mm: make __end_folio_writeback() return void

Rather than check the result of test-and-clear, just check that we have
the writeback bit set at the start.  This wouldn't catch every case, but
it's good enough (and enables the next patch).

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Andreas Dilger <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/filemap.c

@@ -1593,9 +1593,15 @@ EXPORT_SYMBOL(folio_wait_private_2_killable);
 /**
  * folio_end_writeback - End writeback against a folio.
  * @folio: The folio.
+ *
+ * The folio must actually be under writeback.
+ *
+ * Context: May be called from process or interrupt context.
  */
 void folio_end_writeback(struct folio *folio)
 {
+	VM_BUG_ON_FOLIO(!folio_test_writeback(folio), folio);
+
 	/*
 	 * folio_test_clear_reclaim() could be used here but it is an
 	 * atomic operation and overkill in this particular case. Failing
@@ -1615,8 +1621,7 @@ void folio_end_writeback(struct folio *folio)
 	 * reused before the folio_wake().
 	 */
 	folio_get(folio);
-	if (!__folio_end_writeback(folio))
-		BUG();
+	__folio_end_writeback(folio);
 
 	smp_mb__after_atomic();
 	folio_wake(folio, PG_writeback);

mm/internal.h

@@ -105,7 +105,7 @@ static inline void wake_throttle_isolated(pg_data_t *pgdat)
 
 vm_fault_t do_swap_page(struct vm_fault *vmf);
 void folio_rotate_reclaimable(struct folio *folio);
-bool __folio_end_writeback(struct folio *folio);
+void __folio_end_writeback(struct folio *folio);
 void deactivate_file_folio(struct folio *folio);
 void folio_activate(struct folio *folio);
 

mm/page-writeback.c

@@ -2940,11 +2940,10 @@ static void wb_inode_writeback_end(struct bdi_writeback *wb)
 	spin_unlock_irqrestore(&wb->work_lock, flags);
 }
 
-bool __folio_end_writeback(struct folio *folio)
+void __folio_end_writeback(struct folio *folio)
 {
 	long nr = folio_nr_pages(folio);
 	struct address_space *mapping = folio_mapping(folio);
-	bool ret;
 
 	folio_memcg_lock(folio);
 	if (mapping && mapping_use_writeback_tags(mapping)) {
@@ -2953,19 +2952,16 @@ bool __folio_end_writeback(struct folio *folio)
 		unsigned long flags;
 
 		xa_lock_irqsave(&mapping->i_pages, flags);
-		ret = folio_test_clear_writeback(folio);
-		if (ret) {
-			__xa_clear_mark(&mapping->i_pages, folio_index(folio),
-						PAGECACHE_TAG_WRITEBACK);
-			if (bdi->capabilities & BDI_CAP_WRITEBACK_ACCT) {
-				struct bdi_writeback *wb = inode_to_wb(inode);
-
-				wb_stat_mod(wb, WB_WRITEBACK, -nr);
-				__wb_writeout_add(wb, nr);
-				if (!mapping_tagged(mapping,
-						    PAGECACHE_TAG_WRITEBACK))
-					wb_inode_writeback_end(wb);
-			}
+		folio_test_clear_writeback(folio);
+		__xa_clear_mark(&mapping->i_pages, folio_index(folio),
+					PAGECACHE_TAG_WRITEBACK);
+		if (bdi->capabilities & BDI_CAP_WRITEBACK_ACCT) {
+			struct bdi_writeback *wb = inode_to_wb(inode);
+
+			wb_stat_mod(wb, WB_WRITEBACK, -nr);
+			__wb_writeout_add(wb, nr);
+			if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
+				wb_inode_writeback_end(wb);
 		}
 
 		if (mapping->host && !mapping_tagged(mapping,
@@ -2974,15 +2970,13 @@ bool __folio_end_writeback(struct folio *folio)
 
 		xa_unlock_irqrestore(&mapping->i_pages, flags);
 	} else {
-		ret = folio_test_clear_writeback(folio);
-	}
-	if (ret) {
-		lruvec_stat_mod_folio(folio, NR_WRITEBACK, -nr);
-		zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
-		node_stat_mod_folio(folio, NR_WRITTEN, nr);
+		folio_test_clear_writeback(folio);
 	}
+
+	lruvec_stat_mod_folio(folio, NR_WRITEBACK, -nr);
+	zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
+	node_stat_mod_folio(folio, NR_WRITTEN, nr);
 	folio_memcg_unlock(folio);
-	return ret;
 }
 
 bool __folio_start_writeback(struct folio *folio, bool keep_write)