shrink_folio_list() contains a self-contained pageout() dispatch state
machine. Extract it into pageout_one() to reduce the size of
shrink_folio_list() and make the pageout step independently readable.

No functional change.

Signed-off-by: Zhang Peng <bruzzhang@tencent.com>
---
 mm/vmscan.c | 107 ++++++++++++++++++++++++++++++++++++------------------------
 1 file changed, 65 insertions(+), 42 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b31f67801836..456d38eb172c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1161,8 +1161,68 @@ static bool folio_free(struct folio *folio, struct folio_batch *free_folios,
 	return true;
 }
 
+static bool pageout_one(struct folio *folio,
+			struct folio_batch *free_folios,
+			struct scan_control *sc, struct reclaim_stat *stat,
+			struct swap_iocb **plug, struct list_head *folio_list,
+			unsigned int *nr_reclaimed)
+{
+	struct address_space *mapping = folio_mapping(folio);
+	unsigned int nr_pages = folio_nr_pages(folio);
+
+	switch (pageout(folio, mapping, plug, folio_list)) {
+	case PAGE_ACTIVATE:
+		/*
+		 * If shmem folio is split when writeback to swap,
+		 * the tail pages will make their own pass through
+		 * this function and be accounted then.
+		 */
+		if (nr_pages > 1 && !folio_test_large(folio)) {
+			sc->nr_scanned -= (nr_pages - 1);
+			nr_pages = 1;
+		}
+		folio_activate_locked(folio, stat, nr_pages);
+		folio_unlock(folio);
+		return false;
+	case PAGE_KEEP:
+		folio_unlock(folio);
+		return false;
+	case PAGE_SUCCESS:
+		if (nr_pages > 1 && !folio_test_large(folio)) {
+			sc->nr_scanned -= (nr_pages - 1);
+			nr_pages = 1;
+		}
+		stat->nr_pageout += nr_pages;
+
+		if (folio_test_writeback(folio))
+			return false;
+		if (folio_test_dirty(folio))
+			return false;
+
+		/*
+		 * A synchronous write - probably a ramdisk.  Go
+		 * ahead and try to reclaim the folio.
+		 */
+		if (!folio_trylock(folio))
+			return false;
+		if (folio_test_dirty(folio) ||
+		    folio_test_writeback(folio)) {
+			folio_unlock(folio);
+			return false;
+		}
+		mapping = folio_mapping(folio);
+		fallthrough;
+	case PAGE_CLEAN:
+		; /* try to free the folio below */
+	}
+	if (folio_free(folio, free_folios, sc, stat, nr_reclaimed))
+		return true;
+	folio_unlock(folio);
+	return false;
+}
+
 /*
- * shrink_folio_list() returns the number of reclaimed pages
+ * Reclaimed folios are counted in the return value.
  */
 static unsigned int shrink_folio_list(struct list_head *folio_list,
 		struct pglist_data *pgdat, struct scan_control *sc,
@@ -1499,53 +1559,16 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 				goto keep_locked;
 			if (!sc->may_writepage)
 				goto keep_locked;
-
 			/*
 			 * Folio is dirty. Flush the TLB if a writable entry
 			 * potentially exists to avoid CPU writes after I/O
 			 * starts and then write it out here.
 			 */
 			try_to_unmap_flush_dirty();
-			switch (pageout(folio, mapping, &plug, folio_list)) {
-			case PAGE_KEEP:
-				goto keep_locked;
-			case PAGE_ACTIVATE:
-				/*
-				 * If shmem folio is split when writeback to swap,
-				 * the tail pages will make their own pass through
-				 * this function and be accounted then.
-				 */
-				if (nr_pages > 1 && !folio_test_large(folio)) {
-					sc->nr_scanned -= (nr_pages - 1);
-					nr_pages = 1;
-				}
-				goto activate_locked;
-			case PAGE_SUCCESS:
-				if (nr_pages > 1 && !folio_test_large(folio)) {
-					sc->nr_scanned -= (nr_pages - 1);
-					nr_pages = 1;
-				}
-				stat->nr_pageout += nr_pages;
-
-				if (folio_test_writeback(folio))
-					goto keep;
-				if (folio_test_dirty(folio))
-					goto keep;
-
-				/*
-				 * A synchronous write - probably a ramdisk.  Go
-				 * ahead and try to reclaim the folio.
-				 */
-				if (!folio_trylock(folio))
-					goto keep;
-				if (folio_test_dirty(folio) ||
-				    folio_test_writeback(folio))
-					goto keep_locked;
-				mapping = folio_mapping(folio);
-				fallthrough;
-			case PAGE_CLEAN:
-				; /* try to free the folio below */
-			}
+			if (!pageout_one(folio, &free_folios, sc, stat, &plug,
+					 folio_list, &nr_reclaimed))
+				goto keep;
+			continue;
 		}
 
 		if (!folio_free(folio, &free_folios, sc, stat, &nr_reclaimed))

-- 
2.43.7