[PATCH v7 04/26] rust: alloc: implement `Allocator` for `Kmalloc`

Danilo Krummrich
2024-09-11T22:55:22Z

Implement `Allocator` for `Kmalloc`, the kernel's default allocator,
typically used for objects smaller than page size.

All memory allocations made with `Kmalloc` end up in `krealloc()`.

It serves as allocator for the subsequently introduced types `KBox` and
`KVec`.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
rust/kernel/alloc.rs | 2 +-
rust/kernel/alloc/allocator.rs | 89 +++++++++++++++++++++++++++++++++-
2 files changed, 89 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index 6c21bd2edad9..535ed1527034 100644
--- a/rust/kernel/alloc.rs
+++ b/rust/kernel/alloc.rs
@@ -4,7 +4,7 @@

#[cfg(not(test))]
#[cfg(not(testlib))]
-mod allocator;
+pub mod allocator;
pub mod box_ext;
pub mod vec_ext;

diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index e32182f91167..7a93442187a1 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -1,12 +1,28 @@
// SPDX-License-Identifier: GPL-2.0

//! Allocator support.
+//!
+//! Documentation for the kernel's memory allocators can found in the "Memory Allocation Guide"
+//! linked below. For instance, this includes the concept of "get free page" (GFP) flags and the
+//! typical application of the different kernel allocators.
+//!
+//! Reference: <https://docs.kernel.org/core-api/memory-allocation.html>

use super::{flags::*, Flags};
use core::alloc::{GlobalAlloc, Layout};
use core::ptr;
+use core::ptr::NonNull;

-struct Kmalloc;
+use crate::alloc::{AllocError, Allocator};
+use crate::bindings;
+
+/// The contiguous kernel allocator.
+///
+/// `Kmalloc` is typically used for physically contiguous allocations up to page size, but also
+/// supports larger allocations up to `bindings::KMALLOC_MAX_SIZE`, which is hardware specific.
+///
+/// For more details see [self].
+pub struct Kmalloc;

/// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
fn aligned_size(new_layout: Layout) -> usize {
@@ -36,6 +52,77 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags.0) as *mut u8 }
}

+/// # Invariants
+///
+/// One of the following `krealloc`, `vrealloc`, `kvrealloc`.
+struct ReallocFunc(
+ unsafe extern "C" fn(*const core::ffi::c_void, usize, u32) -> *mut core::ffi::c_void,
+);
+
+impl ReallocFunc {
+ // INVARIANT: `krealloc` satisfies the type invariants.
+ const KREALLOC: Self = Self(bindings::krealloc);
+
+ /// # Safety
+ ///
+ /// This method has the same safety requirements as [`Allocator::realloc`].
+ ///
+ /// # Guarantees
+ ///
+ /// This method has the same guarantees as `Allocator::realloc`. Additionally
+ /// - it accepts any pointer to a valid memory allocation allocated by this function.
+ /// - memory allocated by this function remains valid until it is passed to this function.
+ unsafe fn call(
+ &self,
+ ptr: Option<NonNull<u8>>,
+ layout: Layout,
+ flags: Flags,
+ ) -> Result<NonNull<[u8]>, AllocError> {
+ let size = aligned_size(layout);
+ let ptr = match ptr {
+ Some(ptr) => ptr.as_ptr(),
+ None => ptr::null(),
+ };
+
+ // SAFETY:
+ // - `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc` and thus only requires that
+ // `ptr` is NULL or valid.
+ // - `ptr` is either NULL or valid by the safety requirements of this function.
+ //
+ // GUARANTEE:
+ // - `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc`.
+ // - Those functions provide the guarantees of this function.
+ let raw_ptr = unsafe {
+ // If `size == 0` and `ptr != NULL` the memory behind the pointer is freed.
+ self.0(ptr.cast(), size, flags.0).cast()
+ };
+
+ let ptr = if size == 0 {
+ NonNull::dangling()
+ } else {
+ NonNull::new(raw_ptr).ok_or(AllocError)?
+ };
+
+ Ok(NonNull::slice_from_raw_parts(ptr, size))
+ }
+}
+
+// SAFETY: `realloc` delegates to `ReallocFunc::call`, which guarantees that
+// - memory remains valid until it is explicitly freed,
+// - passing a pointer to a valid memory allocation is OK,
+// - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
+unsafe impl Allocator for Kmalloc {
+ #[inline]
+ unsafe fn realloc(
+ ptr: Option<NonNull<u8>>,
+ layout: Layout,
+ flags: Flags,
+ ) -> Result<NonNull<[u8]>, AllocError> {
+ // SAFETY: `ReallocFunc::call` has the same safety requirements as `Allocator::realloc`.
+ unsafe { ReallocFunc::KREALLOC.call(ptr, layout, flags) }
+ }
+}
+
unsafe impl GlobalAlloc for Kmalloc {
unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
// SAFETY: `ptr::null_mut()` is null and `layout` has a non-zero size by the function safety
--
2.46.0