[PATCH v7 07/26] rust: alloc: implement `KVmalloc` allocator

Danilo Krummrich
2024-09-11T22:55:38Z

Implement `Allocator` for `KVmalloc`, an `Allocator` that tries to
allocate memory wth `kmalloc` first and, on failure, falls back to
`vmalloc`.

All memory allocations made with `KVmalloc` end up in
`kvrealloc_noprof()`; all frees in `kvfree()`.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
rust/helpers/slab.c | 6 +++++
rust/kernel/alloc/allocator.rs | 35 +++++++++++++++++++++++++++++
rust/kernel/alloc/allocator_test.rs | 1 +
3 files changed, 42 insertions(+)

diff --git a/rust/helpers/slab.c b/rust/helpers/slab.c
index f043e087f9d6..a842bfbddcba 100644
--- a/rust/helpers/slab.c
+++ b/rust/helpers/slab.c
@@ -7,3 +7,9 @@ rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
{
return krealloc(objp, new_size, flags);
}
+
+void * __must_check __realloc_size(2)
+rust_helper_kvrealloc(const void *p, size_t size, gfp_t flags)
+{
+ return kvrealloc(p, size, flags);
+}
diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index 0f2bc702e8e4..a5d7e66a68db 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -34,6 +34,15 @@
/// For more details see [self].
pub struct Vmalloc;

+/// The kvmalloc kernel allocator.
+///
+/// `KVmalloc` attempts to allocate memory with `Kmalloc` first, but falls back to `Vmalloc` upon
+/// failure. This allocator is typically used when the size for the requested allocation is not
+/// known and may exceed the capabilities of `Kmalloc`.
+///
+/// For more details see [self].
+pub struct KVmalloc;
+
/// 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.
@@ -76,6 +85,9 @@ impl ReallocFunc {
// INVARIANT: `vrealloc` satisfies the type invariants.
const VREALLOC: Self = Self(bindings::vrealloc);

+ // INVARIANT: `kvrealloc` satisfies the type invariants.
+ const KVREALLOC: Self = Self(bindings::kvrealloc);
+
/// # Safety
///
/// This method has the same safety requirements as [`Allocator::realloc`].
@@ -194,6 +206,29 @@ unsafe fn realloc(
}
}

+// 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 KVmalloc {
+ #[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!("KVmalloc 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::KVREALLOC.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 e7bf2982f68f..1b2642c547ec 100644
--- a/rust/kernel/alloc/allocator_test.rs
+++ b/rust/kernel/alloc/allocator_test.rs
@@ -8,6 +8,7 @@

pub struct Kmalloc;
pub type Vmalloc = Kmalloc;
+pub type KVmalloc = Kmalloc;

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