[RFC 2/6] mm/migrate_pages: split unmap_and_move() to _unmap() and _move()

[Huang Ying]

This is a preparation patch to batch the page unmapping and moving
for the normal pages and THP.

In this patch, unmap_and_move() is split to migrate_page_unmap() and
migrate_page_move().  So, we can batch _unmap() and _move() in
different loops later.  To pass some information between unmap and
move, the original unused newpage->mapping and newpage->private are
used.

Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
---
 mm/migrate.c | 164 ++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 122 insertions(+), 42 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 117134f1c6dc..4a81e0bfdbcd 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -976,13 +976,32 @@ static int move_to_new_folio(struct folio *dst, struct folio *src,
 	return rc;
 }
 
-static int __unmap_and_move(struct page *page, struct page *newpage,
+static void __migrate_page_record(struct page *newpage,
+				  int page_was_mapped,
+				  struct anon_vma *anon_vma)
+{
+	newpage->mapping = (struct address_space *)anon_vma;
+	newpage->private = page_was_mapped;
+}
+
+static void __migrate_page_extract(struct page *newpage,
+				   int *page_was_mappedp,
+				   struct anon_vma **anon_vmap)
+{
+	*anon_vmap = (struct anon_vma *)newpage->mapping;
+	*page_was_mappedp = newpage->private;
+	newpage->mapping = NULL;
+	newpage->private = 0;
+}
+
+#define MIGRATEPAGE_UNMAP		1
+
+static int __migrate_page_unmap(struct page *page, struct page *newpage,
 				int force, enum migrate_mode mode)
 {
 	struct folio *folio = page_folio(page);
-	struct folio *dst = page_folio(newpage);
 	int rc = -EAGAIN;
-	bool page_was_mapped = false;
+	int page_was_mapped = 0;
 	struct anon_vma *anon_vma = NULL;
 	bool is_lru = !__PageMovable(page);
 
@@ -1058,8 +1077,8 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 		goto out_unlock;
 
 	if (unlikely(!is_lru)) {
-		rc = move_to_new_folio(dst, folio, mode);
-		goto out_unlock_both;
+		__migrate_page_record(newpage, page_was_mapped, anon_vma);
+		return MIGRATEPAGE_UNMAP;
 	}
 
 	/*
@@ -1085,11 +1104,41 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 		VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma,
 				page);
 		try_to_migrate(folio, 0);
-		page_was_mapped = true;
+		page_was_mapped = 1;
+	}
+
+	if (!page_mapped(page)) {
+		__migrate_page_record(newpage, page_was_mapped, anon_vma);
+		return MIGRATEPAGE_UNMAP;
 	}
 
-	if (!page_mapped(page))
-		rc = move_to_new_folio(dst, folio, mode);
+	if (page_was_mapped)
+		remove_migration_ptes(folio, folio, false);
+
+out_unlock_both:
+	unlock_page(newpage);
+out_unlock:
+	/* Drop an anon_vma reference if we took one */
+	if (anon_vma)
+		put_anon_vma(anon_vma);
+	unlock_page(page);
+out:
+
+	return rc;
+}
+
+static int __migrate_page_move(struct page *page, struct page *newpage,
+			       enum migrate_mode mode)
+{
+	struct folio *folio = page_folio(page);
+	struct folio *dst = page_folio(newpage);
+	int rc;
+	int page_was_mapped = 0;
+	struct anon_vma *anon_vma = NULL;
+
+	__migrate_page_extract(newpage, &page_was_mapped, &anon_vma);
+
+	rc = move_to_new_folio(dst, folio, mode);
 
 	/*
 	 * When successful, push newpage to LRU immediately: so that if it
@@ -1110,14 +1159,11 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 		remove_migration_ptes(folio,
 			rc == MIGRATEPAGE_SUCCESS ? dst : folio, false);
 
-out_unlock_both:
 	unlock_page(newpage);
-out_unlock:
 	/* Drop an anon_vma reference if we took one */
 	if (anon_vma)
 		put_anon_vma(anon_vma);
 	unlock_page(page);
-out:
 	/*
 	 * If migration is successful, decrease refcount of the newpage,
 	 * which will not free the page because new page owner increased
@@ -1129,18 +1175,31 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 	return rc;
 }
 
-/*
- * Obtain the lock on page, remove all ptes and migrate the page
- * to the newly allocated page in newpage.
- */
-static int unmap_and_move(new_page_t get_new_page,
-				   free_page_t put_new_page,
-				   unsigned long private, struct page *page,
-				   int force, enum migrate_mode mode,
-				   enum migrate_reason reason,
-				   struct list_head *ret)
+static void migrate_page_done(struct page *page,
+			      enum migrate_reason reason)
+{
+	/*
+	 * Compaction can migrate also non-LRU pages which are
+	 * not accounted to NR_ISOLATED_*. They can be recognized
+	 * as __PageMovable
+	 */
+	if (likely(!__PageMovable(page)))
+		mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
+				    page_is_file_lru(page), -thp_nr_pages(page));
+
+	if (reason != MR_MEMORY_FAILURE)
+		/* We release the page in page_handle_poison. */
+		put_page(page);
+}
+
+/* Obtain the lock on page, remove all ptes. */
+static int migrate_page_unmap(new_page_t get_new_page, free_page_t put_new_page,
+			      unsigned long private, struct page *page,
+			      struct page **newpagep, int force,
+			      enum migrate_mode mode, enum migrate_reason reason,
+			      struct list_head *ret)
 {
-	int rc = MIGRATEPAGE_SUCCESS;
+	int rc = MIGRATEPAGE_UNMAP;
 	struct page *newpage = NULL;
 
 	if (!thp_migration_supported() && PageTransHuge(page))
@@ -1151,19 +1210,48 @@ static int unmap_and_move(new_page_t get_new_page,
 		ClearPageActive(page);
 		ClearPageUnevictable(page);
 		/* free_pages_prepare() will clear PG_isolated. */
-		goto out;
+		list_del(&page->lru);
+		migrate_page_done(page, reason);
+		return MIGRATEPAGE_SUCCESS;
 	}
 
 	newpage = get_new_page(page, private);
 	if (!newpage)
 		return -ENOMEM;
+	*newpagep = newpage;
 
-	newpage->private = 0;
-	rc = __unmap_and_move(page, newpage, force, mode);
+	rc = __migrate_page_unmap(page, newpage, force, mode);
+	if (rc == MIGRATEPAGE_UNMAP)
+		return rc;
+
+	/*
+	 * A page that has not been migrated will have kept its
+	 * references and be restored.
+	 */
+	/* restore the page to right list. */
+	if (rc != -EAGAIN)
+		list_move_tail(&page->lru, ret);
+
+	if (put_new_page)
+		put_new_page(newpage, private);
+	else
+		put_page(newpage);
+
+	return rc;
+}
+
+/* Migrate the page to the newly allocated page in newpage. */
+static int migrate_page_move(free_page_t put_new_page, unsigned long private,
+			     struct page *page, struct page *newpage,
+			     enum migrate_mode mode, enum migrate_reason reason,
+			     struct list_head *ret)
+{
+	int rc;
+
+	rc = __migrate_page_move(page, newpage, mode);
 	if (rc == MIGRATEPAGE_SUCCESS)
 		set_page_owner_migrate_reason(newpage, reason);
 
-out:
 	if (rc != -EAGAIN) {
 		/*
 		 * A page that has been migrated has all references
@@ -1179,20 +1267,7 @@ static int unmap_and_move(new_page_t get_new_page,
 	 * we want to retry.
 	 */
 	if (rc == MIGRATEPAGE_SUCCESS) {
-		/*
-		 * Compaction can migrate also non-LRU pages which are
-		 * not accounted to NR_ISOLATED_*. They can be recognized
-		 * as __PageMovable
-		 */
-		if (likely(!__PageMovable(page)))
-			mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
-					page_is_file_lru(page), -thp_nr_pages(page));
-
-		if (reason != MR_MEMORY_FAILURE)
-			/*
-			 * We release the page in page_handle_poison.
-			 */
-			put_page(page);
+		migrate_page_done(page, reason);
 	} else {
 		if (rc != -EAGAIN)
 			list_add_tail(&page->lru, ret);
@@ -1405,6 +1480,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 	int pass = 0;
 	bool is_thp = false;
 	struct page *page;
+	struct page *newpage = NULL;
 	struct page *page2;
 	int rc, nr_subpages;
 	LIST_HEAD(ret_pages);
@@ -1493,9 +1569,13 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 			if (PageHuge(page))
 				continue;
 
-			rc = unmap_and_move(get_new_page, put_new_page,
-						private, page, pass > 2, mode,
+			rc = migrate_page_unmap(get_new_page, put_new_page, private,
+						page, &newpage, pass > 2, mode,
 						reason, &ret_pages);
+			if (rc == MIGRATEPAGE_UNMAP)
+				rc = migrate_page_move(put_new_page, private,
+						       page, newpage, mode,
+						       reason, &ret_pages);
 			/*
 			 * The rules are:
 			 *	Success: page will be freed
-- 
2.35.1