On Mon, Feb 15, 2021 at 03:45:01PM +0000, David Howells wrote:

Export rw_verify_area() for so that cachefiles can use it before issuing
call_read_iter() and call_write_iter() to effect async DIO operations
against the cache.  This is analogous to aio_read() and aio_write().

I don't think this is the right thing to do.  Instead of calling
into ->read_iter / ->write_iter directly this should be using helpers.

What prevents you from using vfs_iocb_iter_read and
vfs_iocb_iter_write which seem the right level of abstraction for this?