[API questions for Zhang and -fsdevel/ -api below)

+	unsigned int		blksize = i_blocksize(inode);
+	loff_t			offset_aligned = round_down(offset, blksize);

I think this actually needs to found up instead of rounding down.

+	/*
+	 * Zero the tail of the old EOF block and any space up to the new
+	 * offset.
+	 * In the usual truncate path, xfs_falloc_setsize takes care of
+	 * zeroing those blocks.
+	 */
+	if (offset_aligned > old_size) {
+		trace_xfs_zero_eof(ip, old_size, offset_aligned - old_size);
+		error = xfs_zero_range(ip, old_size, offset_aligned - old_size,
+				NULL, &did_zero);
+		if (error)
+			return error;
+	}

... then this will properly zero from the old i_size to the first block
boundary after the old size.

+	error = xfs_alloc_file_space(ip, offset, len,
+			XFS_ALLOC_FILE_SPACE_WRITE_ZEROES);

... and here we need to pass offset_aligned instead of offset and
a new calculated len based on the last block boundary, and then
zero again after that.  That is assuming FALLOC_FL_WRITE_ZEROES
allows unaligned ranges for file systems.  The block code doesn't,
but I can't quite follow the ext4 code if it does or not, and there
is no mention of FALLOC_FL_WRITE_ZEROES even in the latest man-pages
tree.

Maybe we also want xfstests that try unaligned FALLOC_FL_WRITE_ZEROES
and make sure no existing data before the range is lost and the
entire range is zeroed?

+	if (error)
+		return error;
+
+	/*
+	 * xfs_falloc_setsize() would re-zero the written extents via
+	 * iomap_zero_range(). Use xfs_setfilesize() instead.
+	 * Update in-core i_size first as xfs_setfilesize() clamps the on-disk
+	 * size to it.
+	 */
+	if (new_size > i_size_read(inode))
+		i_size_write(inode, new_size);

I think Sashiko is right that we need a pagecache_isize_extended and
filemap_write_and_wait_range calls here.