mm/filemap: unify dropbehind flag testing and clearing

The read and write side does this a bit differently, unify it such that
the _{read,write} helpers check the bit before locking, and the generic
handler is in charge of clearing the bit and invalidating, once under
the folio lock.

Signed-off-by: Jens Axboe <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: axboe

mm/filemap.c

@@ -1595,7 +1595,11 @@ static void filemap_end_dropbehind(struct folio *folio)
 
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 
-	if (mapping && !folio_test_writeback(folio) && !folio_test_dirty(folio))
+	if (folio_test_writeback(folio) || folio_test_dirty(folio))
+		return;
+	if (!folio_test_clear_dropbehind(folio))
+		return;
+	if (mapping)
 		folio_unmap_invalidate(mapping, folio, 0);
 }
 
@@ -1606,6 +1610,9 @@ static void filemap_end_dropbehind(struct folio *folio)
  */
 static void filemap_end_dropbehind_write(struct folio *folio)
 {
+	if (!folio_test_dropbehind(folio))
+		return;
+
 	/*
 	 * Hitting !in_task() should not happen off RWF_DONTCACHE writeback,
 	 * but can happen if normal writeback just happens to find dirty folios
@@ -1629,8 +1636,6 @@ static void filemap_end_dropbehind_write(struct folio *folio)
  */
 void folio_end_writeback(struct folio *folio)
 {
-	bool folio_dropbehind = false;
-
 	VM_BUG_ON_FOLIO(!folio_test_writeback(folio), folio);
 
 	/*
@@ -1652,14 +1657,11 @@ void folio_end_writeback(struct folio *folio)
 	 * reused before the folio_wake_bit().
 	 */
 	folio_get(folio);
-	if (!folio_test_dirty(folio))
-		folio_dropbehind = folio_test_clear_dropbehind(folio);
 	if (__folio_end_writeback(folio))
 		folio_wake_bit(folio, PG_writeback);
-	acct_reclaim_writeback(folio);
 
-	if (folio_dropbehind)
-		filemap_end_dropbehind_write(folio);
+	filemap_end_dropbehind_write(folio);
+	acct_reclaim_writeback(folio);
 	folio_put(folio);
 }
 EXPORT_SYMBOL(folio_end_writeback);
@@ -2651,8 +2653,7 @@ static void filemap_end_dropbehind_read(struct folio *folio)
 	if (folio_test_writeback(folio) || folio_test_dirty(folio))
 		return;
 	if (folio_trylock(folio)) {
-		if (folio_test_clear_dropbehind(folio))
-			filemap_end_dropbehind(folio);
+		filemap_end_dropbehind(folio);
 		folio_unlock(folio);
 	}
 }