mm: split out the non-present case from copy_one_pte()

This is a purely mechanical split of the copy_one_pte() function.  It's
not immediately obvious when looking at the diff because of the
indentation change, but the way to see what is going on in this commit
is to use the "-w" flag to not show pure whitespace changes, and you see
how the first part of copy_one_pte() is simply lifted out into a
separate function.

And since the non-present case is marked unlikely, don't make the new
function be inlined.  Not that gcc really seems to care, since it looks
like it will inline it anyway due to the whole "single callsite for
static function" logic.  In fact, code generation with the function
split is almost identical to before.  But not marking it inline is the
right thing to do.

This is pure prep-work and cleanup for subsequent changes.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -695,85 +695,98 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
  * covered by this vma.
  */
 
-static inline unsigned long
-copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
+static unsigned long
+copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		pte_t *dst_pte, pte_t *src_pte, struct vm_area_struct *vma,
 		unsigned long addr, int *rss)
 {
 	unsigned long vm_flags = vma->vm_flags;
 	pte_t pte = *src_pte;
 	struct page *page;
+	swp_entry_t entry = pte_to_swp_entry(pte);
+
+	if (likely(!non_swap_entry(entry))) {
+		if (swap_duplicate(entry) < 0)
+			return entry.val;
+
+		/* make sure dst_mm is on swapoff's mmlist. */
+		if (unlikely(list_empty(&dst_mm->mmlist))) {
+			spin_lock(&mmlist_lock);
+			if (list_empty(&dst_mm->mmlist))
+				list_add(&dst_mm->mmlist,
+						&src_mm->mmlist);
+			spin_unlock(&mmlist_lock);
+		}
+		rss[MM_SWAPENTS]++;
+	} else if (is_migration_entry(entry)) {
+		page = migration_entry_to_page(entry);
 
-	/* pte contains position in swap or file, so copy. */
-	if (unlikely(!pte_present(pte))) {
-		swp_entry_t entry = pte_to_swp_entry(pte);
-
-		if (likely(!non_swap_entry(entry))) {
-			if (swap_duplicate(entry) < 0)
-				return entry.val;
-
-			/* make sure dst_mm is on swapoff's mmlist. */
-			if (unlikely(list_empty(&dst_mm->mmlist))) {
-				spin_lock(&mmlist_lock);
-				if (list_empty(&dst_mm->mmlist))
-					list_add(&dst_mm->mmlist,
-							&src_mm->mmlist);
-				spin_unlock(&mmlist_lock);
-			}
-			rss[MM_SWAPENTS]++;
-		} else if (is_migration_entry(entry)) {
-			page = migration_entry_to_page(entry);
-
-			rss[mm_counter(page)]++;
-
-			if (is_write_migration_entry(entry) &&
-					is_cow_mapping(vm_flags)) {
-				/*
-				 * COW mappings require pages in both
-				 * parent and child to be set to read.
-				 */
-				make_migration_entry_read(&entry);
-				pte = swp_entry_to_pte(entry);
-				if (pte_swp_soft_dirty(*src_pte))
-					pte = pte_swp_mksoft_dirty(pte);
-				if (pte_swp_uffd_wp(*src_pte))
-					pte = pte_swp_mkuffd_wp(pte);
-				set_pte_at(src_mm, addr, src_pte, pte);
-			}
-		} else if (is_device_private_entry(entry)) {
-			page = device_private_entry_to_page(entry);
+		rss[mm_counter(page)]++;
 
+		if (is_write_migration_entry(entry) &&
+				is_cow_mapping(vm_flags)) {
 			/*
-			 * Update rss count even for unaddressable pages, as
-			 * they should treated just like normal pages in this
-			 * respect.
-			 *
-			 * We will likely want to have some new rss counters
-			 * for unaddressable pages, at some point. But for now
-			 * keep things as they are.
+			 * COW mappings require pages in both
+			 * parent and child to be set to read.
 			 */
-			get_page(page);
-			rss[mm_counter(page)]++;
-			page_dup_rmap(page, false);
+			make_migration_entry_read(&entry);
+			pte = swp_entry_to_pte(entry);
+			if (pte_swp_soft_dirty(*src_pte))
+				pte = pte_swp_mksoft_dirty(pte);
+			if (pte_swp_uffd_wp(*src_pte))
+				pte = pte_swp_mkuffd_wp(pte);
+			set_pte_at(src_mm, addr, src_pte, pte);
+		}
+	} else if (is_device_private_entry(entry)) {
+		page = device_private_entry_to_page(entry);
 
-			/*
-			 * We do not preserve soft-dirty information, because so
-			 * far, checkpoint/restore is the only feature that
-			 * requires that. And checkpoint/restore does not work
-			 * when a device driver is involved (you cannot easily
-			 * save and restore device driver state).
-			 */
-			if (is_write_device_private_entry(entry) &&
-			    is_cow_mapping(vm_flags)) {
-				make_device_private_entry_read(&entry);
-				pte = swp_entry_to_pte(entry);
-				if (pte_swp_uffd_wp(*src_pte))
-					pte = pte_swp_mkuffd_wp(pte);
-				set_pte_at(src_mm, addr, src_pte, pte);
-			}
+		/*
+		 * Update rss count even for unaddressable pages, as
+		 * they should treated just like normal pages in this
+		 * respect.
+		 *
+		 * We will likely want to have some new rss counters
+		 * for unaddressable pages, at some point. But for now
+		 * keep things as they are.
+		 */
+		get_page(page);
+		rss[mm_counter(page)]++;
+		page_dup_rmap(page, false);
+
+		/*
+		 * We do not preserve soft-dirty information, because so
+		 * far, checkpoint/restore is the only feature that
+		 * requires that. And checkpoint/restore does not work
+		 * when a device driver is involved (you cannot easily
+		 * save and restore device driver state).
+		 */
+		if (is_write_device_private_entry(entry) &&
+		    is_cow_mapping(vm_flags)) {
+			make_device_private_entry_read(&entry);
+			pte = swp_entry_to_pte(entry);
+			if (pte_swp_uffd_wp(*src_pte))
+				pte = pte_swp_mkuffd_wp(pte);
+			set_pte_at(src_mm, addr, src_pte, pte);
 		}
-		goto out_set_pte;
 	}
+	set_pte_at(dst_mm, addr, dst_pte, pte);
+	return 0;
+}
+
+static inline unsigned long
+copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
+		pte_t *dst_pte, pte_t *src_pte, struct vm_area_struct *vma,
+		unsigned long addr, int *rss)
+{
+	unsigned long vm_flags = vma->vm_flags;
+	pte_t pte = *src_pte;
+	struct page *page;
+
+	/* pte contains position in swap or file, so copy. */
+	if (unlikely(!pte_present(pte)))
+		return copy_nonpresent_pte(dst_mm, src_mm,
+					   dst_pte, src_pte, vma,
+					   addr, rss);
 
 	/*
 	 * If it's a COW mapping, write protect it both
@@ -807,7 +820,6 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		rss[mm_counter(page)]++;
 	}
 
-out_set_pte:
 	set_pte_at(dst_mm, addr, dst_pte, pte);
 	return 0;
 }