ext4: unlock unused_pages timely when doing writeback

In mpage_add_bh_to_extent(), when accumulated extents length is greater
than MAX_WRITEPAGES_EXTENT_LEN or buffer head's b_stat is not equal, we
will not continue to search unmapped area for this page, but note this
page is locked, and will only be unlocked in mpage_release_unused_pages()
after ext4_io_submit, if io also is throttled by blk-throttle or similar
io qos, we will hold this page locked for a while, it's unnecessary.

I think the best fix is to refactor mpage_add_bh_to_extent() to let it
return some hints whether to unlock this page, but given that we will
improve dioread_nolock later, we can let it done later, so currently
the simple fix would just call mpage_release_unused_pages() before
ext4_io_submit().
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -2829,12 +2829,12 @@ static int ext4_writepages(struct address_space *mapping,
 		goto unplug;
 	}
 	ret = mpage_prepare_extent_to_map(&mpd);
+	/* Unlock pages we didn't use */
+	mpage_release_unused_pages(&mpd, false);
 	/* Submit prepared bio */
 	ext4_io_submit(&mpd.io_submit);
 	ext4_put_io_end_defer(mpd.io_submit.io_end);
 	mpd.io_submit.io_end = NULL;
-	/* Unlock pages we didn't use */
-	mpage_release_unused_pages(&mpd, false);
 	if (ret < 0)
 		goto unplug;
 
@@ -2902,10 +2902,11 @@ static int ext4_writepages(struct address_space *mapping,
 			handle = NULL;
 			mpd.do_map = 0;
 		}
-		/* Submit prepared bio */
-		ext4_io_submit(&mpd.io_submit);
 		/* Unlock pages we didn't use */
 		mpage_release_unused_pages(&mpd, give_up_on_write);
+		/* Submit prepared bio */
+		ext4_io_submit(&mpd.io_submit);
+
 		/*
 		 * Drop our io_end reference we got from init. We have
 		 * to be careful and use deferred io_end finishing if