[PATCH 04/20] rust: alloc: implement `Allocator` for `Kmalloc`

[Danilo Krummrich <dakr@redhat.com>]

Implement `Allocator` for `Kmalloc`, the kernel's default allocator,
typically used for objects smaller than page size.

All memory allocations made with `Kmalloc` end up in `krealloc()`.

It serves as the default allocator for the subsequently introduced types
`KBox` and `KVec`.

Signed-off-by: Danilo Krummrich <dakr@redhat.com>
---
 rust/kernel/alloc.rs           |  2 +-
 rust/kernel/alloc/allocator.rs | 74 ++++++++++++++++++++++++++++------
 2 files changed, 63 insertions(+), 13 deletions(-)

diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index 462e00982510..8d79cc95dc1e 100644
--- a/rust/kernel/alloc.rs
+++ b/rust/kernel/alloc.rs
@@ -4,7 +4,7 @@
 
 #[cfg(not(test))]
 #[cfg(not(testlib))]
-mod allocator;
+pub mod allocator;
 pub mod box_ext;
 pub mod vec_ext;
 
diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index e47ab8fe21ab..b7c0490f6415 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -5,9 +5,18 @@
 use super::{flags::*, Flags};
 use core::alloc::{GlobalAlloc, Layout};
 use core::ptr;
+use core::ptr::NonNull;
 
-struct Kmalloc;
+use crate::alloc::{AllocError, Allocator};
+use crate::bindings;
 
+/// The contiguous kernel allocator.
+///
+/// The contiguous kernel allocator only ever allocates physically contiguous memory through
+/// `bindings::krealloc`.
+pub struct Kmalloc;
+
+/// 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();
@@ -27,7 +36,7 @@ fn aligned_size(new_layout: Layout) -> usize {
     size
 }
 
-/// Calls `krealloc` with a proper size to alloc a new object aligned to `new_layout`'s alignment.
+/// Calls `krealloc` with a proper size to alloc a new object.
 ///
 /// # Safety
 ///
@@ -48,20 +57,54 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
     }
 }
 
+unsafe impl Allocator for Kmalloc {
+    unsafe fn realloc(
+        &self,
+        old_ptr: *mut u8,
+        _old_size: usize,
+        layout: Layout,
+        flags: Flags,
+    ) -> Result<NonNull<[u8]>, AllocError> {
+        let size = aligned_size(layout);
+
+        // SAFETY: `src` is guaranteed to point to valid memory with a size of at least
+        // `old_size`, which was previously allocated with this `Allocator` or NULL.
+        let raw_ptr = unsafe {
+            // If `size == 0` and `old_ptr != NULL` `krealloc()` frees the memory behind the
+            // pointer.
+            bindings::krealloc(old_ptr.cast(), size, flags.0).cast()
+        };
+
+        let ptr = if size == 0 {
+            NonNull::dangling()
+        } else {
+            NonNull::new(raw_ptr).ok_or(AllocError)?
+        };
+
+        Ok(NonNull::slice_from_raw_parts(ptr, size))
+    }
+}
+
 unsafe impl GlobalAlloc for Kmalloc {
     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
-        // SAFETY: `ptr::null_mut()` is null and `layout` has a non-zero size by the function safety
-        // requirement.
-        unsafe { krealloc_aligned(ptr::null_mut(), layout, GFP_KERNEL) }
+        let this: &dyn Allocator = self;
+
+        match this.alloc(layout, GFP_KERNEL) {
+            Ok(ptr) => ptr.as_ptr().cast(),
+            Err(_) => ptr::null_mut(),
+        }
     }
 
     unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout) {
-        unsafe {
-            bindings::kfree(ptr as *const core::ffi::c_void);
-        }
+        // SAFETY: The safety requirements of `dealloc` are a superset of the ones of
+        // `Allocator::free`.
+        unsafe { self.free(ptr) }
     }
 
     unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
+        let this: &dyn Allocator = self;
+        let old_size = layout.size();
+
         // SAFETY:
         // - `new_size`, when rounded up to the nearest multiple of `layout.align()`, will not
         //   overflow `isize` by the function safety requirement.
@@ -73,13 +116,20 @@ unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut
         //   requirement.
         // - the size of `layout` is not zero because `new_size` is not zero by the function safety
         //   requirement.
-        unsafe { krealloc_aligned(ptr, layout, GFP_KERNEL) }
+        // - `old_size` represents the memory that needs to be preserved.
+        match unsafe { this.realloc(ptr, old_size, layout, GFP_KERNEL) } {
+            Ok(ptr) => ptr.as_ptr().cast(),
+            Err(_) => ptr::null_mut(),
+        }
     }
 
     unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
-        // SAFETY: `ptr::null_mut()` is null and `layout` has a non-zero size by the function safety
-        // requirement.
-        unsafe { krealloc_aligned(ptr::null_mut(), layout, GFP_KERNEL | __GFP_ZERO) }
+        let this: &dyn Allocator = self;
+
+        match this.alloc(layout, GFP_KERNEL | __GFP_ZERO) {
+            Ok(ptr) => ptr.as_ptr().cast(),
+            Err(_) => ptr::null_mut(),
+        }
     }
 }
 
-- 
2.45.2