mm/memory: factor out common code from vm_normal_page_*()

Let's reduce the code duplication and factor out the non-pte/pmd related
magic into __vm_normal_page().

To keep it simpler, check the pfn against both zero folios, which
shouldn't really make a difference.

It's a good question if we can even hit the !CONFIG_ARCH_HAS_PTE_SPECIAL
scenario in the PMD case in practice: but doesn't really matter, as it's
now all unified in vm_normal_page_pfn().

Add kerneldoc for all involved functions.

Note that, as a side product, we now:
* Support the find_special_page special thingy also for PMD
* Don't check for is_huge_zero_pfn() anymore if we have
  CONFIG_ARCH_HAS_PTE_SPECIAL and the PMD is not special. The
  VM_WARN_ON_ONCE would catch any abuse

No functional change intended.

Link: https://lkml.kernel.org/r/20250811112631.759341-10-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Juegren Gross <jgross@suse.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory.c

@@ -614,8 +614,14 @@ static void print_bad_page_map(struct vm_area_struct *vma,
 #define print_bad_pte(vma, addr, pte, page) \
 	print_bad_page_map(vma, addr, pte_val(pte), page, PGTABLE_LEVEL_PTE)
 
-/*
+/**
- * vm_normal_page -- This function gets the "struct page" associated with a pte.
+ * __vm_normal_page() - Get the "struct page" associated with a page table entry.
+ * @vma: The VMA mapping the page table entry.
+ * @addr: The address where the page table entry is mapped.
+ * @pfn: The PFN stored in the page table entry.
+ * @special: Whether the page table entry is marked "special".
+ * @level: The page table level for error reporting purposes only.
+ * @entry: The page table entry value for error reporting purposes only.
  *
  * "Special" mappings do not wish to be associated with a "struct page" (either
  * it doesn't exist, or it exists but they don't want to touch it). In this
@@ -628,10 +634,10 @@ static void print_bad_page_map(struct vm_area_struct *vma,
  * Selected page table walkers (such as GUP) can still identify mappings of the
  * shared zero folios and work with the underlying "struct page".
  *
- * There are 2 broad cases. Firstly, an architecture may define a pte_special()
+ * There are 2 broad cases. Firstly, an architecture may define a "special"
- * pte bit, in which case this function is trivial. Secondly, an architecture
+ * page table entry bit, such as pte_special(), in which case this function is
- * may not have a spare pte bit, which requires a more complicated scheme,
+ * trivial. Secondly, an architecture may not have a spare page table
- * described below.
+ * entry bit, which requires a more complicated scheme, described below.
  *
  * A raw VM_PFNMAP mapping (ie. one that is not COWed) is always considered a
  * special mapping (even if there are underlying and valid "struct pages").
@@ -664,38 +670,39 @@ static void print_bad_page_map(struct vm_area_struct *vma,
  * don't have to follow the strict linearity rule of PFNMAP mappings in
  * order to support COWable mappings.
  *
+ * Return: Returns the "struct page" if this is a "normal" mapping. Returns
+ *	   NULL if this is a "special" mapping.
  */
-struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
+static inline struct page *__vm_normal_page(struct vm_area_struct *vma,
-			    pte_t pte)
+		unsigned long addr, unsigned long pfn, bool special,
+		unsigned long long entry, enum pgtable_level level)
 {
-	unsigned long pfn = pte_pfn(pte);
-
 	if (IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL)) {
-		if (likely(!pte_special(pte)))
+		if (unlikely(special)) {
-			goto check_pfn;
 			if (vma->vm_ops && vma->vm_ops->find_special_page)
 				return vma->vm_ops->find_special_page(vma, addr);
 			if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
 				return NULL;
-		if (is_zero_pfn(pfn))
+			if (is_zero_pfn(pfn) || is_huge_zero_pfn(pfn))
 				return NULL;
 
-		print_bad_pte(vma, addr, pte, NULL);
+			print_bad_page_map(vma, addr, entry, NULL, level);
 			return NULL;
 		}
-
+		/*
-	/* !CONFIG_ARCH_HAS_PTE_SPECIAL case follows: */
+		 * With CONFIG_ARCH_HAS_PTE_SPECIAL, any special page table
-
+		 * mappings (incl. shared zero folios) are marked accordingly.
+		 */
+	} else {
 		if (unlikely(vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))) {
 			if (vma->vm_flags & VM_MIXEDMAP) {
+				/* If it has a "struct page", it's "normal". */
 				if (!pfn_valid(pfn))
 					return NULL;
-			if (is_zero_pfn(pfn))
-				return NULL;
-			goto out;
 			} else {
-			unsigned long off;
+				unsigned long off = (addr - vma->vm_start) >> PAGE_SHIFT;
-			off = (addr - vma->vm_start) >> PAGE_SHIFT;
+
+				/* Only CoW'ed anon folios are "normal". */
 				if (pfn == vma->vm_pgoff + off)
 					return NULL;
 				if (!is_cow_mapping(vma->vm_flags))
@@ -703,24 +710,54 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
 			}
 		}
 
-	if (is_zero_pfn(pfn))
+		if (is_zero_pfn(pfn) || is_huge_zero_pfn(pfn))
-		return NULL;
-
-check_pfn:
-	if (unlikely(pfn > highest_memmap_pfn)) {
-		print_bad_pte(vma, addr, pte, NULL);
 			return NULL;
 	}
 
+	if (unlikely(pfn > highest_memmap_pfn)) {
+		/* Corrupted page table entry. */
+		print_bad_page_map(vma, addr, entry, NULL, level);
+		return NULL;
+	}
 	/*
 	 * NOTE! We still have PageReserved() pages in the page tables.
-	 * eg. VDSO mappings can cause them to exist.
+	 * For example, VDSO mappings can cause them to exist.
 	 */
-out:
+	VM_WARN_ON_ONCE(is_zero_pfn(pfn) || is_huge_zero_pfn(pfn));
-	VM_WARN_ON_ONCE(is_zero_pfn(pfn));
 	return pfn_to_page(pfn);
 }
 
+/**
+ * vm_normal_page() - Get the "struct page" associated with a PTE
+ * @vma: The VMA mapping the @pte.
+ * @addr: The address where the @pte is mapped.
+ * @pte: The PTE.
+ *
+ * Get the "struct page" associated with a PTE. See __vm_normal_page()
+ * for details on "normal" and "special" mappings.
+ *
+ * Return: Returns the "struct page" if this is a "normal" mapping. Returns
+ *	   NULL if this is a "special" mapping.
+ */
+struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
+			    pte_t pte)
+{
+	return __vm_normal_page(vma, addr, pte_pfn(pte), pte_special(pte),
+				pte_val(pte), PGTABLE_LEVEL_PTE);
+}
+
+/**
+ * vm_normal_folio() - Get the "struct folio" associated with a PTE
+ * @vma: The VMA mapping the @pte.
+ * @addr: The address where the @pte is mapped.
+ * @pte: The PTE.
+ *
+ * Get the "struct folio" associated with a PTE. See __vm_normal_page()
+ * for details on "normal" and "special" mappings.
+ *
+ * Return: Returns the "struct folio" if this is a "normal" mapping. Returns
+ *	   NULL if this is a "special" mapping.
+ */
 struct folio *vm_normal_folio(struct vm_area_struct *vma, unsigned long addr,
 			    pte_t pte)
 {
@@ -732,42 +769,37 @@ struct folio *vm_normal_folio(struct vm_area_struct *vma, unsigned long addr,
 }
 
 #ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES
+/**
+ * vm_normal_page_pmd() - Get the "struct page" associated with a PMD
+ * @vma: The VMA mapping the @pmd.
+ * @addr: The address where the @pmd is mapped.
+ * @pmd: The PMD.
+ *
+ * Get the "struct page" associated with a PTE. See __vm_normal_page()
+ * for details on "normal" and "special" mappings.
+ *
+ * Return: Returns the "struct page" if this is a "normal" mapping. Returns
+ *	   NULL if this is a "special" mapping.
+ */
 struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
 				pmd_t pmd)
 {
-	unsigned long pfn = pmd_pfn(pmd);
+	return __vm_normal_page(vma, addr, pmd_pfn(pmd), pmd_special(pmd),
-
+				pmd_val(pmd), PGTABLE_LEVEL_PMD);
-	if (unlikely(pmd_special(pmd)))
-		return NULL;
-
-	if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
-		if (vma->vm_flags & VM_MIXEDMAP) {
-			if (!pfn_valid(pfn))
-				return NULL;
-			goto out;
-		} else {
-			unsigned long off;
-			off = (addr - vma->vm_start) >> PAGE_SHIFT;
-			if (pfn == vma->vm_pgoff + off)
-				return NULL;
-			if (!is_cow_mapping(vma->vm_flags))
-				return NULL;
-		}
 }
 
-	if (is_huge_zero_pfn(pfn))
+/**
-		return NULL;
+ * vm_normal_folio_pmd() - Get the "struct folio" associated with a PMD
-	if (unlikely(pfn > highest_memmap_pfn))
+ * @vma: The VMA mapping the @pmd.
-		return NULL;
+ * @addr: The address where the @pmd is mapped.
-
+ * @pmd: The PMD.
-	/*
+ *
-	 * NOTE! We still have PageReserved() pages in the page tables.
+ * Get the "struct folio" associated with a PTE. See __vm_normal_page()
-	 * eg. VDSO mappings can cause them to exist.
+ * for details on "normal" and "special" mappings.
+ *
+ * Return: Returns the "struct folio" if this is a "normal" mapping. Returns
+ *	   NULL if this is a "special" mapping.
  */
-out:
-	return pfn_to_page(pfn);
-}
-
 struct folio *vm_normal_folio_pmd(struct vm_area_struct *vma,
 				  unsigned long addr, pmd_t pmd)
 {