filemap: Add a version of folio_end_writeback that ignores dropbehind

Filesystems such as NFS may need to defer dropbehind until after their
2-stage writes are done. This adds a helper
folio_end_writeback_no_dropbehind() that allows them to release the
writeback flag without immediately dropping the folio.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

Author: Trond Myklebust

include/linux/pagemap.h

@@ -1221,6 +1221,7 @@ void folio_wait_writeback(struct folio *folio);
 int folio_wait_writeback_killable(struct folio *folio);
 void end_page_writeback(struct page *page);
 void folio_end_writeback(struct folio *folio);
+void folio_end_writeback_no_dropbehind(struct folio *folio);
 void folio_end_dropbehind(struct folio *folio);
 void folio_wait_stable(struct folio *folio);
 void __folio_mark_dirty(struct folio *folio, struct address_space *, int warn);

mm/filemap.c

@@ -1628,14 +1628,15 @@ void folio_end_dropbehind(struct folio *folio)
 EXPORT_SYMBOL_GPL(folio_end_dropbehind);
 
 /**
- * folio_end_writeback - End writeback against a folio.
+ * folio_end_writeback_no_dropbehind - End writeback against a folio.
  * @folio: The folio.
  *
  * The folio must actually be under writeback.
+ * This call is intended for filesystems that need to defer dropbehind.
  *
  * Context: May be called from process or interrupt context.
  */
-void folio_end_writeback(struct folio *folio)
+void folio_end_writeback_no_dropbehind(struct folio *folio)
 {
 	VM_BUG_ON_FOLIO(!folio_test_writeback(folio), folio);
 
@@ -1651,18 +1652,34 @@ void folio_end_writeback(struct folio *folio)
 		folio_rotate_reclaimable(folio);
 	}
 
+	if (__folio_end_writeback(folio))
+		folio_wake_bit(folio, PG_writeback);
+
+	acct_reclaim_writeback(folio);
+}
+EXPORT_SYMBOL_GPL(folio_end_writeback_no_dropbehind);
+
+/**
+ * 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);
+
 	/*
 	 * Writeback does not hold a folio reference of its own, relying
 	 * on truncation to wait for the clearing of PG_writeback.
 	 * But here we must make sure that the folio is not freed and
 	 * reused before the folio_wake_bit().
 	 */
 	folio_get(folio);
-	if (__folio_end_writeback(folio))
-		folio_wake_bit(folio, PG_writeback);
-
+	folio_end_writeback_no_dropbehind(folio);
 	folio_end_dropbehind(folio);
-	acct_reclaim_writeback(folio);
 	folio_put(folio);
 }
 EXPORT_SYMBOL(folio_end_writeback);