linux-kernel - Re: [PATCH v3 01/25] rust: alloc: add `Allocator` trait

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Message-ID: <Zq90QTKqJQWtCnPn@pollux>
Date: Sun, 4 Aug 2024 14:29:53 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
	gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	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,
	acurrid@...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 v3 01/25] rust: alloc: add `Allocator` trait

On Sat, Aug 03, 2024 at 11:21:21PM -0700, Boqun Feng wrote:
> On Thu, Aug 01, 2024 at 02:02:00AM +0200, Danilo Krummrich wrote:
> [...]
> > +/// The kernel's [`Allocator`] trait.
> > +///
> > +/// An implementation of [`Allocator`] can allocate, re-allocate and free memory buffer described
> > +/// via [`Layout`].
> > +///
> > +/// [`Allocator`] is designed to be implemented as a ZST; [`Allocator`] functions do not operate on
> > +/// an object instance.
> > +///
> > +/// # Safety
> > +///
> > +/// Memory returned from an allocator must point to a valid memory buffer and remain valid until
> > +/// it is explicitly freed.
> > +///
> > +/// Any pointer to a memory buffer which is currently allocated must be valid to be passed to any
> > +/// other [`Allocator`] function. The same applies for a NULL pointer.
> > +///
> 
> Are you saying you could kmalloc() a memory buffer and pass it to a
> vfree()? Or am I missing something here?

I will extend it to:

"valid to be passed to any other [`Allocator`] function" of the same type.

> 
> Regards,
> Boqun
> 
> > +/// If `realloc` is called with:
> > +///   - a size of zero, the given memory allocation, if any, must be freed
> > +///   - a NULL pointer, a new memory allocation must be created
> > +pub unsafe trait Allocator {
> > +    /// Allocate memory based on `layout` and `flags`.
> > +    ///
> > +    /// On success, returns a buffer represented as `NonNull<[u8]>` that satisfies the layout
> > +    /// constraints (i.e. minimum size and alignment as specified by `layout`).
> > +    ///
> > +    /// This function is equivalent to `realloc` when called with a NULL pointer.
> > +    fn alloc(layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {
> > +        // SAFETY: Passing a NULL pointer to `realloc` is valid by it's safety requirements and asks
> > +        // for a new memory allocation.
> > +        unsafe { Self::realloc(None, layout, flags) }
> > +    }
> > +
> > +    /// Re-allocate an existing memory allocation to satisfy the requested `layout`. If the
> > +    /// requested size is zero, `realloc` behaves equivalent to `free`.
> > +    ///
> > +    /// If the requested size is larger than the size of the existing allocation, a successful call
> > +    /// to `realloc` guarantees that the new or grown buffer has at least `Layout::size` bytes, but
> > +    /// may also be larger.
> > +    ///
> > +    /// If the requested size is smaller than the size of the existing allocation, `realloc` may or
> > +    /// may not shrink the buffer; this is implementation specific to the allocator.
> > +    ///
> > +    /// On allocation failure, the existing buffer, if any, remains valid.
> > +    ///
> > +    /// The buffer is represented as `NonNull<[u8]>`.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// `ptr` must point to an existing and valid memory allocation created by this allocator
> > +    /// instance.
> > +    ///
> > +    /// Additionally, `ptr` is allowed to be a NULL pointer; in this case a new memory allocation is
> > +    /// created.
> > +    unsafe fn realloc(
> > +        ptr: Option<NonNull<u8>>,
> > +        layout: Layout,
> > +        flags: Flags,
> > +    ) -> Result<NonNull<[u8]>, AllocError>;
> > +
> > +    /// Free an existing memory allocation.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// `ptr` must point to an existing and valid memory allocation created by this `Allocator`
> > +    /// instance.
> > +    unsafe fn free(ptr: NonNull<u8>) {
> > +        // SAFETY: `ptr` is guaranteed to be previously allocated with this `Allocator` or NULL.
> > +        // Calling `realloc` with a buffer size of zero, frees the buffer `ptr` points to.
> > +        let _ = unsafe { Self::realloc(Some(ptr), Layout::new::<()>(), Flags(0)) };
> > +    }
> > +}
> > -- 
> > 2.45.2
> > 
>