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

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Message-ID: <721dff90-aefd-4df9-b383-d62dc54ca454@nvidia.com>
Date: Thu, 30 Oct 2025 18:49:12 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>
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

Hi Danilo,

On 10/30/2025 5:27 PM, Danilo Krummrich wrote:
> 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)?;
> 

Sure. I will use pin_init here.

> 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.
I like your CListHead idea. Let me sketch this out a bit more and see what the
lifetime relationships look like, thanks! - Joel