+	size_t poff = offset_in_folio(folio, *pos);
+	size_t plen = min_t(loff_t, folio_size(folio) - poff, length);

These variables are a little misnamed now.  But I'm not sure what a
better name would be.

+		size_t off = (page - &folio->page) * PAGE_SIZE +
+				bvec->bv_offset;

Another use case for the helper I suggested earlier.

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>