+/*
+ * Unlike the work in write_one_eb(), we rely completely on extent locking.
+ * Page locking is only utilized at minimum to keep the VMM code happy.
+ *
+ * Caller should still call write_one_eb() other than this function directly.
+ * As write_one_eb() has extra preparation before submitting the extent buffer.
+ */
+static int write_one_subpage_eb(struct extent_buffer *eb,
+ struct writeback_control *wbc,
+ struct extent_page_data *epd)
+{
+ struct btrfs_fs_info *fs_info = eb->fs_info;
+ struct page *page = eb->pages[0];
+ unsigned int write_flags = wbc_to_write_flags(wbc) | REQ_META;
+ bool no_dirty_ebs = false;
+ int ret;
+
+ /* clear_page_dirty_for_io() in subpage helper needs page locked */
+ lock_page(page);
+ btrfs_subpage_set_writeback(fs_info, page, eb->start, eb->len);
+
+ /* Check if this is the last dirty bit to update nr_written */
+ no_dirty_ebs = btrfs_subpage_clear_and_test_dirty(fs_info, page,
+ eb->start, eb->len);
+ if (no_dirty_ebs)
+ clear_page_dirty_for_io(page);
+
+ ret = submit_extent_page(REQ_OP_WRITE | write_flags, wbc, page,
+ eb->start, eb->len, eb->start - page_offset(page),
+ &epd->bio, end_bio_extent_buffer_writepage, 0, 0, 0,
+ false);
+ if (ret) {
+ btrfs_subpage_clear_writeback(fs_info, page, eb->start, eb->len);
+ set_btree_ioerr(page, eb);
+ unlock_page(page);
+
+ if (atomic_dec_and_test(&eb->io_pages))
+ end_extent_buffer_writeback(eb);
+ return -EIO;
+ }
+ unlock_page(page);
+ /*
+ * Submission finished without problem, if no range of the page is
+ * dirty anymore, we have submitted a page. Update nr_written in wbc.
+ */
+ if (no_dirty_ebs)
+ update_nr_written(wbc, 1);
+ return ret;
+}
+