Re: [PATCH v2] rust: add a sample alloc usage

["Danilo Krummrich" <dakr@kernel.org>]

(Cc: Lorenzo, Vlastimil, Liam, Uladzislau)

On Thu Jul 17, 2025 at 11:50 AM CEST, Hui Zhu wrote:
> diff --git a/samples/rust/Makefile b/samples/rust/Makefile
> index bd2faad63b4f..7c3e68d9ada5 100644
> --- a/samples/rust/Makefile
> +++ b/samples/rust/Makefile
> @@ -10,6 +10,7 @@ obj-$(CONFIG_SAMPLE_RUST_DRIVER_PLATFORM)	+= rust_driver_platform.o
>  obj-$(CONFIG_SAMPLE_RUST_DRIVER_FAUX)		+= rust_driver_faux.o
>  obj-$(CONFIG_SAMPLE_RUST_DRIVER_AUXILIARY)	+= rust_driver_auxiliary.o
>  obj-$(CONFIG_SAMPLE_RUST_CONFIGFS)		+= rust_configfs.o
> +obj-$(CONFIG_SAMPLE_RUST_ALLOC)		+= rust_alloc.o

I think adding an example for large alignment is fine, but let's do this in a
doc-test on VBox in rust/kernel/alloc/kbox.rs. I think adding a separate module
for this is overkill.

Note that doc-tests are executed on boot if CONFIG_RUST_KERNEL_DOCTESTS=y.

> diff --git a/samples/rust/rust_alloc.rs b/samples/rust/rust_alloc.rs
> new file mode 100644
> index 000000000000..61efde37b5f2
> --- /dev/null
> +++ b/samples/rust/rust_alloc.rs
> @@ -0,0 +1,88 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (c) 2025, Kylin Software
> +
> +//! Rust alloc sample.
> +
> +use kernel::bindings;
> +use kernel::prelude::*;
> +
> +module! {
> +    type: RustAlloc,
> +    name: "rust_alloc",
> +    authors: ["Rust for Linux Contributors"],
> +    description: "Rust alloc sample",
> +    license: "GPL",
> +}
> +
> +const VBOX_SIZE: usize = 1024;
> +const VBOX_LARGE_ALIGN: usize = bindings::PAGE_SIZE * 4;

Please use kernel::page::PAGE_SIZE instead.

> +const KVEC_VAL: [usize; 3] = [10, 20, 30];
> +
> +#[repr(align(128))]
> +struct VboxBlob([u8; VBOX_SIZE]);
> +
> +// This structure is used to test the allocation of alignments larger
> +// than PAGE_SIZE.
> +// Since this is not yet supported, avoid accessing the contents of
> +// the structure for now.
> +#[allow(dead_code)]
> +#[repr(align(8192))]
> +struct VboxLargeAlignBlob([u8; VBOX_LARGE_ALIGN]);
> +
> +struct RustAlloc {
> +    vbox_blob: VBox<VboxBlob>,
> +    kvec_blob: KVec<usize>,
> +}
> +
> +fn check_align(addr: usize, align: usize) -> bool {
> +    debug_assert!(align.is_power_of_two());
> +    if addr & (align - 1) != 0 {
> +        pr_err!("Address {:#x} is not aligned with {:#x}.\n", addr, align);
> +        false
> +    } else {
> +        true
> +    }
> +}

I think check_align() is unnecessary -- we don't need to test this in this
context.

Instead, I suggest to add some doc-tests to the `Allocator` impls for `Kmalloc`,
`Vmalloc` and `KVmalloc` in rust/kernel/alloc/allocator.rs testing that they
return the expected alignment. This would be a valuable test case for those. If
you're interested in adding them, please feel free to do so. :)

> +impl kernel::Module for RustAlloc {
> +    fn init(_module: &'static ThisModule) -> Result<Self> {
> +        pr_info!("Rust allocator sample (init)\n");
> +
> +        let vbox_blob = VBox::<VboxBlob>::new_uninit(GFP_KERNEL)?;
> +        if !check_align(vbox_blob.as_ptr() as usize, 128) {
> +            return Err(EINVAL);
> +        }
> +        let vbox_blob = vbox_blob.write(VboxBlob([0xfeu8; VBOX_SIZE]));

Given that you initialize it with a value anyways, you can just do

	let blob = VBox::new(Blob([0xfeu8; SIZE]), GFP_KERNEL)?;

instead.

> +        if let Ok(_) = VBox::<VboxLargeAlignBlob>::new_uninit(GFP_KERNEL) {
> +            pr_err!("Allocations for VBox with alignments larger than PAGE_SIZE should fail, but here it succeeded.\n");
> +            return Err(EINVAL);
> +        }

This can just be

	assert!(VBox::<VboxLargeAlignBlob>::new_uninit(GFP_KERNEL).is_err());

within a doc-test.

> +        let mut kvec_blob = KVec::new();
> +        kvec_blob.extend_from_slice(&KVEC_VAL, GFP_KERNEL)?;

We already have doc-tests for KVec::new() and KVec::extend_from_slice() in
rust/kernel/alloc/kvec.rs.

> +        Ok(Self {
> +            vbox_blob,
> +            kvec_blob,
> +        })
> +    }
> +}
> +
> +impl Drop for RustAlloc {
> +    fn drop(&mut self) {
> +        pr_info!("Rust allocator sample (exit)\n");
> +
> +        // check the values
> +        for b in self.vbox_blob.0.as_slice().iter() {
> +            if *b != 0xfeu8 {
> +                pr_err!("vbox_blob contains wrong values\n");
> +            }
> +        }
> +
> +        if self.kvec_blob.as_slice() != KVEC_VAL {
> +            pr_err!("kvec_blob contains wrong values\n");
> +        }

Here you're basically testing Vec::as_slice(). We don't have doc-tests for
as_slice() and as_slice_mut() in rust/kernel/alloc/kvec.rs. Please feel free to
add them. :)

> +    }
> +}