Message-ID: <cfcdaa42-6a62-4a17-a8dc-2382e88774dc@proton.me>
Date: Wed, 14 Aug 2024 16:13:06 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>, ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...sung.com, aliceryhl@...gle.com, akpm@...ux-foundation.org
Cc: daniel.almeida@...labora.com, faith.ekstrand@...labora.com, boris.brezillon@...labora.com, lina@...hilina.net, mcanal@...lia.com, zhiw@...dia.com, cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com, ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 01/26] rust: alloc: add `Allocator` trait

On 12.08.24 20:22, Danilo Krummrich wrote:
> Add a kernel specific `Allocator` trait, that in contrast to the one in
> Rust's core library doesn't require unstable features and supports GFP
> flags.
> 
> Subsequent patches add the following trait implementors: `Kmalloc`,
> `Vmalloc` and `KVmalloc`.
> 
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
> ---
>  rust/kernel/alloc.rs | 81 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 81 insertions(+)
> 
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index 1966bd407017..194745498a75 100644
> --- a/rust/kernel/alloc.rs
> +++ b/rust/kernel/alloc.rs
> @@ -11,6 +11,7 @@
>  /// Indicates an allocation error.
>  #[derive(Copy, Clone, PartialEq, Eq, Debug)]
>  pub struct AllocError;
> +use core::{alloc::Layout, ptr::NonNull};
> 
>  /// Flags to be used when allocating memory.
>  ///
> @@ -86,3 +87,83 @@ pub mod flags {
>      /// small allocations.
>      pub const GFP_NOWAIT: Flags = Flags(bindings::GFP_NOWAIT);
>  }
> +
> +/// The kernel's [`Allocator`] trait.
> +///
> +/// An implementation of [`Allocator`] can allocate, re-allocate and free memory buffer described
> +/// via [`Layout`].
> +///
> +/// [`Allocator`] is designed to be implemented as a ZST; [`Allocator`] functions do not operate on
> +/// an object instance.
> +///
> +/// In order to be able to support `#[derive(SmartPointer)]` later on, we need to avoid a design
> +/// that requires an `Allocator` to be instantiated, hence its functions must not contain any kind
> +/// of `self` parameter.
> +///
> +/// # Safety
> +///
> +/// Memory returned from an allocator must point to a valid memory buffer and remain valid until
> +/// it is explicitly freed.

I wouldn't say that an allocator "returns memory", and in general I
don't think the structure of the safety comment here is nice, how about
the following: we put "Implementers must ensure that all trait functions
abide by the guarantees documented in the `# Guarantees` sections."...

> +///
> +/// Any pointer to a memory buffer which is currently allocated must be valid to be passed to any
> +/// other [`Allocator`] function of the same type.
> +///
> +/// If `realloc` is called with:
> +///   - a size of zero, the given memory allocation, if any, must be freed
> +///   - `None`, a new memory allocation must be created
> +pub unsafe trait Allocator {
> +    /// Allocate memory based on `layout` and `flags`.
> +    ///
> +    /// On success, returns a buffer represented as `NonNull<[u8]>` that satisfies the layout
> +    /// constraints (i.e. minimum size and alignment as specified by `layout`).
> +    ///
> +    /// This function is equivalent to `realloc` when called with `None`.

... Then we can add this here:

    /// # Guarantees
    ///
    /// When the return value is `Ok(ptr)`, then `ptr` is
    /// - valid for writes (and reads after the memory has been initialized) for `layout.size()` bytes,
    ///   until it is passed to [`Allocator::free`] or [`Allocator::realloc`],
    /// - aligned to `layout.align()`,
    /// - is valid for reads, if `flags.contains(flags::__GFP_ZERO)`,

Do we need to handle other flags?
Also IIRC the memory given to us by C is considered initialized by Rust
(though it has a non-deterministic value), so we might have an
unconditional "valid for reads". Am I correct?


> +    fn alloc(layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {
> +        // SAFETY: Passing `None` to `realloc` is valid by it's safety requirements and asks for a
> +        // new memory allocation.
> +        unsafe { Self::realloc(None, layout, flags) }
> +    }
> +
> +    /// Re-allocate an existing memory allocation to satisfy the requested `layout`. If the
> +    /// requested size is zero, `realloc` behaves equivalent to `free`.

I don't think we want to include the second sentence in the short
description of this function, please add an empty line in between.

> +    ///
> +    /// If the requested size is larger than the size of the existing allocation, a successful call
> +    /// to `realloc` guarantees that the new or grown buffer has at least `Layout::size` bytes, but
> +    /// may also be larger.
> +    ///
> +    /// If the requested size is smaller than the size of the existing allocation, `realloc` may or
> +    /// may not shrink the buffer; this is implementation specific to the allocator.
> +    ///
> +    /// On allocation failure, the existing buffer, if any, remains valid.
> +    ///
> +    /// The buffer is represented as `NonNull<[u8]>`.
> +    ///
> +    /// # Safety
> +    ///
> +    /// If `ptr = Some(p)`, then `p` must point to an existing and valid memory allocation created

I don't like the single `=` (I might have written it in haste myself),
how about `==` or we use if-let syntax?

> +    /// by this allocator. The alignment encoded in `layout` must be smaller than or equal to the
> +    /// alignment requested in the previous `alloc` or `realloc` call of the same allocation.
> +    ///
> +    /// Additionally, `ptr` is allowed to be `None`; in this case a new memory allocation is
> +    /// created.

This Safety section does not talk about the case `layout.size() == 0`,
but it should have the same requirement as `free()`.

Also add a `# Guarantees` section here:

    /// # Guarantees
    ///
    /// This function has the same guarantees as [`Allocator::alloc`]. When `ptr == Some(p)`, then it
    /// additionally has the following:
    /// - when `Ok(ret_ptr)` is the return value, then
    ///   `ret_ptr[0..min(layout.size(), old_size)] == p[0..min(layout.size(), old_size)]`, where
    ///   `old_size` is the size of the allocation that `p` points at.
    /// - when the return value is `Err(AllocError)`, then `p` is still valid.

---
Cheers,
Benno

> +    unsafe fn realloc(
> +        ptr: Option<NonNull<u8>>,
> +        layout: Layout,
> +        flags: Flags,
> +    ) -> Result<NonNull<[u8]>, AllocError>;

> +
> +    /// Free an existing memory allocation.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must point to an existing and valid memory allocation created by this `Allocator` and
> +    /// must not be a dangling pointer.
> +    ///
> +    /// The memory allocation at `ptr` must never again be read from or written to.
> +    unsafe fn free(ptr: NonNull<u8>) {
> +        // SAFETY: The caller guarantees that `ptr` points at a valid allocation created by this
> +        // allocator. We are passing a `Layout` with the smallest possible alignment, so it is
> +        // smaller than or equal to the alignment previously used with this allocation.
> +        let _ = unsafe { Self::realloc(Some(ptr), Layout::new::<()>(), Flags(0)) };
> +    }
> +}
> --
> 2.45.2
> 

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