btrfs: factor uncompressed async extent submission code into a new helper

Introduce a new helper, submit_uncompressed_range(), for async cow cases
where we fallback to COW.

There are some new updates introduced to the helper:

- Proper locked_page detection
  It's possible that the async_extent range doesn't cover the locked
  page.  In that case we shouldn't unlock the locked page.

  In the new helper, we will ensure that we only unlock the locked page
  when:

  * The locked page covers part of the async_extent range
  * The locked page is not unlocked by cow_file_range() nor
    extent_write_locked_range()

  This also means extra comments are added focusing on the page locking.

- Add extra comment on some rare parameter used.
  We use @unlock_page = 0 for cow_file_range(), where only two call
  sites doing the same thing, including the new helper.

  It's definitely worth some comments.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -839,6 +839,43 @@ static void free_async_extent_pages(struct async_extent *async_extent)
 	async_extent->pages = NULL;
 }
 
+static int submit_uncompressed_range(struct btrfs_inode *inode,
+				     struct async_extent *async_extent,
+				     struct page *locked_page)
+{
+	u64 start = async_extent->start;
+	u64 end = async_extent->start + async_extent->ram_size - 1;
+	unsigned long nr_written = 0;
+	int page_started = 0;
+	int ret;
+
+	/*
+	 * Call cow_file_range() to run the delalloc range directly, since we
+	 * won't go to NOCOW or async path again.
+	 *
+	 * Also we call cow_file_range() with @unlock_page == 0, so that we
+	 * can directly submit them without interruption.
+	 */
+	ret = cow_file_range(inode, locked_page, start, end, &page_started,
+			     &nr_written, 0);
+	/* Inline extent inserted, page gets unlocked and everything is done */
+	if (page_started) {
+		ret = 0;
+		goto out;
+	}
+	if (ret < 0) {
+		if (locked_page)
+			unlock_page(locked_page);
+		goto out;
+	}
+
+	ret = extent_write_locked_range(&inode->vfs_inode, start, end);
+	/* All pages will be unlocked, including @locked_page */
+out:
+	kfree(async_extent);
+	return ret;
+}
+
 static int submit_one_async_extent(struct btrfs_inode *inode,
 				   struct async_chunk *async_chunk,
 				   struct async_extent *async_extent,
@@ -848,37 +885,28 @@ static int submit_one_async_extent(struct btrfs_inode *inode,
 	struct btrfs_root *root = inode->root;
 	struct btrfs_fs_info *fs_info = root->fs_info;
 	struct btrfs_key ins;
+	struct page *locked_page = NULL;
 	struct extent_map *em;
 	int ret = 0;
 	u64 start = async_extent->start;
 	u64 end = async_extent->start + async_extent->ram_size - 1;
 
-	lock_extent(io_tree, start, end);
-
-	/* We have fallback to uncompressed write */
-	if (!async_extent->pages) {
-		int page_started = 0;
-		unsigned long nr_written = 0;
+	/*
+	 * If async_chunk->locked_page is in the async_extent range, we need to
+	 * handle it.
+	 */
+	if (async_chunk->locked_page) {
+		u64 locked_page_start = page_offset(async_chunk->locked_page);
+		u64 locked_page_end = locked_page_start + PAGE_SIZE - 1;
 
-		/*
-		 * Call cow_file_range() to run the delalloc range directly,
-		 * since we won't go to nocow or async path again.
-		 */
-		ret = cow_file_range(inode, async_chunk->locked_page,
-				     start, end, &page_started, &nr_written, 0);
-		/*
-		 * If @page_started, cow_file_range() inserted an inline extent
-		 * and took care of all the unlocking and IO for us.
-		 * Otherwise, we need to submit all those pages down to the
-		 * drive.
-		 */
-		if (!page_started && !ret)
-			extent_write_locked_range(&inode->vfs_inode, start, end);
-		else if (ret && async_chunk->locked_page)
-			unlock_page(async_chunk->locked_page);
-		kfree(async_extent);
-		return ret;
+		if (!(start >= locked_page_end || end <= locked_page_start))
+			locked_page = async_chunk->locked_page;
 	}
+	lock_extent(io_tree, start, end);
+
+	/* We have fall back to uncompressed write */
+	if (!async_extent->pages)
+		return submit_uncompressed_range(inode, async_extent, locked_page);
 
 	ret = btrfs_reserve_extent(root, async_extent->ram_size,
 				   async_extent->compressed_size,