Re: [PATCH v3 1/3] rust: block: introduce `kernel::block::mq` module

[Benno Lossin]

On 01.06.24 13:11, Andreas Hindborg wrote:
> Benno Lossin <benno.lossin@xxxxxxxxx> writes:
>> On 01.06.24 10:18, Andreas Hindborg wrote:
>>> +    /// This function is called by the C kernel. A pointer to this function is
>>> +    /// installed in the `blk_mq_ops` vtable for the driver.
>>> +    ///
>>> +    /// # Safety
>>> +    ///
>>> +    /// This function may only be called by blk-mq C infrastructure.
>>> +    unsafe extern "C" fn commit_rqs_callback(_hctx: *mut bindings::blk_mq_hw_ctx) {
>>> +        T::commit_rqs()
>>> +    }
>>> +
>>> +    /// This function is called by the C kernel. It is not currently
>>> +    /// implemented, and there is no way to exercise this code path.
>>
>> Is it also possible to completely remove it? ie use `None` in the
>> VTABLE, or will the C side error?
> 
> No, this pointer is not allowed to be null. It must be a callable
> function, hence the stub. It will be populated soon enough when I send
> patches for the remote completion logic.

Makes sense.

[...]

>>> +impl<T: Operations> TagSet<T> {
>>> +    /// Try to create a new tag set
>>> +    pub fn try_new(
>>> +        nr_hw_queues: u32,
>>> +        num_tags: u32,
>>> +        num_maps: u32,
>>> +    ) -> impl PinInit<Self, error::Error> {
>>> +        try_pin_init!( TagSet {
>>> +            // INVARIANT: We initialize `inner` here and it is valid after the
>>> +            // initializer has run.
>>> +            inner <- unsafe {kernel::init::pin_init_from_closure(move |place: *mut Opaque<bindings::blk_mq_tag_set>| -> Result<()> {
>>> +                let place = place.cast::<bindings::blk_mq_tag_set>();
>>> +
>>> +                // SAFETY: pin_init_from_closure promises that `place` is writable, and
>>> +                // zeroes is a valid bit pattern for this structure.
>>> +                core::ptr::write_bytes(place, 0, 1);
>>> +
>>> +                /// For a raw pointer to a struct, write a struct field without
>>> +                /// creating a reference to the field
>>> +                macro_rules! write_ptr_field {
>>> +                    ($target:ident, $field:ident, $value:expr) => {
>>> +                        ::core::ptr::write(::core::ptr::addr_of_mut!((*$target).$field), $value)
>>> +                    };
>>> +                }
>>> +
>>> +                // SAFETY: pin_init_from_closure promises that `place` is writable
>>> +                    write_ptr_field!(place, ops, OperationsVTable::<T>::build());
>>> +                    write_ptr_field!(place, nr_hw_queues , nr_hw_queues);
>>> +                    write_ptr_field!(place, timeout , 0); // 0 means default which is 30 * HZ in C
>>> +                    write_ptr_field!(place, numa_node , bindings::NUMA_NO_NODE);
>>> +                    write_ptr_field!(place, queue_depth , num_tags);
>>> +                    write_ptr_field!(place, cmd_size , core::mem::size_of::<RequestDataWrapper>().try_into()?);
>>> +                    write_ptr_field!(place, flags , bindings::BLK_MQ_F_SHOULD_MERGE);
>>> +                    write_ptr_field!(place, driver_data , core::ptr::null_mut::<core::ffi::c_void>());
>>> +                    write_ptr_field!(place, nr_maps , num_maps);
>>
>> Did something not work with my suggestion?
> 
> I did not want to change it if we are rewriting it to `Opaque::init`
> in a cycle or two, which I think is a better solution.

Ah I was suggesting to do it now, but emulate the `Opaque::init`
function (I should have been clearer about that).
I tried to do exactly that, but failed to easily implement it, since the
fields of `blk_mq_tag_set` include some structs, which of course do not
implement `Zeroable`.

Instead I came up with the following solution, which I find a lot nicer:

    pub fn try_new(
        nr_hw_queues: u32,
        num_tags: u32,
        num_maps: u32,
    ) -> impl PinInit<Self, error::Error> {
        // SAFETY: `blk_mq_tag_set` only contains integers and pointers, which all are allowed to be 0.
        let tag_set: bindings::blk_mq_tag_set = unsafe { core::mem::zeroed() };
        let tag_set = bindings::blk_mq_tag_set {
            ops: OperationsVTable::<T>::build(),
            nr_hw_queues,
            timeout: 0, // 0 means default which is 30Hz in C
            numa_node: bindings::NUMA_NO_NODE,
            queue_depth: num_tags,
            cmd_size: core::mem::size_of::<RequestDataWrapper>().try_into()?,
            flags: bindings::BLK_MQ_F_SHOULD_MERGE,
            driver_data: core::ptr::null_mut::<core::ffi::c_void>(),
            nr_maps: num_maps,
            ..tag_set
        };
        try_pin_init!(TagSet {
            inner <- PinInit::<_, error::Error>::pin_chain(Opaque::new(tag_set), |tag_set| {
                // SAFETY: we do not move out of `tag_set`.
                let tag_set = unsafe { Pin::get_unchecked_mut(tag_set) };
                // SAFETY: `tag_set` is a reference to an initialized `blk_mq_tag_set`.
                error::to_result(unsafe { bindings::blk_mq_alloc_tag_set(tag_set) })
            }),
            _p: PhantomData,
        })
    }

I haven't tested it though, let me know if you have any problems.

---
Cheers,
Benno