Re: [PATCH v7 13/26] rust: alloc: implement kernel `Vec` type

Benno Lossin
2024-09-26T13:47:17Z

On 12.09.24 00:52, Danilo Krummrich wrote:
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> new file mode 100644
> index 000000000000..631a44e19f35
> --- /dev/null
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -0,0 +1,638 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Implementation of [`Vec`].
> +
> +use super::{
> + allocator::{KVmalloc, Kmalloc, Vmalloc},
> + AllocError, Allocator, Box, Flags,
> +};
> +use core::{
> + fmt,
> + marker::PhantomData,
> + mem::{ManuallyDrop, MaybeUninit},
> + ops::Deref,
> + ops::DerefMut,
> + ops::Index,
> + ops::IndexMut,
> + ptr::NonNull,
> + slice,
> + slice::SliceIndex,
> +};
> +
> +/// Create a [`Vec`] containing the arguments.

I would change this to [`KVec`].

> +///
> +/// # Examples
> +///
> +/// ```
> +/// let mut v = kernel::kvec![];
> +/// v.push(1, GFP_KERNEL)?;
> +/// assert_eq!(v, [1]);
> +///
> +/// let mut v = kernel::kvec![1; 3]?;
> +/// v.push(4, GFP_KERNEL)?;
> +/// assert_eq!(v, [1, 1, 1, 4]);
> +///
> +/// let mut v = kernel::kvec![1, 2, 3]?;
> +/// v.push(4, GFP_KERNEL)?;
> +/// assert_eq!(v, [1, 2, 3, 4]);
> +///
> +/// # Ok::<(), Error>(())
> +/// ```
> +#[macro_export]
> +macro_rules! kvec {
> + () => (
> + $crate::alloc::KVec::new()
> + );
> + ($elem:expr; $n:expr) => (
> + $crate::alloc::KVec::from_elem($elem, $n, GFP_KERNEL)
> + );
> + ($($x:expr),+ $(,)?) => (
> + match $crate::alloc::KBox::new_uninit(GFP_KERNEL) {
> + Ok(b) => Ok($crate::alloc::KVec::from($crate::alloc::KBox::write(b, [$($x),+]))),
> + Err(e) => Err(e),
> + }
> + );
> +}
> +
> +/// The kernel's [`Vec`] type.
> +///
> +/// A contiguous growable array type with contents allocated with the kernel's allocators (e.g.
> +/// `Kmalloc`, `Vmalloc` or `KVmalloc`), written `Vec<T, A>`.

Can you turn these into links?

> +///
> +/// For non-zero-sized values, a [`Vec`] will use the given allocator `A` for its allocation. For
> +/// the most common allocators the type aliases `KVec`, `VVec` and `KVVec` exist.

Ditto.

> +///
> +/// For zero-sized types the [`Vec`]'s pointer must be `dangling_mut::<T>`; no memory is allocated.
> +///
> +/// Generally, [`Vec`] consists of a pointer that represents the vector's backing buffer, the
> +/// capacity of the vector (the number of elements that currently fit into the vector), it's length
> +/// (the number of elements that are currently stored in the vector) and the `Allocator` type used
> +/// to allocate (and free) the backing buffer.
> +///
> +/// A [`Vec`] can be deconstructed into and (re-)constructed from it's previously named raw parts
> +/// and manually modified.
> +///
> +/// [`Vec`]'s backing buffer gets, if required, automatically increased (re-allocated) when elements
> +/// are added to the vector.
> +///
> +/// # Invariants
> +///
> +/// - `self.ptr` is always properly aligned and either points to memory allocated with `A` or, for
> +/// zero-sized types, is a dangling, well aligned pointer.
> +///
> +/// - `self.len` always represents the exact number of elements stored in the vector.
> +///
> +/// - `self.cap` represents the absolute number of elements that can be stored within the vector
> +/// without re-allocation. However, it is legal for the backing buffer to be larger than
> +/// `size_of<T>` times the capacity.
> +///
> +/// - The `Allocator` type `A` of the vector is the exact same `Allocator` type the backing buffer
> +/// was allocated with (and must be freed with).
> +pub struct Vec<T, A: Allocator> {
> + ptr: NonNull<T>,
> + /// Represents the actual buffer size as `cap` times `size_of::<T>` bytes.
> + ///
> + /// Note: This isn't quite the same as `Self::capacity`, which in contrast returns the number of
> + /// elements we can still store without reallocating.
> + ///
> + /// # Invariants
> + ///
> + /// `cap` must be in the `0..=isize::MAX` range.
> + cap: usize,
> + len: usize,
> + _p: PhantomData,
> +}

[...]

> + /// Appends an element to the back of the [`Vec`] instance.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// let mut v = KVec::new();
> + /// v.push(1, GFP_KERNEL)?;
> + /// assert_eq!(&v, &[1]);
> + ///
> + /// v.push(2, GFP_KERNEL)?;
> + /// assert_eq!(&v, &[1, 2]);
> + /// # Ok::<(), Error>(())
> + /// ```
> + pub fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
> + Vec::reserve(self, 1, flags)?;
> +
> + // SAFETY:
> + // - `self.len` is smaller than `self.capacity` and hence, the resulting pointer is
> + // guaranteed to be part of the same allocated object.
> + // - `self.len` can not overflow `isize`.
> + let ptr = unsafe { self.as_mut_ptr().add(self.len) };
> +
> + // SAFETY:
> + // - `ptr` is properly aligned and valid for writes.
> + unsafe { core::ptr::write(ptr, v) };

Why not use `self.spare_capacity_mut()[0].write(v);`?

If you want to avoid the bounds check, you can do

let first = self.spare_capacity_mut().first();
// SAFETY: the call to `Vec::reserve` above ensures that `spare_capacity_mut()` is non-empty.
unsafe { first.unwrap_unchecked() }.write(v);

> +
> + // SAFETY: We just initialised the first spare entry, so it is safe to increase the length
> + // by 1. We also know that the new length is <= capacity because of the previous call to
> + // `reserve` above.
> + unsafe { self.set_len(self.len() + 1) };
> + Ok(())
> + }
> +
> + /// Creates a new [`Vec`] instance with at least the given capacity.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// let v = KVec::<u32>::with_capacity(20, GFP_KERNEL)?;
> + ///
> + /// assert!(v.capacity() >= 20);
> + /// # Ok::<(), Error>(())
> + /// ```
> + pub fn with_capacity(capacity: usize, flags: Flags) -> Result<Self, AllocError> {
> + let mut v = Vec::new();
> +
> + Self::reserve(&mut v, capacity, flags)?;
> +
> + Ok(v)
> + }
> +
> + /// Pushes clones of the elements of slice into the [`Vec`] instance.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// let mut v = KVec::new();
> + /// v.push(1, GFP_KERNEL)?;
> + ///
> + /// v.extend_from_slice(&[20, 30, 40], GFP_KERNEL)?;
> + /// assert_eq!(&v, &[1, 20, 30, 40]);
> + ///
> + /// v.extend_from_slice(&[50, 60], GFP_KERNEL)?;
> + /// assert_eq!(&v, &[1, 20, 30, 40, 50, 60]);
> + /// # Ok::<(), Error>(())
> + /// ```
> + pub fn extend_from_slice(&mut self, other: &[T], flags: Flags) -> Result<(), AllocError>
> + where
> + T: Clone,

This method can be moved into the other impl block below, it already has
the `T: Clone` bound.

> + {
> + self.reserve(other.len(), flags)?;
> + for (slot, item) in core::iter::zip(self.spare_capacity_mut(), other) {
> + slot.write(item.clone());
> + }
> +
> + // SAFETY:
> + // - `other.len()` spare entries have just been initialized, so it is safe to increase
> + // the length by the same number.
> + // - `self.len() + other.len() <= self.capacity()` is guaranteed by the preceding `reserve`
> + // call.
> + unsafe { self.set_len(self.len() + other.len()) };
> + Ok(())
> + }
> +
> + /// Creates a Vec<T, A> from a pointer, a length and a capacity using the allocator `A`.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// let mut v = kernel::kvec![1, 2, 3]?;
> + /// v.reserve(1, GFP_KERNEL)?;
> + ///
> + /// let (mut ptr, mut len, cap) = v.into_raw_parts();
> + ///
> + /// // SAFETY: We've just reserved memory for another element.
> + /// unsafe { ptr.add(len).write(4) };
> + /// len += 1;
> + ///
> + /// // SAFETY: We only wrote an additional element at the end of the `KVec`'s buffer and
> + /// // correspondingly increased the length of the `KVec` by one. Otherwise, we construct it
> + /// // from the exact same raw parts.
> + /// let v = unsafe { KVec::from_raw_parts(ptr, len, cap) };
> + ///
> + /// assert_eq!(v, [1, 2, 3, 4]);
> + ///
> + /// # Ok::<(), Error>(())
> + /// ```
> + ///
> + /// # Safety
> + ///
> + /// If `T` is a ZST:
> + ///
> + /// - `ptr` must be a dangling, well aligned pointer.
> + ///
> + /// Otherwise:
> + ///
> + /// - `ptr` must have been allocated with the allocator `A`.
> + /// - `ptr` must satisfy or exceed the alignment requirements of `T`.
> + /// - `ptr` must point to memory with a size of at least `size_of::<T>() * capacity`.
> + /// bytes.
> + /// - The allocated size in bytes must not be larger than `isize::MAX`.
> + /// - `length` must be less than or equal to `capacity`.
> + /// - The first `length` elements must be initialized values of type `T`.
> + ///
> + /// It is also valid to create an empty `Vec` passing a dangling pointer for `ptr` and zero for
> + /// `cap` and `len`.
> + pub unsafe fn from_raw_parts(ptr: *mut T, length: usize, capacity: usize) -> Self {
> + let cap = if Self::is_zst() { 0 } else { capacity };
> +
> + Self {
> + // SAFETY: By the safety requirements, `ptr` is either dangling or pointing to a valid
> + // memory allocation, allocated with `A`.
> + ptr: unsafe { NonNull::new_unchecked(ptr) },
> + cap,
> + len: length,
> + _p: PhantomData::,
> + }

Would be nice to have `debug_assert!(length <= capacity)` here. But feel
free to make that a good-first-issue instead of including it in the next
version. (there are probably more asserts elsewhere)

> + }
> +
> + /// Consumes the `Vec<T, A>` and returns its raw components `pointer`, `length` and `capacity`.
> + ///
> + /// This will not run the destructor of the contained elements and for non-ZSTs the allocation
> + /// will stay alive indefinitely. Use [`Vec::from_raw_parts`] to recover the [`Vec`], drop the
> + /// elements and free the allocation, if any.
> + pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
> + let mut me = ManuallyDrop::new(self);
> + let len = me.len();
> + let capacity = me.capacity();
> + let ptr = me.as_mut_ptr();
> + (ptr, len, capacity)
> + }

[...]

> +macro_rules! impl_slice_eq {
> + ([$($vars:tt)*] $lhs:ty, $rhs:ty) => {

You could wrap the entire pattern in "$()*", same for the entire body
and then...

> + impl<T, U, $($vars)*> PartialEq<$rhs> for $lhs
> + where
> + T: PartialEq,
> + {
> + #[inline]
> + fn eq(&self, other: &$rhs) -> bool { self[..] == other[..] }
> + }
> + }
> +}
> +
> +impl_slice_eq! { [A1: Allocator, A2: Allocator] Vec<T, A1>, Vec<U, A2> }
> +impl_slice_eq! { [A: Allocator] Vec<T, A>, &[U] }
> +impl_slice_eq! { [A: Allocator] Vec<T, A>, &mut [U] }
> +impl_slice_eq! { [A: Allocator] &[T], Vec<U, A> }
> +impl_slice_eq! { [A: Allocator] &mut [T], Vec<U, A> }
> +impl_slice_eq! { [A: Allocator] Vec<T, A>, [U] }
> +impl_slice_eq! { [A: Allocator] [T], Vec<U, A> }
> +impl_slice_eq! { [A: Allocator, const N: usize] Vec<T, A>, [U; N] }
> +impl_slice_eq! { [A: Allocator, const N: usize] Vec<T, A>, &[U; N] }

...we could have a single `impl_slice_eq` invocation here:

impl_slice_eq! {
[A1: Allocator, A2: Allocator] Vec<T, A1>, Vec<U, A2>
[A: Allocator] Vec<T, A>, &[U]
[A: Allocator] Vec<T, A>, &mut [U]
[A: Allocator] &[T], Vec<U, A>
[A: Allocator] &mut [T], Vec<U, A>
[A: Allocator] Vec<T, A>, [U]
[A: Allocator] [T], Vec<U, A>
[A: Allocator, const N: usize] Vec<T, A>, [U; N]
[A: Allocator, const N: usize] Vec<T, A>, &[U; N]
}

Not a huge improvement, but I think it makes it a bit nicer to read.

---
Cheers,
Benno

> diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
> index d5f2fe42d093..80223cdaa485 100644
> --- a/rust/kernel/prelude.rs
> +++ b/rust/kernel/prelude.rs
> @@ -14,7 +14,7 @@
> #[doc(no_inline)]
> pub use core::pin::Pin;
>
> -pub use crate::alloc::{flags::*, vec_ext::VecExt, Box, KBox, KVBox, VBox};
> +pub use crate::alloc::{flags::*, vec_ext::VecExt, Box, KBox, KVBox, KVVec, KVec, VBox, VVec};
>
> #[doc(no_inline)]
> pub use alloc::vec::Vec;
> --
> 2.46.0
>