[PATCH v3 01/25] rust: alloc: add `Allocator` trait

Danilo Krummrich
2024-08-01T00:06:57Z

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`.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
rust/kernel/alloc.rs | 73 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 73 insertions(+)

diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index 1966bd407017..b79dd2c49277 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 +87,75 @@ 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.
+///
+/// # Safety
+///
+/// Memory returned from an allocator must point to a valid memory buffer and remain valid until
+/// it is explicitly freed.
+///
+/// Any pointer to a memory buffer which is currently allocated must be valid to be passed to any
+/// other [`Allocator`] function. The same applies for a NULL pointer.
+///
+/// If `realloc` is called with:
+/// - a size of zero, the given memory allocation, if any, must be freed
+/// - a NULL pointer, 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 a NULL pointer.
+ fn alloc(layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {
+ // SAFETY: Passing a NULL pointer 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`.
+ ///
+ /// 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
+ ///
+ /// `ptr` must point to an existing and valid memory allocation created by this allocator
+ /// instance.
+ ///
+ /// Additionally, `ptr` is allowed to be a NULL pointer; in this case a new memory allocation is
+ /// created.
+ 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`
+ /// instance.
+ unsafe fn free(ptr: NonNull<u8>) {
+ // SAFETY: `ptr` is guaranteed to be previously allocated with this `Allocator` or NULL.
+ // Calling `realloc` with a buffer size of zero, frees the buffer `ptr` points to.
+ let _ = unsafe { Self::realloc(Some(ptr), Layout::new::<()>(), Flags(0)) };
+ }
+}
--
2.45.2