Re: [RFC][PATCH] iov_iter: Add extraction functions

[David Howells <dhowells@redhat.com>]

Al Viro <viro@zeniv.linux.org.uk> wrote:

> IDGI.  Essentially, you are passing a callback disguised as enum, only to
> lose any type safety.

Granted, but this is how struct iov_iter works.  I'm keeping the enum inside
the internal functions where it can't be got at by external code.

Now, I could just copy the code three times, once for iter=>bvec, once for
iter=>sg and once for iter=>rdma.  I can also grant that iter=>bvec is really
only needed for UBUF/IOVEC-class iterators; I think I can make the assumption
that kernel-supplied buffers are good for the lifetime of the operation.

> How is it better than "iov_iter_get_pages2() into a
> fixed-sized array and handle the result" done in a loop?

Note that iov_iter_get_pages2() doesn't handle KVEC-class iterators, which
this code does - for kmalloc'd, vmalloc'd and vmap'd memory and for global and
stack variables.  What I've written gets the physical addresses but
doesn't/can't pin it (which probably means I can't just move my code into
iov_iter_get_pages2()).

Further, my code also treats multipage folios as single units which
iov_iter_get_pages2() also doesn't - at least from XARRAY-class iterators.

The UBUF-/IOVEC-extraction code doesn't handle multipage folios because
get_user_pages_fast() doesn't - though perhaps it will need to at some point.

David