Message-ID: <87ikysor3q.fsf@metaspace.dk>
Date: Sat, 01 Jun 2024 13:11:05 +0200
From: Andreas Hindborg <nmi@...aspace.dk>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Jens Axboe <axboe@...nel.dk>,  Christoph Hellwig <hch@....de>,  Keith
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Subject: Re: [PATCH v3 1/3] rust: block: introduce `kernel::block::mq` module

Benno Lossin <benno.lossin@...ton.me> writes:

> On 01.06.24 10:18, Andreas Hindborg wrote:
>> From: Andreas Hindborg <a.hindborg@...sung.com>
>
> [...]
>
>> +impl<T: Operations> GenDisk<T, Initialized> {
>> +    /// Try to create a new `GenDisk`.
>> +    pub fn try_new(tagset: Arc<TagSet<T>>) -> Result<Self> {
>> +        let lock_class_key = crate::sync::LockClassKey::new();
>> +
>> +        // SAFETY: `tagset.raw_tag_set()` points to a valid and initialized tag set
>> +        let gendisk = from_err_ptr(unsafe {
>> +            bindings::__blk_mq_alloc_disk(
>> +                tagset.raw_tag_set(),
>> +                core::ptr::null_mut(), // TODO: We can pass queue limits right here
>> +                core::ptr::null_mut(),
>> +                lock_class_key.as_ptr(),
>> +            )
>> +        })?;
>> +
>> +        const TABLE: bindings::block_device_operations = bindings::block_device_operations {
>> +            submit_bio: None,
>> +            open: None,
>> +            release: None,
>> +            ioctl: None,
>> +            compat_ioctl: None,
>> +            check_events: None,
>> +            unlock_native_capacity: None,
>> +            getgeo: None,
>> +            set_read_only: None,
>> +            swap_slot_free_notify: None,
>> +            report_zones: None,
>> +            devnode: None,
>> +            alternative_gpt_sector: None,
>> +            get_unique_id: None,
>> +            // TODO: Set to THIS_MODULE. Waiting for const_refs_to_static feature to
>> +            // be merged (unstable in rustc 1.78 which is staged for linux 6.10)
>> +            // https://github.com/rust-lang/rust/issues/119618
>
> AFAIK the 1.78 upgrade already is in rust-next (and also should appear
> in v6.10-rc2, right?) do you have this on your radar?

I am tracking this and I plan to add support in a later patch.

>
>> +            owner: core::ptr::null_mut(),
>> +            pr_ops: core::ptr::null_mut(),
>> +            free_disk: None,
>> +            poll_bio: None,
>> +        };
>> +
>> +        // SAFETY: gendisk is a valid pointer as we initialized it above
>> +        unsafe { (*gendisk).fops = &TABLE };
>> +
>> +        // INVARIANT: `gendisk` was initialized above.
>> +        // INVARIANT: `gendisk.queue.queue_data` is set to `data` in the call to
>> +        // `__blk_mq_alloc_disk` above.
>> +        Ok(GenDisk {
>> +            tagset,
>> +            gendisk,
>> +            _phantom: PhantomData,
>> +        })
>> +    }
>
> [...]
>
>> +    /// 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.

>
>> +    ///
>> +    /// # Safety
>> +    ///
>> +    /// This function may only be called by blk-mq C infrastructure.
>> +    unsafe extern "C" fn complete_callback(_rq: *mut bindings::request) {}
>
> [...]
>
>> +impl<'a> RawWriter<'a> {
>> +    /// Create a new `RawWriter` instance.
>> +    fn new(buffer: &'a mut [u8]) -> Result<RawWriter<'a>> {
>> +        *(buffer.last_mut().ok_or(EINVAL)?) = 0;
>> +
>> +        // INVARIANT: We null terminated the buffer above.
>> +        Ok(Self { buffer, pos: 0 })
>> +    }
>> +
>> +    pub(crate) fn from_array<const N: usize>(
>> +        a: &'a mut [core::ffi::c_char; N],
>> +    ) -> Result<RawWriter<'a>> {
>
> You could change the return type to be `RawWriter<'a>` and check using
> `build_assert!` that `N > 0`. Then you can also call `unwrap_unchecked`
> on the result that you get below.
>
> I don't know if we want that, but it is a possibility.

I guess we could potentially make the type generic over a const buffer
size. But let's put a pin in that for now. I'll look into that down the road.

>
>> +        Self::new(
>> +            // SAFETY: the buffer of `a` is valid for read and write as `u8` for
>> +            // at least `N` bytes.
>> +            unsafe { core::slice::from_raw_parts_mut(a.as_mut_ptr().cast::<u8>(), N) },
>> +        )
>> +    }
>> +}
>
> [...]
>
>> +/// Store the result of `op(target.load())` in target, returning new value of
>> +/// taret.
>> +fn atomic_relaxed_op_return(target: &AtomicU64, op: impl Fn(u64) -> u64) -> u64 {
>> +    let old = target.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |x| Some(op(x)));
>> +
>> +    // SAFETY: Because the operation passed to `fetch_update` above always
>> +    // return `Some`, `old` will always be `Ok`.
>> +    let old = unsafe { old.unwrap_unchecked() };
>> +
>> +    op(old)
>> +}
>> +
>> +/// Store the result of `op(target.load)` in `target` if `target.load() !=
>> +/// pred`, returning previous value of target
>
> The function returns a bool, not a u64 (value). From the body I read
> that you return whether the value was updated.

Thanks 👍

>
>> +fn atomic_relaxed_op_unless(target: &AtomicU64, op: impl Fn(u64) -> u64, pred: u64) -> bool {
>> +    let x = target.load(Ordering::Relaxed);
>> +    loop {
>> +        if x == pred {
>> +            break;
>> +        }
>> +        if target
>> +            .compare_exchange_weak(x, op(x), Ordering::Relaxed, Ordering::Relaxed)
>> +            .is_ok()
>> +        {
>> +            break;
>> +        }
>
> If this fails, you are not re-reading the value of `target`, so if
> someone else just set it to `pred`, you will still continue to try to
> set it from `x` to `op(x)`, but it might never have the value `x` again.
> This would lead to a potentially infinite loop, right?

Yea, that is not good. I should have moved the assignment of `x` into the loop

>
>> +    }
>
> Do you think you can also implement this using `fetch_update`? I guess
> this would do what you want, right?:
>
>     target.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |x| {
>         if x == pred {
>             None
>         } else {
>             Some(op(x))
>         }
>     }).is_ok()

Makes sense, I will steal that.

>
>> +
>> +    x == pred
>> +}
>
> [...]
>
>> +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.


Best regards,
Andreas

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