Re: [RFC PATCH 03/11] rust: block: introduce `kernel::block::mq` module

[Benno Lossin]

On 23.01.24 19:39, Andreas Hindborg (Samsung) wrote:
>>>>> +/// A generic block device
>>>>> +///
>>>>> +/// # Invariants
>>>>> +///
>>>>> +///  - `gendisk` must always point to an initialized and valid `struct gendisk`.
>>>>> +pub struct GenDisk<T: Operations> {
>>>>> +    _tagset: Arc<TagSet<T>>,
>>>>> +    gendisk: *mut bindings::gendisk,
>>>>
>>>> Why are these two fields not embedded? Shouldn't the user decide where
>>>> to allocate?
>>>
>>> The `TagSet` can be shared between multiple `GenDisk`. Using an `Arc`
>>> seems resonable?
>>>
>>> For the `gendisk` field, the allocation is done by C and the address
>>> must be stable. We are owning the pointee and must drop it when it goes out
>>> of scope. I could do this:
>>>
>>> #[repr(transparent)]
>>> struct GenDisk(Opaque<bindings::gendisk>);
>>>
>>> struct UniqueGenDiskRef {
>>>       _tagset: Arc<TagSet<T>>,
>>>       gendisk: Pin<&'static mut GenDisk>,
>>>
>>> }
>>>
>>> but it seems pointless. `struct GenDisk` would not be pub in that case. What do you think?
>>
>> Hmm, I am a bit confused as to how you usually use a `struct gendisk`.
>> You said that a `TagSet` might be shared between multiple `GenDisk`s,
>> but that is not facilitated by the C side?
>>
>> Is it the case that on the C side you create a struct containing a
>> tagset and a gendisk for every block device you want to represent?
> 
> Yes, but the `struct tag_set` can be shared between multiple `struct
> gendisk`.
> 
> Let me try to elaborate:
> 
> In C you would first allocate a `struct tag_set` and partially
> initialize it. The allocation can be dynamic, static or part of existing
> allocation. You would then partially initialize the structure and finish
> the initialization by calling `blk_mq_alloc_tag_set()`. This populates
> the rest of the structure which includes more dynamic allocations.
> 
> You then allocate a `struct gendisk` by calling `blk_mq_alloc_disk()`,
> passing in a pointer to the `struct tag_set` you just created. This
> function will return a pointer to a `struct gendisk` on success.
> 
> In the Rust abstractions, we allocate the `TagSet`:
> 
> #[pin_data(PinnedDrop)]
> #[repr(transparent)]
> pub struct TagSet<T: Operations> {
>      #[pin]
>      inner: Opaque<bindings::blk_mq_tag_set>,
>      _p: PhantomData<T>,
> }
> 
> with `PinInit` [^1]. The initializer will partially initialize the struct and
> finish the initialization like C does by calling
> `blk_mq_alloc_tag_set()`. We now need a place to point the initializer.
> `Arc::pin_init()` is that place for now. It allows us to pass the
> `TagSet` reference to multiple `GenDisk` if required. Maybe we could be
> generic over `Deref<TagSet>` in the future. Bottom line is that we need
> to hold on to that `TagSet` reference until the `GenDisk` is dropped.

I see, thanks for the elaborate explanation! I now think that using `Arc`
makes sense.

> `struct tag_set` is not reference counted on the C side. C
> implementations just take care to keep it alive, for instance by storing
> it next to a pointer to `struct gendisk` that it is servicing.

This is interesting, is this also done in the case where it is shared
among multiple `struct gendisk`s?
Does this have some deeper reason? Or am I right to assume that creating
`Gendisk`/`TagSet` is done rarely (i.e. only at initialization of the
driver)?

>> And you decided for the Rust abstractions that you want to have only a
>> single generic struct for any block device, distinguished by the generic
>> parameter?
> 
> Yes, we have a single generic struct (`GenDisk`) representing the C
> `struct gendisk`, and a single generic struct (`TagSet`) representing
> the C `struct tag_set`. These are both generic over `T: Operations`.
> `Operations` represent a C vtable (`struct blk_mq_ops`) attached to the
> `struct tag_set`. This vtable is provided by the driver and holds
> function pointers that allow the kernel to perform actions such as queue
> IO requests with the driver. A C driver can instantiate multiple `struct
> gendisk` and service them with the same `struct tag_set` and thereby the
> same vtable. Or it can use separate tag sets and the same vtable. Or a
> separate tag_set and vtable for each gendisk.
> 
>> I think these kinds of details would be nice to know. Not only for
>> reviewers, but also for veterans of the C APIs.
> 
> I should write some module level documentation clarifying the use of
> these types. The null block driver is a simple example, but it is just
> code. I will include more docs in the next version.

Thanks a lot for explaining!

-- 
Cheers,
Benno