rust: alloc: implement Vmalloc allocator

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>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20240911225449.152928-7-dakr@kernel.org

Author: Danilo Krummrich

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"

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);
+}

rust/kernel/alloc/allocator.rs

@@ -15,6 +15,7 @@ use core::ptr::NonNull;
 
 use crate::alloc::{AllocError, Allocator};
 use crate::bindings;
+use crate::pr_warn;
 
 /// The contiguous kernel allocator.
 ///
@@ -24,6 +25,26 @@ use crate::bindings;
 /// 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.
+    let layout = new_layout.pad_to_align();
+
+    // Note that `layout.size()` (after padding) is guaranteed to be a multiple of `layout.align()`
+    // which together with the slab guarantees means the `krealloc` will return a properly aligned
+    // object (see comments in `kmalloc()` for more information).
+    layout.size()
+}
+
 /// Calls `krealloc` with a proper size to alloc a new object aligned to `new_layout`'s alignment.
 ///
 /// # Safety
@@ -58,6 +79,9 @@ 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`].
@@ -153,6 +177,29 @@ unsafe impl GlobalAlloc for Kmalloc {
     }
 }
 
+// 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;
 

rust/kernel/alloc/allocator_test.rs

@@ -7,6 +7,7 @@ use core::alloc::Layout;
 use core::ptr::NonNull;
 
 pub struct Kmalloc;
+pub type Vmalloc = Kmalloc;
 
 unsafe impl Allocator for Kmalloc {
     unsafe fn realloc(