Message-ID: <92ef0fb2-aa5a-451a-a79c-2d81e562da41@proton.me>
Date: Tue, 10 Sep 2024 19:42:58 +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 v6 04/26] rust: alloc: implement `Allocator` for `Kmalloc`

On 10.09.24 15:37, Danilo Krummrich wrote:
> On Tue, Sep 10, 2024 at 01:11:35PM +0000, Benno Lossin wrote:
>> On 03.09.24 13:48, Danilo Krummrich wrote:
>>> On Fri, Aug 30, 2024 at 02:45:35PM +0000, Benno Lossin wrote:
>>>> On 30.08.24 00:04, Danilo Krummrich wrote:
>>>>> On Thu, Aug 29, 2024 at 06:32:42PM +0000, Benno Lossin wrote:
>>>>>> On 16.08.24 02:10, Danilo Krummrich wrote:
>>>>>>> +///
>>>>>>> +/// For more details see [self].
>>>>>>> +pub struct Kmalloc;
>>>>>>>
>>>>>>>  /// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
>>>>>>>  fn aligned_size(new_layout: Layout) -> usize {
>>>>>>> @@ -36,6 +52,60 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
>>>>>>>      unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags.0) as *mut u8 }
>>>>>>>  }
>>>>>>>
>>>>>>> +/// # Invariants
>>>>>>> +///
>>>>>>> +/// One of the following `krealloc`, `vrealloc`, `kvrealloc`.
>>>>>>> +struct ReallocFunc(
>>>>>>> +    unsafe extern "C" fn(*const core::ffi::c_void, usize, u32) -> *mut core::ffi::c_void,
>>>>>>> +);
>>>>>>> +
>>>>>>> +impl ReallocFunc {
>>>>>>> +    // INVARIANT: `krealloc` satisfies the type invariants.
>>>>>>> +    const KREALLOC: Self = Self(bindings::krealloc);
>>>>>>> +
>>>>>>> +    /// # Safety
>>>>>>> +    ///
>>>>>>> +    /// This method has the same safety requirements as [`Allocator::realloc`].
>>>>>>> +    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.
>>>>>>
>>>>>> You need some justification as to why calling the three allowed
>>>>>> functions here.
>>>>>
>>>>> What kind of justification do I need? Can you please share some more details on
>>>>> what you think is missing here?
>>>>
>>>> So, you are calling a function pointer to an `unsafe` function. This
>>>> means that through some invariant you have to know what the safety
>>>> requirements are (otherwise how can you guarantee that this is OK?). You
>>>> have the invariant that the pointer points at one of the three functions
>>>> mentioned above. What are the safety requirements of those functions? I
>>>> would assume that the only one is that `ptr` is valid. So you can use:
>>>>
>>>>     // SAFETY:
>>>>     // - `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc` and thus only requires that `ptr` is
>>>>     //   NULL or valid.
>>>
>>> I'm fine adding it, but I'd like to understand why you think it's required in
>>> the safety comment here? Isn't this implicit by being the type invariant?
>>
>> You are calling a function pointer to an `unsafe` function that takes a
>> raw pointer. Without this comment it is not clear what the function
>> pointer's safety requirements are for the raw pointer parameter.
> 
> That's my point, isn't this implicitly clear by the type invariant? If needed,
> shouldn't it be:

I would argue that it is not implicitly clear, since to the reader of
just that unsafe block it's totally unclear that `self.0` has such an
invariant. They would have to read the type definition.

> // INVARIANT:
> // - `self.0` is one of [...]
> //
> // SAFETY:
> // - `ptr` is either NULL or [...]
> 
>>
>>>>     // - `ptr` is either NULL or valid by the safety requirements of this function.
>>>
>>> This is the part I already have.
>>
>> I kept it to ensure that you also keep it.

[...]

>>>>>>> +    #[inline]
>>>>>>> +    unsafe fn realloc(
>>>>>>> +        ptr: Option<NonNull<u8>>,
>>>>>>> +        layout: Layout,
>>>>>>> +        flags: Flags,
>>>>>>> +    ) -> Result<NonNull<[u8]>, AllocError> {
>>>>>>> +        // SAFETY: `ReallocFunc::call` has the same safety requirements as `Allocator::realloc`.
>>>>>>> +        unsafe { ReallocFunc::KREALLOC.call(ptr, layout, flags) }
>>>>>>> +    }
>>>>>>> +}
>>>>
>>>> Oh one more thing, I know that you already have a lot of patches in this
>>>> series, but could you split this one into two? So the first one should
>>>> introduce `ReallocFunc` and the second one add the impl for `Kmalloc`?
>>>> I managed to confuse me twice because of that :)
>>>
>>> Generally, I'm fine with that, but I'm not sure if I can avoid an intermediate
>>> compiler warning about unused code doing that.
>>
>> You can just use `#[expect(dead_code)]` for that in the intermediate
>> patches.
> 
> I usually try to avoid that, because it can be misleading when bisecting things.
> 
> If the temporarily unused code contains a bug, your bisection doesn't end up at
> this patch, but some other patch that starts using it.

I don't think it's a problem in this case, since the two patches are
directly next to each other and you're not changing existing code, just
splitting up the addition of new code.

---
Cheers,
Benno

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