[PATCH v7 06/26] rust: alloc: implement `Vmalloc` allocator

Danilo Krummrich
2024-09-11T22:55:33Z

Implement `Allocator` for `Vmalloc`, the kernel's virtually contiguous
allocator, typically used for larger objects, (much) larger than page
size.

All memory allocations made with `Vmalloc` end up in `vrealloc()`.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
rust/helpers/helpers.c | 1 +
rust/helpers/vmalloc.c | 9 ++++++++
rust/kernel/alloc/allocator.rs | 36 +++++++++++++++++++++++++++++
rust/kernel/alloc/allocator_test.rs | 1 +
4 files changed, 47 insertions(+)
create mode 100644 rust/helpers/vmalloc.c

diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 30f40149f3a9..20a0c69d5cc7 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -22,5 +22,6 @@
#include "spinlock.c"
#include "task.c"
#include "uaccess.c"
+#include "vmalloc.c"
#include "wait.c"
#include "workqueue.c"
diff --git a/rust/helpers/vmalloc.c b/rust/helpers/vmalloc.c
new file mode 100644
index 000000000000..80d34501bbc0
--- /dev/null
+++ b/rust/helpers/vmalloc.c
@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/vmalloc.h>
+
+void * __must_check __realloc_size(2)
+rust_helper_vrealloc(const void *p, size_t size, gfp_t flags)
+{
+ return vrealloc(p, size, flags);
+}
diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index 7a93442187a1..0f2bc702e8e4 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -15,6 +15,7 @@

use crate::alloc::{AllocError, Allocator};
use crate::bindings;
+use crate::pr_warn;

/// The contiguous kernel allocator.
///
@@ -24,6 +25,15 @@
/// For more details see [self].
pub struct Kmalloc;

+/// The virtually contiguous kernel allocator.
+///
+/// `Vmalloc` allocates pages from the page level allocator and maps them into the contiguous kernel
+/// virtual space. It is typically used for large allocations. The memory allocated with this
+/// allocator is not physically contiguous.
+///
+/// For more details see [self].
+pub struct Vmalloc;
+
/// 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.
@@ -63,6 +73,9 @@ impl ReallocFunc {
// INVARIANT: `krealloc` satisfies the type invariants.
const KREALLOC: Self = Self(bindings::krealloc);

+ // INVARIANT: `vrealloc` satisfies the type invariants.
+ const VREALLOC: Self = Self(bindings::vrealloc);
+
/// # Safety
///
/// This method has the same safety requirements as [`Allocator::realloc`].
@@ -158,6 +171,29 @@ unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
}
}

+// 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 Vmalloc {
+ #[inline]
+ unsafe fn realloc(
+ ptr: Option<NonNull<u8>>,
+ layout: Layout,
+ flags: Flags,
+ ) -> Result<NonNull<[u8]>, AllocError> {
+ // TODO: Support alignments larger than PAGE_SIZE.
+ if layout.align() > bindings::PAGE_SIZE {
+ pr_warn!("Vmalloc does not support alignments larger than PAGE_SIZE yet.\n");
+ return Err(AllocError);
+ }
+
+ // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
+ // allocated with this `Allocator`.
+ unsafe { ReallocFunc::VREALLOC.call(ptr, layout, flags) }
+ }
+}
+
#[global_allocator]
static ALLOCATOR: Kmalloc = Kmalloc;

diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
index 4785efc474a7..e7bf2982f68f 100644
--- a/rust/kernel/alloc/allocator_test.rs
+++ b/rust/kernel/alloc/allocator_test.rs
@@ -7,6 +7,7 @@
use core::ptr::NonNull;

pub struct Kmalloc;
+pub type Vmalloc = Kmalloc;

unsafe impl Allocator for Kmalloc {
unsafe fn realloc(
--
2.46.0