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

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Message-ID: <17bcfe21-1af3-4247-98d1-480944be400e@proton.me>
Date: Wed, 14 Aug 2024 17:02:53 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>
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 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.

---
Cheers,
Benno