Re: [PATCH v4 06/28] rust: alloc: implement `Vmalloc` allocator

[Benno Lossin]

On 05.08.24 17:19, Danilo Krummrich wrote:
> Implement `Allocator` for `Vmalloc`, the kernel's virtually contiguous
> allocator, typically used for larger objects, (much) larger than page
> size.
> 
> All memory allocations made with `Vmalloc` end up in `vrealloc()`.
> 
> Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
> ---
>  rust/helpers.c                      |  7 +++++++
>  rust/kernel/alloc/allocator.rs      | 32 +++++++++++++++++++++++++++++
>  rust/kernel/alloc/allocator_test.rs |  1 +
>  3 files changed, 40 insertions(+)
> 
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 9f7275493365..7406943f887d 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -33,6 +33,7 @@
>  #include <linux/sched/signal.h>
>  #include <linux/slab.h>
>  #include <linux/spinlock.h>
> +#include <linux/vmalloc.h>
>  #include <linux/wait.h>
>  #include <linux/workqueue.h>
> 
> @@ -199,6 +200,12 @@ void *rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_krealloc);
> 
> +void *rust_helper_vrealloc(const void *p, size_t size, gfp_t flags)
> +{
> +	return vrealloc(p, size, flags);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_vrealloc);
> +
>  /*
>   * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
>   * use it in contexts where Rust expects a `usize` like slice (array) indices.
> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> index c6ad1dd59dd0..bb55895cbd03 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -9,6 +9,7 @@
> 
>  use crate::alloc::{AllocError, Allocator};
>  use crate::bindings;
> +use crate::pr_warn;
> 
>  /// The contiguous kernel allocator.
>  ///
> @@ -16,6 +17,12 @@
>  /// `bindings::krealloc`.
>  pub struct Kmalloc;
> 
> +/// The virtually contiguous kernel allocator.
> +///
> +/// The vmalloc allocator allocates pages from the page level allocator and maps them into the
> +/// contiguous kernel virtual space.
> +pub struct Vmalloc;

One thing that I should also have mentioned for `Kmalloc`, do we want
these types to also have values? I don't think that we need them to be,
so we could declare them as `pub enum Vmalloc {}`.

> +
>  /// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
>  fn aligned_size(new_layout: Layout) -> usize {
>      // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
> @@ -61,6 +68,11 @@ fn krealloc() -> Self {
>          Self(bindings::krealloc)
>      }
> 
> +    // INVARIANT: `vrealloc` satisfies the type invariants.

This should be moved down a line.

---
Cheers,
Benno

> +    fn vrealloc() -> Self {
> +        Self(bindings::vrealloc)
> +    }
> +
>      /// # Safety
>      ///
>      /// This method has the exact same safety requirements as `Allocator::realloc`.
> @@ -141,6 +153,26 @@ unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
>      }
>  }
> 
> +unsafe impl Allocator for Vmalloc {
> +    unsafe fn realloc(
> +        ptr: Option<NonNull<u8>>,
> +        layout: Layout,
> +        flags: Flags,
> +    ) -> Result<NonNull<[u8]>, AllocError> {
> +        let realloc = ReallocFunc::vrealloc();
> +
> +        // TODO: Support alignments larger than PAGE_SIZE.
> +        if layout.align() > bindings::PAGE_SIZE {
> +            pr_warn!("Vmalloc does not support alignments larger than PAGE_SIZE yet.\n");
> +            return Err(AllocError);
> +        }
> +
> +        // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
> +        // allocated with this `Allocator`.
> +        unsafe { realloc.call(ptr, layout, flags) }

I am a bit confused, for `Kmalloc`, you manually returned
`NonNull::dangling` when allocating a zero-sized allocation, but here
you don't?

---
Cheers,
Benno

> +    }
> +}
> +
>  #[global_allocator]
>  static ALLOCATOR: Kmalloc = Kmalloc;
> 
> diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> index 4785efc474a7..e7bf2982f68f 100644
> --- a/rust/kernel/alloc/allocator_test.rs
> +++ b/rust/kernel/alloc/allocator_test.rs
> @@ -7,6 +7,7 @@
>  use core::ptr::NonNull;
> 
>  pub struct Kmalloc;
> +pub type Vmalloc = Kmalloc;
> 
>  unsafe impl Allocator for Kmalloc {
>      unsafe fn realloc(
> --
> 2.45.2
>