mm/memory: factor out copying the actual PTE in copy_present_pte()

Let's prepare for further changes.

Link: https://lkml.kernel.org/r/20240129124649.189745-12-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: David S. Miller <davem@davemloft.net>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Naveen N. Rao <naveen.n.rao@linux.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Russell King (Oracle) <linux@armlinux.org.uk>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory.c

@@ -930,6 +930,29 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
 	return 0;
 }
 
+static inline void __copy_present_pte(struct vm_area_struct *dst_vma,
+		struct vm_area_struct *src_vma, pte_t *dst_pte, pte_t *src_pte,
+		pte_t pte, unsigned long addr)
+{
+	struct mm_struct *src_mm = src_vma->vm_mm;
+
+	/* If it's a COW mapping, write protect it both processes. */
+	if (is_cow_mapping(src_vma->vm_flags) && pte_write(pte)) {
+		ptep_set_wrprotect(src_mm, addr, src_pte);
+		pte = pte_wrprotect(pte);
+	}
+
+	/* If it's a shared mapping, mark it clean in the child. */
+	if (src_vma->vm_flags & VM_SHARED)
+		pte = pte_mkclean(pte);
+	pte = pte_mkold(pte);
+
+	if (!userfaultfd_wp(dst_vma))
+		pte = pte_clear_uffd_wp(pte);
+
+	set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte);
+}
+
 /*
  * Copy one pte.  Returns 0 if succeeded, or -EAGAIN if one preallocated page
  * is required to copy this pte.
@@ -939,23 +962,23 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
 		 pte_t *dst_pte, pte_t *src_pte, unsigned long addr, int *rss,
 		 struct folio **prealloc)
 {
-	struct mm_struct *src_mm = src_vma->vm_mm;
-	unsigned long vm_flags = src_vma->vm_flags;
 	pte_t pte = ptep_get(src_pte);
 	struct page *page;
 	struct folio *folio;
 
 	page = vm_normal_page(src_vma, addr, pte);
-	if (page)
+	if (unlikely(!page))
+		goto copy_pte;
+
 	folio = page_folio(page);
-	if (page && folio_test_anon(folio)) {
+	folio_get(folio);
+	if (folio_test_anon(folio)) {
 		/*
 		 * If this page may have been pinned by the parent process,
 		 * copy the page immediately for the child so that we'll always
 		 * guarantee the pinned page won't be randomly replaced in the
 		 * future.
 		 */
-		folio_get(folio);
 		if (unlikely(folio_try_dup_anon_rmap_pte(folio, page, src_vma))) {
 			/* Page may be pinned, we have to copy. */
 			folio_put(folio);
@@ -963,34 +986,14 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
 						 addr, rss, prealloc, page);
 		}
 		rss[MM_ANONPAGES]++;
-	} else if (page) {
-		folio_get(folio);
+		VM_WARN_ON_FOLIO(PageAnonExclusive(page), folio);
+	} else {
 		folio_dup_file_rmap_pte(folio, page);
 		rss[mm_counter_file(folio)]++;
 	}
 
-	/*
-	 * If it's a COW mapping, write protect it both
-	 * in the parent and the child
-	 */
-	if (is_cow_mapping(vm_flags) && pte_write(pte)) {
-		ptep_set_wrprotect(src_mm, addr, src_pte);
-		pte = pte_wrprotect(pte);
-	}
-	VM_BUG_ON(page && folio_test_anon(folio) && PageAnonExclusive(page));
-
-	/*
-	 * If it's a shared mapping, mark it clean in
-	 * the child
-	 */
-	if (vm_flags & VM_SHARED)
-		pte = pte_mkclean(pte);
-	pte = pte_mkold(pte);
-
-	if (!userfaultfd_wp(dst_vma))
-		pte = pte_clear_uffd_wp(pte);
-
-	set_pte_at(dst_vma->vm_mm, addr, dst_pte, pte);
+copy_pte:
+	__copy_present_pte(dst_vma, src_vma, dst_pte, src_pte, pte, addr);
 	return 0;
 }