linux-kernel - Re: [PATCH RFC 3/4] rust: drm: Add DRM buddy allocator bindings

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Message-Id: <DDVZ4L08QMIR.GFMG544BYQEO@kernel.org>
Date: Thu, 30 Oct 2025 22:27:46 +0100
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Joel Fernandes" <joelagnelf@...dia.com>
Cc: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <dri-devel@...ts.freedesktop.org>, "David Airlie" <airlied@...il.com>,
 <acourbot@...dia.com>, "Alistair Popple" <apopple@...dia.com>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun
 Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Simona Vetter" <simona@...ll.ch>,
 "Maarten Lankhorst" <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard"
 <mripard@...nel.org>, "Thomas Zimmermann" <tzimmermann@...e.de>, "John
 Hubbard" <jhubbard@...dia.com>, "Timur Tabi" <ttabi@...dia.com>,
 <joel@...lfernandes.org>, "Elle Rhumsaa" <elle@...thered-steel.dev>,
 "Daniel Almeida" <daniel.almeida@...labora.com>, "Andrea Righi"
 <arighi@...dia.com>, "Philipp Stanner" <phasta@...nel.org>,
 <nouveau@...ts.freedesktop.org>
Subject: Re: [PATCH RFC 3/4] rust: drm: Add DRM buddy allocator bindings

On Thu Oct 30, 2025 at 8:06 PM CET, Joel Fernandes wrote:
> +    ///
> +    /// Returns an [`AllocatedBlocks`] structure that owns the allocated blocks and automatically
> +    /// frees them when dropped. Allocation of `list_head` uses the `gfp` flags passed.
> +    pub fn alloc_blocks(
> +        &self,
> +        start: usize,
> +        end: usize,
> +        size: usize,
> +        min_block_size: usize,
> +        flags: BuddyFlags,
> +        gfp: Flags,
> +    ) -> Result<AllocatedBlocks<'_>> {
> +        // Allocate list_head on the heap.
> +        let mut list_head = KBox::new(bindings::list_head::default(), gfp)?;
> +
> +        // SAFETY: list_head is valid and heap-allocated.
> +        unsafe {
> +            bindings::INIT_LIST_HEAD(&mut *list_head as *mut _);
> +        }

Not a full review, but a quick drive-by comment:

bindings::list_head has to be pinned in memory it should be

	let list_head = KBox::pin_init(Opaque::ffi_init(|slot: *mut bindings::list_head| {
	    // SAFETY: `slot` is a valid pointer to uninitialized memory.
	    unsafe { bindings::INIT_LIST_HEAD(slot) };
	}), gfp)?;

if you're doing it by hand, but as mentioned in a previous patch, I think it
would be nice to have a transparent wrapper type, CListHead, for this.