linux-kernel - Re: [PATCH v10 3/4] rust: add support for NUMA ids in allocations

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Message-Id: <5BDED328-4E79-4E39-95C3-21C3FB80593C@konsulko.se>
Date: Sat, 5 Jul 2025 09:17:10 +0200
From: Vitaly Wool <vitaly.wool@...sulko.se>
To: Danilo Krummrich <dakr@...nel.org>
Cc: linux-mm@...ck.org,
 akpm@...ux-foundation.org,
 linux-kernel@...r.kernel.org,
 Uladzislau Rezki <urezki@...il.com>,
 Alice Ryhl <aliceryhl@...gle.com>,
 rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v10 3/4] rust: add support for NUMA ids in allocations



> On Jul 4, 2025, at 5:23 PM, Danilo Krummrich <dakr@...nel.org> wrote:
> 
> On Wed, Jul 02, 2025 at 06:09:10PM +0200, Vitaly Wool wrote:
>> /// The kernel's [`Allocator`] trait.
>> ///
>> /// An implementation of [`Allocator`] can allocate, re-allocate and free memory buffers described
>> @@ -148,7 +172,7 @@ pub unsafe trait Allocator {
>>     ///
>>     /// When the return value is `Ok(ptr)`, then `ptr` is
>>     /// - valid for reads and writes for `layout.size()` bytes, until it is passed to
>> -    ///   [`Allocator::free`] or [`Allocator::realloc`],
>> +    ///   [`Allocator::free`], [`Allocator::realloc`] or [`Allocator::realloc_node`],
>>     /// - aligned to `layout.align()`,
>>     ///
>>     /// Additionally, `Flags` are honored as documented in
>> @@ -159,7 +183,41 @@ fn alloc(layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {
>>         unsafe { Self::realloc(None, layout, Layout::new::<()>(), flags) }
>>     }
>> 
>> -    /// Re-allocate an existing memory allocation to satisfy the requested `layout`.
>> +    /// Allocate memory based on `layout`, `flags` and `nid`.
>> +    ///
>> +    /// 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_node` when called with `None`.
>> +    ///
>> +    /// # Guarantees
>> +    ///
>> +    /// When the return value is `Ok(ptr)`, then `ptr` is
>> +    /// - valid for reads and writes for `layout.size()` bytes, until it is passed to
>> +    ///   [`Allocator::free`], [`Allocator::realloc`] or [`Allocator::realloc_node`],
>> +    /// - aligned to `layout.align()`,
>> +    ///
>> +    /// Additionally, `Flags` are honored as documented in
>> +    /// <https://docs.kernel.org/core-api/mm-api.html#mm-api-gfp-flags>.
>> +    fn alloc_node(
>> +        layout: Layout,
>> +        flags: Flags,
>> +        nid: NumaNode,
>> +    ) -> Result<NonNull<[u8]>, AllocError> {
>> +        // SAFETY: Passing `None` to `realloc_node` is valid by its safety requirements and
>> +        // asks for a new memory allocation.
>> +        unsafe { Self::realloc_node(None, layout, Layout::new::<()>(), flags, nid) }
>> +    }
>> +
>> +    /// Re-allocate an existing memory allocation to satisfy the requested `layout` and
>> +    /// a specific NUMA node request to allocate the memory for.
>> +    ///
>> +    /// Systems employing a Non Uniform Memory Access (NUMA) architecture contain collections of
>> +    /// hardware resources including processors, memory, and I/O buses, that comprise what is
>> +    /// commonly known as a NUMA node.
>> +    ///
>> +    /// `nid` stands for NUMA id, i. e. NUMA node identifier, which is a non-negative
>> +    /// integer if a node needs to be specified, or NO_NODE if the caller doesn't care.
>>     ///
>>     /// If the requested size is zero, `realloc` behaves equivalent to `free`.
>>     ///
>> @@ -191,13 +249,27 @@ fn alloc(layout: Layout, flags: Flags) -> Result<NonNull<[u8]>, AllocError> {
>>     ///   and old size, i.e. `ret_ptr[0..min(layout.size(), old_layout.size())] ==
>>     ///   p[0..min(layout.size(), old_layout.size())]`.
>>     /// - when the return value is `Err(AllocError)`, then `ptr` is still valid.
>> -    unsafe fn realloc(
>> +    unsafe fn realloc_node(
>>         ptr: Option<NonNull<u8>>,
>>         layout: Layout,
>>         old_layout: Layout,
>>         flags: Flags,
>> +        nid: NumaNode,
>>     ) -> Result<NonNull<[u8]>, AllocError>;
>> 
>> +    /// Re-allocate an existing memory allocation to satisfy the requested `layout`. This
>> +    /// function works exactly as realloc_node() but it doesn't give the ability to specify
>> +    /// the NUMA node in the call.
>> +    unsafe fn realloc(
>> +        ptr: Option<NonNull<u8>>,
>> +        layout: Layout,
>> +        old_layout: Layout,
>> +        flags: Flags,
>> +    ) -> Result<NonNull<[u8]>, AllocError> {
>> +        // SAFETY: guaranteed by realloc_node()
>> +        unsafe { Self::realloc_node(ptr, layout, old_layout, flags, NumaNode::NO_NODE) }
>> +    }
> 
> I think Alice suggested to just drop alloc_node() and realloc_node() and make
> alloc() and realloc() always take a NumaNode argument.
> 
> I don't have a strong preference, but keeping only alloc() and realloc() for
> seems indeed simpler, so let's remove the _node() variants.

I don’t have a strong preference either but if I modify alloc() and realloc(), I’ll have to change KVec and KBox in the same patch, would that be okay with you?

~Vitaly

<snip>