rust: alloc: implement `KVmalloc` allocator

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>

3 files changed, 39 insertions, 0 deletions

diff --git a/rust/helpers.c b/rust/helpers.c
index 96f8b37a668b..9a7604e3a749 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -256,6 +256,13 @@ rust_helper_vrealloc(const void *p, size_t size, gfp_t flags)
 }
 EXPORT_SYMBOL_GPL(rust_helper_vrealloc);
 
+void * __must_check __realloc_size(2)
+rust_helper_kvrealloc(const void *p, size_t size, gfp_t flags)
+{
+	return kvrealloc(p, size, flags);
+}
+EXPORT_SYMBOL_GPL(rust_helper_kvrealloc);
+
 void rust_helper_rb_link_node(struct rb_node *node, struct rb_node *parent,
 			      struct rb_node **rb_link)
 {
diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index 09a322e9168d..0183fbac1ada 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -34,6 +34,15 @@ pub struct Kmalloc;
 /// 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 requsted 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`].
@@ -173,6 +185,25 @@ unsafe impl Allocator for Vmalloc {
     }
 }
 
+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 @@ use core::ptr::NonNull;
 
 pub struct Kmalloc;
 pub type Vmalloc = Kmalloc;
+pub type KVmalloc = Kmalloc;
 
 unsafe impl Allocator for Kmalloc {
     unsafe fn realloc(