rust: alloc: replace aligned_size() with Kmalloc::aligned_layout()

aligned_size() dates back to when Rust did support kmalloc() only, but
is now used in ReallocFunc::call() and hence for all allocators.

However, the additional padding applied by aligned_size() is only
required by the kmalloc() allocator backend.

Hence, replace aligned_size() with Kmalloc::aligned_layout() and use it
for the affected allocators, i.e. kmalloc() and kvmalloc(), only.

While at it, make Kmalloc::aligned_layout() public, such that Rust
abstractions, which have to call subsystem specific kmalloc() based
allocation primitives directly, can make use of it.

Fixes: 8a799831fc ("rust: alloc: implement `ReallocFunc`")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250731154919.4132-2-dakr@kernel.org
[ Remove `const` from Kmalloc::aligned_layout(). - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/alloc/allocator.rs

@@ -43,17 +43,6 @@
 /// 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.
-    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()
-}
-
 /// # Invariants
 ///
 /// One of the following: `krealloc`, `vrealloc`, `kvrealloc`.
@@ -88,7 +77,7 @@ unsafe fn call(
         old_layout: Layout,
         flags: Flags,
     ) -> Result<NonNull<[u8]>, AllocError> {
-        let size = aligned_size(layout);
+        let size = layout.size();
         let ptr = match ptr {
             Some(ptr) => {
                 if old_layout.size() == 0 {
@@ -123,6 +112,17 @@ unsafe fn call(
     }
 }
 
+impl Kmalloc {
+    /// Returns a [`Layout`] that makes [`Kmalloc`] fulfill the requested size and alignment of
+    /// `layout`.
+    pub fn aligned_layout(layout: Layout) -> Layout {
+        // Note that `layout.size()` (after padding) is guaranteed to be a multiple of
+        // `layout.align()` which together with the slab guarantees means that `Kmalloc` will return
+        // a properly aligned object (see comments in `kmalloc()` for more information).
+        layout.pad_to_align()
+    }
+}
+
 // 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,
@@ -135,6 +135,8 @@ unsafe fn realloc(
         old_layout: Layout,
         flags: Flags,
     ) -> Result<NonNull<[u8]>, AllocError> {
+        let layout = Kmalloc::aligned_layout(layout);
+
         // SAFETY: `ReallocFunc::call` has the same safety requirements as `Allocator::realloc`.
         unsafe { ReallocFunc::KREALLOC.call(ptr, layout, old_layout, flags) }
     }
@@ -176,6 +178,10 @@ unsafe fn realloc(
         old_layout: Layout,
         flags: Flags,
     ) -> Result<NonNull<[u8]>, AllocError> {
+        // `KVmalloc` may use the `Kmalloc` backend, hence we have to enforce a `Kmalloc`
+        // compatible layout.
+        let layout = Kmalloc::aligned_layout(layout);
+
         // 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");