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@xxxxxxxxxx>
Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
---
rust/kernel/alloc.rs | 112 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 112 insertions(+)
diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index 1966bd407017..6c21bd2edad9 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 +87,114 @@ 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 buffers 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
+///
+/// - A memory allocation returned from an allocator must remain valid until it is explicitly freed.
+///
+/// - Any pointer to a valid memory allocation must be valid to be passed to any other [`Allocator`]
+/// function of the same type.
+///
+/// - Implementers must ensure that all trait functions abide by the guarantees documented in the
+/// `# Guarantees` sections.
+//
+// Note that `Allocator::{realloc,free}` don't have an `old_layout` argument (like stdlib's
+// corresponding `Allocator` trait functions have), since the implemented (kernel) allocators
+// neither need nor honor such an argument. Thus, it would be misleading to make this API require it
+// anyways.
+//
+// More generally, this trait isn't intended for implementers to encode a lot of semantics, but
+// rather provide a thin generalization layer for the kernel's allocators.
+//
+// Depending on future requirements, the requirements for this trait may change as well and
+// implementing allocators that need to encode more semantics may become desirable.
+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`.
+ ///
+ /// # Guarantees
+ ///
+ /// When the return value is `Ok(ptr)`, then `ptr` is
+ /// - valid for reads and writes for `layout.size()` bytes, until it is passed to
+ /// [`Allocator::free`] or [`Allocator::realloc`],
+ /// - aligned to `layout.align()`,
+ ///
+ /// Additionally, `Flags` are honored as documented in
+ /// <https://docs.kernel.org/core-api/mm-api.html#mm-api-gfp-flags>.
+ 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`.
+ ///
+ /// 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
+ /// 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.
+ ///
+ /// # Guarantees
+ ///
+ /// This function has the same guarantees as [`Allocator::alloc`]. When `ptr == Some(p)`, then
+ /// it additionally guarantees that:
+ /// - the contents of the memory pointed to by `p` are preserved up to the lesser of the new
+ /// and old size,
+ /// and old size, i.e.
+ /// `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.
+ 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)) };
+ }
+}
--
2.46.0
