Re: [PATCH v2 2/3] rust: block: add rnull, Rust null_blk implementation

[Benno Lossin]

On 29.05.24 15:00, Andreas Hindborg wrote:
> Benno Lossin <benno.lossin@xxxxxxxxx> writes:
> 
>> On 21.05.24 16:03, Andreas Hindborg wrote:
>>> From: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
>>>
>>> This patch adds an initial version of the Rust null block driver.
>>>
>>> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
>>> ---
>>>  drivers/block/Kconfig   |  9 +++++
>>>  drivers/block/Makefile  |  3 ++
>>>  drivers/block/rnull.rs  | 86 +++++++++++++++++++++++++++++++++++++++++
>>>  rust/kernel/block/mq.rs |  4 +-
>>>  4 files changed, 101 insertions(+), 1 deletion(-)
>>>  create mode 100644 drivers/block/rnull.rs
>>>
>>> diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
>>> index 5b9d4aaebb81..ed209f4f2798 100644
>>> --- a/drivers/block/Kconfig
>>> +++ b/drivers/block/Kconfig
>>> @@ -354,6 +354,15 @@ config VIRTIO_BLK
>>>  	  This is the virtual block driver for virtio.  It can be used with
>>>            QEMU based VMMs (like KVM or Xen).  Say Y or M.
>>>
>>> +config BLK_DEV_RUST_NULL
>>> +	tristate "Rust null block driver (Experimental)"
>>> +	depends on RUST
>>> +	help
>>> +	  This is the Rust implementation of the null block driver. For now it
>>> +	  is only a minimal stub.
>>> +
>>> +	  If unsure, say N.
>>> +
>>>  config BLK_DEV_RBD
>>>  	tristate "Rados block device (RBD)"
>>>  	depends on INET && BLOCK
>>> diff --git a/drivers/block/Makefile b/drivers/block/Makefile
>>> index 101612cba303..1105a2d4fdcb 100644
>>> --- a/drivers/block/Makefile
>>> +++ b/drivers/block/Makefile
>>> @@ -9,6 +9,9 @@
>>>  # needed for trace events
>>>  ccflags-y				+= -I$(src)
>>>
>>> +obj-$(CONFIG_BLK_DEV_RUST_NULL) += rnull_mod.o
>>> +rnull_mod-y := rnull.o
>>> +
>>>  obj-$(CONFIG_MAC_FLOPPY)	+= swim3.o
>>>  obj-$(CONFIG_BLK_DEV_SWIM)	+= swim_mod.o
>>>  obj-$(CONFIG_BLK_DEV_FD)	+= floppy.o
>>> diff --git a/drivers/block/rnull.rs b/drivers/block/rnull.rs
>>> new file mode 100644
>>> index 000000000000..1d6ab6f0f26f
>>> --- /dev/null
>>> +++ b/drivers/block/rnull.rs
>>> @@ -0,0 +1,86 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +
>>> +//! This is a Rust implementation of the C null block driver.
>>> +//!
>>> +//! Supported features:
>>> +//!
>>> +//! - blk-mq interface
>>> +//! - direct completion
>>> +//! - block size 4k
>>> +//!
>>> +//! The driver is not configurable.
>>> +
>>> +use kernel::{
>>> +    alloc::flags,
>>> +    block::mq::{
>>> +        self,
>>> +        gen_disk::{self, GenDisk},
>>> +        Operations, TagSet,
>>> +    },
>>> +    error::Result,
>>> +    new_mutex, pr_info,
>>> +    prelude::*,
>>> +    sync::{Arc, Mutex},
>>> +    types::ARef,
>>> +};
>>> +
>>> +module! {
>>> +    type: NullBlkModule,
>>> +    name: "rnull_mod",
>>> +    author: "Andreas Hindborg",
>>> +    license: "GPL v2",
>>> +}
>>> +
>>> +struct NullBlkModule {
>>> +    _disk: Pin<Box<Mutex<GenDisk<NullBlkDevice, gen_disk::Added>>>>,
>>> +}
>>> +
>>> +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 })
        }
    }

>>> +    let block_size: u16 = 4096;
>>> +    if block_size % 512 != 0 || !(512..=4096).contains(&block_size) {
>>> +        return Err(kernel::error::code::EINVAL);
>>> +    }
>>> +
>>> +    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()
>>> +}
>>> +
>>> +impl kernel::Module for NullBlkModule {
>>> +    fn init(_module: &'static ThisModule) -> Result<Self> {
>>> +        pr_info!("Rust null_blk loaded\n");
>>> +        let tagset = Arc::pin_init(TagSet::try_new(1, 256, 1), flags::GFP_KERNEL)?;
>>> +        let disk = Box::pin_init(
>>> +            new_mutex!(add_disk(tagset)?, "nullb:disk"),
>>> +            flags::GFP_KERNEL,
>>> +        )?;
>>> +
>>> +        Ok(Self { _disk: disk })
>>> +    }
>>> +}
>>> +
>>> +struct NullBlkDevice;
>>> +
>>> +#[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.

>>> +
>>> +        Ok(())
>>> +    }
>>> +
>>> +    fn commit_rqs() {}
>>> +
>>> +    fn complete(rq: ARef<mq::Request<Self>>) {
>>
>> Am I correct in thinking that this function is never actually called,
>> since all requests that are queued are immediately ended?
> 
> Yes, re my other email. It is a callback that cannot be triggered for
> now. I will move it to a later patch where it belongs.
> 
>>
>>> +        mq::Request::end_ok(rq)
>>> +            .map_err(|_e| kernel::error::code::EIO)
>>> +            .expect("Failed to complete request")
>>> +    }
>>> +}
>>> diff --git a/rust/kernel/block/mq.rs b/rust/kernel/block/mq.rs
>>> index efbd2588791b..54e032bbdffd 100644
>>> --- a/rust/kernel/block/mq.rs
>>> +++ b/rust/kernel/block/mq.rs
>>> @@ -51,6 +51,7 @@
>>>  //!
>>>  //! ```rust
>>>  //! use kernel::{
>>> +//!     alloc::flags,
>>>  //!     block::mq::*,
>>>  //!     new_mutex,
>>>  //!     prelude::*,
>>> @@ -77,7 +78,8 @@
>>>  //!     }
>>>  //! }
>>>  //!
>>> -//! let tagset: Arc<TagSet<MyBlkDevice>> = Arc::pin_init(TagSet::try_new(1, 256, 1))?;
>>> +//! let tagset: Arc<TagSet<MyBlkDevice>> =
>>> +//!     Arc::pin_init(TagSet::try_new(1, 256, 1), flags::GFP_KERNEL)?;
>>
>> This change should probably be in the patch before (seems like an
>> artifact from rebasing).
> 
> Yes, thank you for spotting that. I thought I checked that this was
> building, so this is strange to me. Maybe I failed to build the
> doctests between the two patches. It is weird that kernel robot did not
> pick this up. Maybe it is not building doctests?

Hmm, that is strange...

---
Cheers,
Benno