rust: alloc: implement Allocator for Kmalloc

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>
Link: https://lore.kernel.org/r/20240911225449.152928-5-dakr@kernel.org

Author: Danilo Krummrich

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;
 

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;
 
 /// Calls `krealloc` with a proper size to alloc a new object aligned to `new_layout`'s alignment.
 ///
@@ -31,6 +47,77 @@ 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