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

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Message-ID: <ZrzmFHGXrXxSqXzt@cassiopeiae>
Date: Wed, 14 Aug 2024 19:15:00 +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 05:02:53PM +0000, Benno Lossin wrote:
> On 14.08.24 18:59, Danilo Krummrich wrote:
> > On Wed, Aug 14, 2024 at 04:21:38PM +0000, Benno Lossin wrote:
> >> On 12.08.24 20:22, Danilo Krummrich wrote:
> >>> +    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.
> 
> But you can just return early above? I would prefer the check be done
> before `self.0` is called.

No, we can't return early, we need `self.0` to be called, because if `size == 0`
we free the given allocation, if any.

> 
> ---
> Cheers,
> Benno
>