Re: [PATCH v3 2/3] rust: block: add rnull, Rust null_blk implementation

[Benno Lossin]

On 01.06.24 10:18, 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 | 78 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 90 insertions(+)
>  create mode 100644 drivers/block/rnull.rs

I left two comments below, but it already seems fine to me:

Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>

> 
> 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..c69ea7e7436e
> --- /dev/null
> +++ b/drivers/block/rnull.rs
> @@ -0,0 +1,78 @@
> +// 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>>>>,
> +}
> +
> +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 = {
> +            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::GenDisk::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()
> +        }?;

Personally, I would prefer to put the `?` into the line above.

> +
> +        let disk = Box::pin_init(new_mutex!(disk, "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("Fatal error - expected to be able to end request");

I expected something more along the lines of: "expected to be able to
end request, since `NullBlkDevice` never takes refcounts on Requests and
as such the ARef must be unique, but `end_ok` only fails if that is not
the case". But maybe that would fit better in a comment, what do you
think?

> +
> +        Ok(())
> +    }
> +
> +    fn commit_rqs() {}
> +}
> --
> 2.45.1
>