writeback: Move I_DIRTY_PAGES handling

Instead of clearing I_DIRTY_PAGES and resetting it when we didn't succeed in
writing them all, just clear the bit only when we succeeded writing all the
pages. We also move the clearing of the bit close to other i_state handling to
separate it from writeback list handling. This is desirable because list
handling will differ for flusher thread and other writeback_single_inode()
callers in future. No filesystem plays any tricks with I_DIRTY_PAGES (like
checking it in ->writepages or ->write_inode implementation) so this movement
is safe.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>

fs/fs-writeback.c

@@ -385,7 +385,6 @@ writeback_single_inode(struct inode *inode, struct bdi_writeback *wb,
 
 	/* Set I_SYNC, reset I_DIRTY_PAGES */
 	inode->i_state |= I_SYNC;
-	inode->i_state &= ~I_DIRTY_PAGES;
 	spin_unlock(&inode->i_lock);
 	spin_unlock(&wb->list_lock);
 
@@ -408,6 +407,9 @@ writeback_single_inode(struct inode *inode, struct bdi_writeback *wb,
 	 * write_inode()
 	 */
 	spin_lock(&inode->i_lock);
+	/* Clear I_DIRTY_PAGES if we've written out all dirty pages */
+	if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
+		inode->i_state &= ~I_DIRTY_PAGES;
 	dirty = inode->i_state & I_DIRTY;
 	inode->i_state &= ~(I_DIRTY_SYNC | I_DIRTY_DATASYNC);
 	spin_unlock(&inode->i_lock);
@@ -435,7 +437,6 @@ writeback_single_inode(struct inode *inode, struct bdi_writeback *wb,
 			 * We didn't write back all the pages.  nfs_writepages()
 			 * sometimes bales out without doing anything.
 			 */
-			inode->i_state |= I_DIRTY_PAGES;
 			if (wbc->nr_to_write <= 0) {
 				/*
 				 * slice used up: queue for next turn