Re: [RFC PATCH v2 2/4] rust: io_uring: introduce rust abstraction for io-uring cmd

[Caleb Sander Mateos <csander@purestorage.com>]

On Fri, Aug 8, 2025 at 2:56 AM Sidong Yang <sidong.yang@furiosa.ai> wrote:
>
> On Wed, Aug 06, 2025 at 03:38:24PM +0200, Benno Lossin wrote:
> > On Wed Aug 6, 2025 at 2:38 PM CEST, Daniel Almeida wrote:
> > > Hi Benno,
> > >
> > >> On 2 Aug 2025, at 07:52, Benno Lossin <lossin@kernel.org> wrote:
> > >>
> > >> On Fri Aug 1, 2025 at 3:48 PM CEST, Daniel Almeida wrote:
> > >>>> On 27 Jul 2025, at 12:03, Sidong Yang <sidong.yang@furiosa.ai> wrote:
> > >>>> +    #[inline]
> > >>>> +    pub fn pdu(&mut self) -> &mut MaybeUninit<[u8; 32]> {
> > >>>
> > >>> Why MaybeUninit? Also, this is a question for others, but I don´t think
> > >>> that `u8`s can ever be uninitialized as all byte values are valid for `u8`.
> > >>
> > >> `u8` can be uninitialized. Uninitialized doesn't just mean "can take any
> > >> bit pattern", but also "is known to the compiler as being
> > >> uninitialized". The docs of `MaybeUninit` explain it like this:
> > >>
> > >>    Moreover, uninitialized memory is special in that it does not have a
> > >>    fixed value ("fixed" meaning "it won´t change without being written
> > >>    to"). Reading the same uninitialized byte multiple times can give
> > >>    different results.
> > >>
> > >> But the return type probably should be `&mut [MaybeUninit<u8>; 32]`
> > >> instead.
> > >
> > >
> > > Right, but I guess the question then is why would we ever need to use
> > > MaybeUninit here anyways.
> > >
> > > It's a reference to a C array. Just treat that as initialized.
> >
> > AFAIK C uninitialized memory also is considered uninitialized in Rust.
> > So if this array is not properly initialized on the C side, this would
> > be the correct type. If it is initialized, then just use `&mut [u8; 32]`.
>
> pdu field is memory chunk for driver can use it freely. The driver usually
> saves a private data and read or modify it on the other context. using
> just `&mut [u8;32]` would be simple and easy to use.

MaybeUninit is correct. The io_uring/uring_cmd layer doesn't
initialize the pdu field. struct io_uring_cmd is overlaid with struct
io_kiocb's cmd field. struct io_kiocb's are allocated using
kmem_cache_alloc(_bulk)() in __io_alloc_req_refill(). io_preinit_req()
is called to initialize each struct io_kiocb but doesn't initialize
the cmd field. As Sidong said, it's uninitialized memory for the
->uring_cmd() implementation to use however it wants for the duration
of the command.

Best,
Caleb