linux-kernel - Re: [PATCH v4 3/4] rust: support large alignments in allocations

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Message-Id: <72365C10-D984-4BC5-A081-B058C1619D06@konsulko.se>
Date: Thu, 26 Jun 2025 18:29:24 +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 v4 3/4] rust: support large alignments in allocations



> On Jun 26, 2025, at 2:36 PM, Danilo Krummrich <dakr@...nel.org> wrote:
> 
> On Thu, Jun 26, 2025 at 10:36:42AM +0200, Vitaly Wool wrote:
>> void * __must_check __realloc_size(2)
>> -rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
>> +rust_helper_krealloc(const void *objp, size_t new_size, unsigned long align, gfp_t flags)
>> {
>> return krealloc(objp, new_size, flags);
>> }
>> 
>> void * __must_check __realloc_size(2)
>> -rust_helper_kvrealloc(const void *p, size_t size, gfp_t flags)
>> +rust_helper_kvrealloc(const void *p, size_t size, unsigned long align, gfp_t flags)
>> {
>> return kvrealloc(p, size, flags);
>> }
> 
> I think you forgot to add comments explaining why we have the additional
> discarded align argument.
> 
> Also please keep those helpers as they are. You can write an identical inline
> function in Rust that discards the align argument and calls bindings::krealloc,
> etc.
> 
> For instance:
> 
> unsafe extern "C" fn krealloc_align(
>    ptr: *const c_void,
>    size: usize,
>    _align: c_ulong
>    flags: u32,
> ) -> *mut c_void {
>    bindings::krealloc(ptr, size, flags)
> }
> 

Ugh. This is indeed a mistake from my side but I don’t quite agree with your variant here too.
The thing is that the new patchset has a patch #2 which adds kvrealloc_node and realloc_node so this chunk IMO should have looked like

-rust_helper_kvrealloc(const void *p, size_t size, gfp_t flags)
+rust_helper_kvrealloc(const void *p, size_t size, unsigned long align, gfp_t flags)
 {
 -      return kvrealloc(p, size, flags);
 +      return kvrealloc_node(p, size, align, flags, NUMA_NO_NODE);

 }

…exactly like for vmalloc, see also my comment below.

>> diff --git a/rust/helpers/vmalloc.c b/rust/helpers/vmalloc.c
>> index 80d34501bbc0..4618c0b79283 100644
>> --- a/rust/helpers/vmalloc.c
>> +++ b/rust/helpers/vmalloc.c
>> @@ -3,7 +3,7 @@
>> #include <linux/vmalloc.h>
>> 
>> void * __must_check __realloc_size(2)
>> -rust_helper_vrealloc(const void *p, size_t size, gfp_t flags)
>> +rust_helper_vrealloc(const void *p, size_t size, unsigned long align, gfp_t flags)
>> {
>> - return vrealloc(p, size, flags);
>> + return vrealloc_node(p, size, align, flags, NUMA_NO_NODE);
>> }
> 
> Same here, just make this a "real" helper for vrealloc_node() and create a Rust
> function vrealloc_align() like in the example above.

Wait, why? What’s the use of vrealloc() if it doesn’t provide the align functionality that we need?
> 
>> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
>> index aa2dfa9dca4c..a0d78c497974 100644
>> --- a/rust/kernel/alloc/allocator.rs
>> +++ b/rust/kernel/alloc/allocator.rs
>> @@ -58,7 +58,7 @@ fn aligned_size(new_layout: Layout) -> usize {
>> ///
>> /// One of the following: `krealloc`, `vrealloc`, `kvrealloc`.
>> struct ReallocFunc(
>> -    unsafe extern "C" fn(*const crate::ffi::c_void, usize, u32) -> *mut crate::ffi::c_void,
>> +    unsafe extern "C" fn(*const crate::ffi::c_void, usize, usize, u32) -> *mut crate::ffi::c_void,
> 
> Should be c_ulong instead of usize.
> 

Noted.

>> );
>> 
>> impl ReallocFunc {
>> @@ -110,7 +110,7 @@ unsafe fn call(
>>         // - Those functions provide the guarantees of this function.
>>         let raw_ptr = unsafe {
>>             // If `size == 0` and `ptr != NULL` the memory behind the pointer is freed.
>> -            self.0(ptr.cast(), size, flags.0).cast()
>> +            self.0(ptr.cast(), size, layout.align(), flags.0).cast()
>>         };
>> 
>>         let ptr = if size == 0 {
>> @@ -152,12 +152,6 @@ unsafe fn realloc(
>>         old_layout: Layout,
>>         flags: Flags,
>>     ) -> Result<NonNull<[u8]>, AllocError> {
>> -        // TODO: Support alignments larger than PAGE_SIZE.
>> -        if layout.align() > bindings::PAGE_SIZE {
>> -            pr_warn!("Vmalloc does not support alignments larger than PAGE_SIZE yet.\n");
>> -            return Err(AllocError);
>> -        }
>> -
>>         // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
>>         // allocated with this `Allocator`.
>>         unsafe { ReallocFunc::VREALLOC.call(ptr, layout, old_layout, flags) }
>> @@ -176,12 +170,6 @@ unsafe fn realloc(
>>         old_layout: Layout,
>>         flags: Flags,
>>     ) -> Result<NonNull<[u8]>, AllocError> {
>> -        // TODO: Support alignments larger than PAGE_SIZE.
>> -        if layout.align() > bindings::PAGE_SIZE {
>> -            pr_warn!("KVmalloc does not support alignments larger than PAGE_SIZE yet.\n");
>> -            return Err(AllocError);
>> -        }
> 
> Didn't you propose to use VREALLOC if layout.align() > bindings::PAGE_SIZE?
> 

I did, and this is what happens on the C side now, please see the #2 patch in series.
I think it’s better this way because of uniformity but I don’t have a strong opinion on this.

>>         // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
>>         // allocated with this `Allocator`.
>>         unsafe { ReallocFunc::KVREALLOC.call(ptr, layout, old_layout, flags) }
>