linux-kernel - Re: [PATCH v2 2/3] rust: block: add rnull, Rust null

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Message-ID: <87r0dhnl9p.fsf@metaspace.dk>
Date: Sat, 01 Jun 2024 10:02:26 +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 v2 2/3] rust: block: add rnull, Rust null_blk
 implementation

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

[...]

>>>> +
>>>> +fn add_disk(tagset: Arc<TagSet<NullBlkDevice>>) -> Result<GenDisk<NullBlkDevice, gen_disk::Added>> {
>>>
>>> Any reason that this is its own function and not in the
>>> `NullBlkModule::init` function?
>> 
>> Would you embed it inside the `init` function? To what end? I don't
>> think that would read well.
>
> I just found it strange that you have this extracted into its own
> function, since I just expected it to be present in the init function.
> Does this look really that bad?:
>
>     impl kernel::Module for NullBlkModule {
>         fn init(_module: &'static ThisModule) -> Result<Self> {
>             pr_info!("Rust null_blk loaded\n");
>             let block_size: u16 = 4096;
>             if block_size % 512 != 0 ||
> !(512..=4096).contains(&block_size) {
>                 return Err(kernel::error::code::EINVAL);
>             }
>
>             let disk = {
>                 let tagset = Arc::pin_init(TagSet::try_new(1, 256, 1),
> flags::GFP_KERNEL)?;
>                 let mut disk = gen_disk::try_new(tagset)?;
>                 disk.set_name(format_args!("rnullb{}", 0))?;
>                 disk.set_capacity_sectors(4096 << 11);
>                 disk.set_queue_logical_block_size(block_size.into());
>                 disk.set_queue_physical_block_size(block_size.into());
>                 disk.set_rotational(false);
>                 disk.add()
>             };
>             let disk = Box::pin_init(
>                 new_mutex!(disk, "nullb:disk"),
>                 flags::GFP_KERNEL,
>             )?;
>
>             Ok(Self { _disk: disk })
>         }
>     }

I don't mind either way. I guess we could combine it.

[...]

>>>> +#[vtable]
>>>> +impl Operations for NullBlkDevice {
>>>> +    #[inline(always)]
>>>> +    fn queue_rq(rq: ARef<mq::Request<Self>>, _is_last: bool) -> Result {
>>>> +        mq::Request::end_ok(rq)
>>>> +            .map_err(|_e| kernel::error::code::EIO)
>>>> +            .expect("Failed to complete request");
>>>
>>> This error would only happen if `rq` is not the only ARef to that
>>> request, right?
>> 
>> Yes, it should never happen. If it happens, something is seriously
>> broken and panic is adequate.
>> 
>> Other drivers might want to retry later or something, but in this case
>> it should just work.
>
> In that case, I think the error message should reflect that and not just
> be a generic "failed to complete request" error.

Right, that is a good point.


Best regards,
Andreas