()" failed to apply to 6.1-stable tree