btrfs: remove the nr_ret parameter from __extent_writepage_io()

The parameter @nr_ret is used to tell the caller how many sectors have
been submitted for IO.

Then callers check @nr_ret value to determine if we need to manually
clear the PAGECACHE_TAG_DIRTY, as if we submitted no sector (e.g. all
sectors are beyond i_size) there is no folio_start_writeback() called thus
PAGECACHE_TAG_DIRTY tag will not be cleared.

Remove this parameter by:

- Moving the btrfs_folio_clear_writeback() call into
__extent_writepage_io()
So that if we didn't submit any IO, then manually call
btrfs_folio_set_writeback() to clear PAGECACHE_TAG_DIRTY when
the page is no longer dirty.

- Use a bool to record if we have submitted any sector
Instead of an int.

- Use subpage compatible helpers to end folio writeback.
This brings no change to the behavior, just for the sake of consistency.

As for the call site inside __extent_writepage(), we're always called
for the whole page, so the existing full page helper
folio_(start|end)_writeback() is totally fine.

For the call site inside extent_write_locked_range(), although we can
have subpage range, folio_start_writeback() will only clear
PAGECACHE_TAG_DIRTY if the page is no longer dirty, and the full folio
will still be dirty if there is any subpage dirty range.
Only when the last dirty subpage sector is cleared, the
folio_start_writeback() will clear PAGECACHE_TAG_DIRTY.

So no matter if we call the full page or subpage helper, the result
is still the same, then just use the subpage helpers for consistency.

Signed-off-by: Qu Wenruo <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/extent_io.c

···
 						    struct folio *folio,
 						    u64 start, u32 len,
 						    struct btrfs_bio_ctrl *bio_ctrl,
-						    loff_t i_size, int *nr_ret)
+						    loff_t i_size)
 {
 	struct btrfs_fs_info *fs_info = inode->root->fs_info;
 	unsigned long range_bitmap = 0;
···
 	 */
 	unsigned long dirty_bitmap = 1;
 	unsigned int bitmap_size = 1;
+	bool submitted_io = false;
 	const u64 folio_start = folio_pos(folio);
 	u64 cur;
 	int bit;
 	int ret = 0;
-	int nr = 0;
 
 	ASSERT(start >= folio_start &&
 	       start + len <= folio_start + folio_size(folio));
···
 		}
 		ret = submit_one_sector(inode, folio, cur, bio_ctrl, i_size);
 		if (ret < 0)
-			goto out_error;
-		nr++;
+			goto out;
+		submitted_io = true;
 	}
 
 	btrfs_folio_assert_not_dirty(fs_info, folio, start, len);
-	*nr_ret = nr;
-	return 0;
-
-out_error:
+out:
 	/*
-	 * If we finish without problem, we should not only clear folio dirty,
-	 * but also empty subpage dirty bits
+	 * If we didn't submitted any sector (>= i_size), folio dirty get
+	 * cleared but PAGECACHE_TAG_DIRTY is not cleared (only cleared
+	 * by folio_start_writeback() if the folio is not dirty).
+	 *
+	 * Here we set writeback and clear for the range. If the full folio
+	 * is no longer dirty then we clear the PAGECACHE_TAG_DIRTY tag.
 	 */
-	*nr_ret = nr;
+	if (!submitted_io) {
+		btrfs_folio_set_writeback(fs_info, folio, start, len);
+		btrfs_folio_clear_writeback(fs_info, folio, start, len);
+	}
 	return ret;
 }
 
···
 	struct inode *inode = folio->mapping->host;
 	const u64 page_start = folio_pos(folio);
 	int ret;
-	int nr = 0;
 	size_t pg_offset;
 	loff_t i_size = i_size_read(inode);
 	unsigned long end_index = i_size >> PAGE_SHIFT;
···
 		goto done;
 
 	ret = __extent_writepage_io(BTRFS_I(inode), folio, folio_pos(folio),
-				    PAGE_SIZE, bio_ctrl, i_size, &nr);
+				    PAGE_SIZE, bio_ctrl, i_size);
 	if (ret == 1)
 		return 0;
 
 	bio_ctrl->wbc->nr_to_write--;
 
 done:
-	if (nr == 0) {
-		/* make sure the mapping tag for page dirty gets cleared */
-		folio_start_writeback(folio);
-		folio_end_writeback(folio);
-	}
 	if (ret) {
 		btrfs_mark_ordered_io_finished(BTRFS_I(inode), folio,
 					       page_start, PAGE_SIZE, !ret);
···
 		u64 cur_end = min(round_down(cur, PAGE_SIZE) + PAGE_SIZE - 1, end);
 		u32 cur_len = cur_end + 1 - cur;
 		struct folio *folio;
-		int nr = 0;
 
 		folio = __filemap_get_folio(mapping, cur >> PAGE_SHIFT, 0, 0);
 
···
 			ASSERT(folio_test_dirty(folio));
 
 		ret = __extent_writepage_io(BTRFS_I(inode), folio, cur, cur_len,
-					    &bio_ctrl, i_size, &nr);
+					    &bio_ctrl, i_size);
 		if (ret == 1)
 			goto next_page;
 
-		/* Make sure the mapping tag for page dirty gets cleared. */
-		if (nr == 0) {
-			btrfs_folio_set_writeback(fs_info, folio, cur, cur_len);
-			btrfs_folio_clear_writeback(fs_info, folio, cur, cur_len);
-		}
 		if (ret) {
 			btrfs_mark_ordered_io_finished(BTRFS_I(inode), folio,
 						       cur, cur_len, !ret);