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

[Gary Guo]

On Fri, 23 Jun 2023 10:35:00 -0700
Boqun Feng <boqun.feng@xxxxxxxxx> wrote:

> On Thu, Jun 22, 2023 at 09:24:40PM +0200, Björn Roy Baron via B4 Relay wrote:
> > From: Björn Roy Baron <bjorn3_gh@xxxxxxxxxxxxxx>
> > 
> > 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  
> 
> Nice! Although I think we need to do the simialr size adjustment as:
> 
> 	https://lore.kernel.org/rust-for-linux/20230613164258.3831917-1-boqun.feng@xxxxxxxxx/
> 
> so I applied your patch onto my patch and came up with the following:

The diff LGTM.

> 
> --------------------------------->8  
> diff --git a/rust/kernel/allocator.rs b/rust/kernel/allocator.rs
> index ce7a06bf7589..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) {
> @@ -37,23 +59,30 @@ unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout) {
>          }
>      }
>  
> -    unsafe fn realloc(&self, ptr: *mut u8, _layout: Layout, new_size: usize) -> *mut u8 {
> -        unsafe {
> -            bindings::krealloc(
> -                ptr as *const core::ffi::c_void,
> -                new_size,
> -                bindings::GFP_KERNEL,
> -            ) as *mut u8
> -        }
> +    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 {
> -            bindings::krealloc(
> -                core::ptr::null(),
> -                layout.size(),
> +            self.krealloc_with_flags(
> +                ptr::null_mut(),
> +                layout,
>                  bindings::GFP_KERNEL | bindings::__GFP_ZERO,
> -            ) as *mut u8
> +            )
>          }
>      }
>  }
> 
> 
> Regards,
> Boqun
> 
> > Signed-off-by: Björn Roy Baron <bjorn3_gh@xxxxxxxxxxxxxx>
> > ---
> >  rust/kernel/allocator.rs | 20 ++++++++++++++++++++
> >  1 file changed, 20 insertions(+)
> > 
> > diff --git a/rust/kernel/allocator.rs b/rust/kernel/allocator.rs
> > index 397a3dd57a9b..e0a27b1326b5 100644
> > --- a/rust/kernel/allocator.rs
> > +++ b/rust/kernel/allocator.rs
> > @@ -21,6 +21,26 @@ 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 {
> > +        unsafe {
> > +            bindings::krealloc(
> > +                ptr as *const core::ffi::c_void,
> > +                new_size,
> > +                bindings::GFP_KERNEL,
> > +            ) as *mut u8
> > +        }
> > +    }
> > +
> > +    unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
> > +        unsafe {
> > +            bindings::krealloc(
> > +                core::ptr::null(),
> > +                layout.size(),
> > +                bindings::GFP_KERNEL | bindings::__GFP_ZERO,
> > +            ) as *mut u8
> > +        }
> > +    }
> >  }
> >  
> >  #[global_allocator]
> > 
> > ---
> > base-commit: d2e3115d717197cb2bc020dd1f06b06538474ac3
> > change-id: 20230622-global_alloc_methods-abf5b5e38dba
> > 
> > Best regards,
> > -- 
> > Björn Roy Baron <bjorn3_gh@xxxxxxxxxxxxxx>
> >