Re: [PATCH v2] rust: alloc: Add realloc and alloc_zeroed to the GlobalAlloc impl

[Gary Guo]

On Sun, 25 Jun 2023 16:25:28 -0700
Boqun Feng <boqun.feng@xxxxxxxxx> wrote:

> While there are default impls for these methods, using the respective C
> api's is faster. Currently neither the existing nor these new
> GlobalAlloc method implementations are actually called. Instead the
> __rust_* function defined below the GlobalAlloc impl are used. With
> rustc 1.71 these functions will be gone and all allocation calls will go
> through the GlobalAlloc implementation.
> 
> Link: https://github.com/Rust-for-Linux/linux/issues/68
> Signed-off-by: Björn Roy Baron <bjorn3_gh@xxxxxxxxxxxxxx>
> [boqun: add size adjustment for alignment requirement]
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>

Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>

> ---
> Miguel, I fold my diff into Björn's original patch and make a v2, it
> relies on the other patch:
> 
> 	https://lore.kernel.org/rust-for-linux/6e61f06f-2411-0bcb-926b-0a6927096f20@xxxxxxxxx/
> 
> So this v2 (if all goes well) is targeted for v6.6, JFYI.
> 
> v1 -> v2:
> 
> *	Add size adjustment for align requirement.
> 
>  rust/kernel/allocator.rs | 59 ++++++++++++++++++++++++++++++++++++----
>  1 file changed, 54 insertions(+), 5 deletions(-)
> 
> diff --git a/rust/kernel/allocator.rs b/rust/kernel/allocator.rs
> index 66575cf87ce2..af723c2924dc 100644
> --- a/rust/kernel/allocator.rs
> +++ b/rust/kernel/allocator.rs
> @@ -9,8 +9,17 @@
>  
>  struct KernelAllocator;
>  
> -unsafe impl GlobalAlloc for KernelAllocator {
> -    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
> +impl KernelAllocator {
> +    /// # Safety
> +    ///
> +    /// * `ptr` can be either null or a pointer which has been allocated by this allocator.
> +    /// * `layout` must have a non-zero size.
> +    unsafe fn krealloc_with_flags(
> +        &self,
> +        ptr: *mut u8,
> +        layout: Layout,
> +        flags: bindings::gfp_t,
> +    ) -> *mut u8 {
>          // Customized layouts from `Layout::from_size_align()` can have size < align, so pads first.
>          let layout = layout.pad_to_align();
>  
> @@ -26,9 +35,22 @@ unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
>              size = size.next_power_of_two();
>          }
>  
> -        // `krealloc()` is used instead of `kmalloc()` because the latter is
> -        // an inline function and cannot be bound to as a result.
> -        unsafe { bindings::krealloc(ptr::null(), size, bindings::GFP_KERNEL) as *mut u8 }
> +        // SAFETY:
> +        //
> +        // * `ptr` is either null or a pointer returned from a previous k{re}alloc() by the function
> +        //   safety requirement.
> +        //
> +        // * `size` is greater than 0 since it's either a `layout.size()` (which cannot be zero
> +        //    according to the function safety requirement) or a result from `next_power_of_two()`.
> +        unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags) as *mut u8 }
> +    }
> +}
> +
> +unsafe impl GlobalAlloc for KernelAllocator {
> +    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 { self.krealloc_with_flags(ptr::null_mut(), layout, bindings::GFP_KERNEL) }
>      }
>  
>      unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout) {
> @@ -36,6 +58,33 @@ unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout) {
>              bindings::kfree(ptr as *const core::ffi::c_void);
>          }
>      }
> +
> +    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
> +        // SAFETY:
> +        // * `new_size` when rounded up to the nearest multiple of `layout.align()`, will not
> +        //   overflow `isize` by the function safety requirement.
> +        // * `layout.align()` is a proper alignment (i.e. not zero and must be a power of two).
> +        let layout = unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
> +
> +        // SAFETY:
> +        // * `ptr` is either null or a pointer allocated by this allocator by function safety
> +        //   requirement.
> +        // * the size of `layout` is not zero because `new_size` is not zero by function safety
> +        //   requirement.
> +        unsafe { self.krealloc_with_flags(ptr, layout, bindings::GFP_KERNEL) }
> +    }
> +
> +    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 {
> +            self.krealloc_with_flags(
> +                ptr::null_mut(),
> +                layout,
> +                bindings::GFP_KERNEL | bindings::__GFP_ZERO,
> +            )
> +        }
> +    }
>  }
>  
>  #[global_allocator]