linux-kernel - Re: [PATCH] mm, slab: extend kmalloc() alignment for non power-of-two sizes

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Date: Tue, 2 Jul 2024 14:41:11 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Alice Ryhl <aliceryhl@...gle.com>, linux-mm@...ck.org,
	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Kees Cook <keescook@...omium.org>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH] mm, slab: extend kmalloc() alignment for non
 power-of-two sizes

On Tue, Jul 02, 2024 at 11:18:36PM +0200, Vlastimil Babka wrote:
> On 7/2/24 6:40 PM, Alice Ryhl wrote:
> > On Tue, Jul 2, 2024 at 5:58 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> > Thanks!
> > 
> > Since this change is motivated by Rust, why not also include the Rust
> > change in this patch? You would need to remove the if inside
> > krealloc_aligned in rust/kernel/alloc/allocator.rs and update the
> > comments.
> 
> Right, thanks. Does this look ok? (not tested as I don't have the
> environment working)
> 

Thanks! This overall looks good to me.

> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> index 229642960cd1..c619acb8b285 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -18,23 +18,16 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
>      // 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 guarantee means the `krealloc` will return a properly aligned
> +    // object (see comments in `kmalloc()` for more information).
>      let mut size = layout.size();

The `mut` needs to be removed since no need to modify `size` afterwards.

>  
> -    if layout.align() > bindings::ARCH_SLAB_MINALIGN {
> -        // The alignment requirement exceeds the slab guarantee, thus try to enlarge the size
> -        // to use the "power-of-two" size/alignment guarantee (see comments in `kmalloc()` for
> -        // more information).
> -        //
> -        // Note that `layout.size()` (after padding) is guaranteed to be a multiple of
> -        // `layout.align()`, so `next_power_of_two` gives enough alignment guarantee.
> -        size = size.next_power_of_two();
> -    }
> -
>      // 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()`.
> +    //   according to the function safety requirement)

The word "either" needs to be removed as well.

With these changes,

Reviewed-by: Boqun Feng <boqun.feng@...il.com>

Regards,
Boqun

>      unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags.0) as *mut u8 }
>  }
>  
>