blkdev_get_block
err = block_write_full_folio(folio, wbc, blkdev_get_block);
return block_read_full_folio(folio, blkdev_get_block);
mpage_readahead(rac, blkdev_get_block);
return block_write_begin(mapping, pos, len, foliop, blkdev_get_block);