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

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Message-ID: <00c48c22-52c2-454b-a5c3-15b6b1caa0a3@proton.me>
Date: Wed, 14 Aug 2024 21:07:53 +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 19:15, Danilo Krummrich wrote:
> 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.

Oh yeah of course... I don't know how I arrived at that comment...

---
Cheers,
Benno