linux-kernel - Re: [PATCH v4 01/20] rust: dma: expose the count and size of CoherentAllocation

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Message-Id: <DA2FX8C1HZ59.13SM6DQ1JVFQ5@nvidia.com>
Date: Thu, 22 May 2025 14:29:35 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Boqun Feng" <boqun.feng@...il.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <benno.lossin@...ton.me>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "Danilo Krummrich" <dakr@...nel.org>, "David Airlie" <airlied@...il.com>,
 "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>, "Ben Skeggs" <bskeggs@...dia.com>, "Joel Fernandes"
 <joelagnelf@...dia.com>, "Timur Tabi" <ttabi@...dia.com>, "Alistair Popple"
 <apopple@...dia.com>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <nouveau@...ts.freedesktop.org>,
 <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 01/20] rust: dma: expose the count and size of
 CoherentAllocation

On Wed May 21, 2025 at 9:43 PM JST, Boqun Feng wrote:
> On Wed, May 21, 2025 at 03:44:56PM +0900, Alexandre Courbot wrote:
>> These properties are very useful to have and should be accessible.
>> 
>> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
>> ---
>>  rust/kernel/dma.rs | 18 ++++++++++++++++++
>>  1 file changed, 18 insertions(+)
>> 
>> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
>> index 605e01e35715667f93297fd9ec49d8e7032e0910..2a60eefa47dfc1f836c30ee342e26c6ff3e9b13a 100644
>> --- a/rust/kernel/dma.rs
>> +++ b/rust/kernel/dma.rs
>> @@ -129,6 +129,10 @@ pub mod attrs {
>>  //
>>  // Hence, find a way to revoke the device resources of a `CoherentAllocation`, but not the
>>  // entire `CoherentAllocation` including the allocated memory itself.
>> +//
>> +// # Invariants
>> +//
>> +// The size in bytes of the allocation is equal to `size_of::<T> * count()`.
>>  pub struct CoherentAllocation<T: AsBytes + FromBytes> {
>>      dev: ARef<Device>,
>>      dma_handle: bindings::dma_addr_t,
>> @@ -201,6 +205,20 @@ pub fn alloc_coherent(
>>          CoherentAllocation::alloc_attrs(dev, count, gfp_flags, Attrs(0))
>>      }
>>  
>> +    /// Returns the number of elements `T` in this allocation.
>> +    ///
>> +    /// Note that this is not the size of the allocation in bytes, which is provided by
>> +    /// [`Self::size`].
>> +    pub fn count(&self) -> usize {
>> +        self.count
>> +    }
>> +
>> +    /// Returns the size in bytes of this allocation.
>> +    pub fn size(&self) -> usize {
>> +        // As per the invariants of `CoherentAllocation`.
>> +        self.count * core::mem::size_of::<T>()
>
> I think we need a comment or even an invariant saying this multiply
> cannot overflow.

I'll add the following invariant:

    `size_of::<T> * count` fits into a `usize`.

and refer to it here. Does it work?