[PATCH] ext4: unlock unused_pages timely when doing writeback

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

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>
---
 fs/ext4/inode.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 34d7e0703cc6..83a0d7c76736 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2836,12 +2836,12 @@ 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;
 
@@ -2909,10 +2909,11 @@ 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
-- 
2.17.2