linux-kernel - Re: [PATCH v5 04/26] rust: alloc: implement `Allocator` for `Kmalloc`

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Message-ID: <ZrziXqB5zExntYfW@cassiopeiae>
Date: Wed, 14 Aug 2024 18:59:10 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	a.hindborg@...sung.com, aliceryhl@...gle.com,
	akpm@...ux-foundation.org, daniel.almeida@...labora.com,
	faith.ekstrand@...labora.com, boris.brezillon@...labora.com,
	lina@...hilina.net, mcanal@...lia.com, zhiw@...dia.com,
	cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com,
	ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 04/26] rust: alloc: implement `Allocator` for `Kmalloc`

On Wed, Aug 14, 2024 at 04:21:38PM +0000, Benno Lossin wrote:
> On 12.08.24 20:22, Danilo Krummrich wrote:
> > +    /// # Safety
> > +    ///
> > +    /// This method has the same safety requirements as `Allocator::realloc`.
> 
> Please make this a link.

Sure.

> 
> > +    unsafe fn call(
> > +        &self,
> > +        ptr: Option<NonNull<u8>>,
> > +        layout: Layout,
> > +        flags: Flags,
> > +    ) -> Result<NonNull<[u8]>, AllocError> {
> > +        let size = aligned_size(layout);
> > +        let ptr = match ptr {
> > +            Some(ptr) => ptr.as_ptr(),
> > +            None => ptr::null(),
> > +        };
> > +
> > +        // SAFETY: `ptr` is either NULL or valid by the safety requirements of this function.
> > +        let raw_ptr = unsafe {
> > +            // If `size == 0` and `ptr != NULL` the memory behind the pointer is freed.
> > +            self.0(ptr.cast(), size, flags.0).cast()
> > +        };
> > +
> > +        let ptr = if size == 0 {
> 
> Why do you do this check *after* calling `self.0`?

Because I need `raw_ptr` in the else case below.

> 
> ---
> Cheers,
> Benno
> 
> > +            NonNull::dangling()
> > +        } else {
> > +            NonNull::new(raw_ptr).ok_or(AllocError)?
> > +        };
> > +
> > +        Ok(NonNull::slice_from_raw_parts(ptr, size))
> > +    }
> > +}
>