mm/writeback: Add folio_start_writeback()

Rename set_page_writeback() to folio_start_writeback() to match
folio_end_writeback().  Do not bother with wrappers that return void;
callers are perfectly capable of ignoring return values.

Add wrappers for set_page_writeback(), set_page_writeback_keepwrite() and
test_set_page_writeback() for compatibililty with existing filesystems.
The main advantage of this patch is getting the statistics right,
although it does eliminate a couple of calls to compound_head().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Howells <dhowells@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>

include/linux/page-flags.h

@@ -657,21 +657,22 @@ static __always_inline void SetPageUptodate(struct page *page)
 
 CLEARPAGEFLAG(Uptodate, uptodate, PF_NO_TAIL)
 
-int __test_set_page_writeback(struct page *page, bool keep_write);
+bool __folio_start_writeback(struct folio *folio, bool keep_write);
+bool set_page_writeback(struct page *page);
 
-#define test_set_page_writeback(page)			\
-	__test_set_page_writeback(page, false)
-#define test_set_page_writeback_keepwrite(page)	\
-	__test_set_page_writeback(page, true)
-
-static inline void set_page_writeback(struct page *page)
-{
-	test_set_page_writeback(page);
-}
+#define folio_start_writeback(folio)			\
+	__folio_start_writeback(folio, false)
+#define folio_start_writeback_keepwrite(folio)	\
+	__folio_start_writeback(folio, true)
 
 static inline void set_page_writeback_keepwrite(struct page *page)
 {
-	test_set_page_writeback_keepwrite(page);
+	folio_start_writeback_keepwrite(page_folio(page));
+}
+
+static inline bool test_set_page_writeback(struct page *page)
+{
+	return set_page_writeback(page);
 }
 
 __PAGEFLAG(Head, head, PF_ANY) CLEARPAGEFLAG(Head, head, PF_ANY)

mm/folio-compat.c

@@ -71,3 +71,9 @@ void migrate_page_copy(struct page *newpage, struct page *page)
 }
 EXPORT_SYMBOL(migrate_page_copy);
 #endif
+
+bool set_page_writeback(struct page *page)
+{
+	return folio_start_writeback(page_folio(page));
+}
+EXPORT_SYMBOL(set_page_writeback);

mm/page-writeback.c

@@ -2811,21 +2811,23 @@ bool __folio_end_writeback(struct folio *folio)
 	return ret;
 }
 
-int __test_set_page_writeback(struct page *page, bool keep_write)
+bool __folio_start_writeback(struct folio *folio, bool keep_write)
 {
-	struct address_space *mapping = page_mapping(page);
-	int ret, access_ret;
+	long nr = folio_nr_pages(folio);
+	struct address_space *mapping = folio_mapping(folio);
+	bool ret;
+	int access_ret;
 
-	lock_page_memcg(page);
+	folio_memcg_lock(folio);
 	if (mapping && mapping_use_writeback_tags(mapping)) {
-		XA_STATE(xas, &mapping->i_pages, page_index(page));
+		XA_STATE(xas, &mapping->i_pages, folio_index(folio));
 		struct inode *inode = mapping->host;
 		struct backing_dev_info *bdi = inode_to_bdi(inode);
 		unsigned long flags;
 
 		xas_lock_irqsave(&xas, flags);
 		xas_load(&xas);
-		ret = TestSetPageWriteback(page);
+		ret = folio_test_set_writeback(folio);
 		if (!ret) {
 			bool on_wblist;
 
@@ -2836,43 +2838,42 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
 			if (bdi->capabilities & BDI_CAP_WRITEBACK_ACCT) {
 				struct bdi_writeback *wb = inode_to_wb(inode);
 
-				inc_wb_stat(wb, WB_WRITEBACK);
+				wb_stat_mod(wb, WB_WRITEBACK, nr);
 				if (!on_wblist)
 					wb_inode_writeback_start(wb);
 			}
 
 			/*
-			 * We can come through here when swapping anonymous
-			 * pages, so we don't necessarily have an inode to track
-			 * for sync.
+			 * We can come through here when swapping
+			 * anonymous folios, so we don't necessarily
+			 * have an inode to track for sync.
 			 */
 			if (mapping->host && !on_wblist)
 				sb_mark_inode_writeback(mapping->host);
 		}
-		if (!PageDirty(page))
+		if (!folio_test_dirty(folio))
 			xas_clear_mark(&xas, PAGECACHE_TAG_DIRTY);
 		if (!keep_write)
 			xas_clear_mark(&xas, PAGECACHE_TAG_TOWRITE);
 		xas_unlock_irqrestore(&xas, flags);
 	} else {
-		ret = TestSetPageWriteback(page);
+		ret = folio_test_set_writeback(folio);
 	}
 	if (!ret) {
-		inc_lruvec_page_state(page, NR_WRITEBACK);
-		inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
+		lruvec_stat_mod_folio(folio, NR_WRITEBACK, nr);
+		zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, nr);
 	}
-	unlock_page_memcg(page);
-	access_ret = arch_make_page_accessible(page);
+	folio_memcg_unlock(folio);
+	access_ret = arch_make_folio_accessible(folio);
 	/*
 	 * If writeback has been triggered on a page that cannot be made
 	 * accessible, it is too late to recover here.
 	 */
-	VM_BUG_ON_PAGE(access_ret != 0, page);
+	VM_BUG_ON_FOLIO(access_ret != 0, folio);
 
 	return ret;
-
 }
-EXPORT_SYMBOL(__test_set_page_writeback);
+EXPORT_SYMBOL(__folio_start_writeback);
 
 /**
  * folio_wait_writeback - Wait for a folio to finish writeback.