[PATCH v3 04/25] rust: alloc: implement `Allocator` for `Kmalloc`

Danilo Krummrich
2024-08-01T00:07:14Z

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 | 66 ++++++++++++++++++++++++++++++++--
2 files changed, 65 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index b79dd2c49277..8cabc888393b 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 10774c51ae26..397ae5bcc043 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -5,9 +5,18 @@
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.
+///
+/// The contiguous kernel allocator only ever allocates physically contiguous memory through
+/// `bindings::krealloc`.
+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 {
// Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
let layout = new_layout.pad_to_align();
@@ -18,7 +27,7 @@ fn aligned_size(new_layout: Layout) -> usize {
layout.size()
}

-/// Calls `krealloc` with a proper size to alloc a new object aligned to `new_layout`'s alignment.
+/// Calls `krealloc` with a proper size to alloc a new object.
///
/// # Safety
///
@@ -39,6 +48,59 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
}
}

+struct ReallocFunc(
+ // INVARIANT: One of the following `krealloc`, `vrealloc`, `kvrealloc`.
+ unsafe extern "C" fn(*const core::ffi::c_void, usize, u32) -> *mut core::ffi::c_void,
+);
+
+impl ReallocFunc {
+ fn krealloc() -> Self {
+ Self(bindings::krealloc)
+ }
+
+ // SAFETY: `call` has the exact same safety requirements as `Allocator::realloc`.
+ 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: `ptr` is valid by the safety requirements 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))
+ }
+}
+
+unsafe impl Allocator for Kmalloc {
+ unsafe fn realloc(
+ ptr: Option<NonNull<u8>>,
+ layout: Layout,
+ flags: Flags,
+ ) -> Result<NonNull<[u8]>, AllocError> {
+ let realloc = ReallocFunc::krealloc();
+
+ // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
+ // allocated with this `Allocator`.
+ unsafe { realloc.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
--
2.45.2