Re: rust compile failure in next-20260730

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 8/4/26 12:10, Gary Guo wrote:
> On Mon Aug 3, 2026 at 10:04 PM BST, Vlastimil Babka (SUSE) wrote:
>> On 8/3/26 17:32, Gary Guo wrote:
>>> On Mon Aug 3, 2026 at 4:10 PM BST, Vlastimil Babka (SUSE) wrote:
>>>> On 8/3/26 16:23, Boqun Feng wrote:
>>>>> On Mon, Aug 03, 2026 at 02:57:49PM +0100, Gary Guo wrote:
>>>>>> 
>>>>>> We could also unconditionally use `kvfree_rcu_head` here, and
>>>>>> add
>>>>>> 
>>>>>>     #[cfg(not(CONFIG_KVFREE_RCU_BATCHED))]
>>>>>>     pub type kvfree_rcu_head = callback_head;
>>>>>> 
>>>>>> to bindings.rs?
>>>>>> 
>>>>> 
>>>>> This option is currently not maintainable unless it becomes a
>>>>> maintainer-aware way to handle things like this.
>>>>> 
>>>>>> (Or even better, changing `#define` to `typedef` so bindgen takes care of
>>>>>> everything).
>>>>>> 
>>>>> 
>>>>> Yes, this is better IMO, but it's up to slab maintainers. :-)
>>>>
>>>> Can you elaborate a bit please, how would that look like?
>>> 
>>> I was thinking of doing `typedef struct rcu_head kvfree_rcu_head;` but of course
>>> that didn't work because you can't use typedef to create `kvfree_rcu_head` :)
>>> 
>>> However, something like this could work?
>>> 
>>>     #ifdef CONFIG_KVFREE_RCU_BATCHED
>>>     ...
>>>     #else
>>>     struct kvfree_rcu_head {
>>>         struct rcu_head head;
>>>     };
>>>     #endif
>>> 
>>> and everywhere add a cast everywhere that expects kvfree_rcu_head == rcu_head.
>>> 
>>> but this would indeed be more complex :(
>>
>> So you mean like this? Doesn't seem so complex and seems to compile here
>> with CONFIG_KVFREE_RCU_BATCHED both disabled and enabled.
> 
> I thought that a lot more places have to be updated, but it looks from your diff
> below that this is simple enough.

Thanks. The slab/for-next branch now includes the slab changes.
The merge commit should thus perform the rust/kernel/sync/poll.rs change.

> Best,
> Gary
> 
>> I can apply the slab part in slab tree, but AFAICS the poll.rs
>> change still needs to be done in the merge commit. Unless there's
>> some way to make things conditional on whether kvfree_rcu_head exists?
>>
>> diff --git a/include/linux/types.h b/include/linux/types.h
>> index 79bf419c69e4..53e0adca4b9f 100644
>> --- a/include/linux/types.h
>> +++ b/include/linux/types.h
>> @@ -262,7 +262,9 @@ struct kvfree_rcu_head {
>>  	struct kvfree_rcu_head *next;
>>  };
>>  #else
>> -#define kvfree_rcu_head rcu_head
>> +struct kvfree_rcu_head {
>> +	struct rcu_head head;
>> +};
>>  #endif
>>  
>>  typedef void (*rcu_callback_t)(struct rcu_head *head);
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index 64845ac81b79..aecbe9b9df4c 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -1325,11 +1325,11 @@ EXPORT_SYMBOL_GPL(kfree_call_rcu_nolock);
>>  
>>  #ifndef CONFIG_KVFREE_RCU_BATCHED
>>  
>> -void kvfree_call_rcu(struct rcu_head *head, void *ptr)
>> +void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr)
>>  {
>>  	if (head) {
>>  		kasan_record_aux_stack(ptr);
>> -		call_rcu(head, kvfree_rcu_cb);
>> +		call_rcu(&head->head, kvfree_rcu_cb);
>>  		return;
>>  	}
>>  
>> diff --git a/rust/kernel/sync/poll.rs b/rust/kernel/sync/poll.rs
>> index 684dfa242b1a..f3cdf95db12d 100644
>> --- a/rust/kernel/sync/poll.rs
>> +++ b/rust/kernel/sync/poll.rs
>> @@ -124,7 +124,7 @@ pub struct PollCondVarBox {
>>  struct PollCondVarBoxInner {
>>      #[pin]
>>      inner: PollCondVar,
>> -    rcu: Opaque<bindings::callback_head>,
>> +    rcu: Opaque<bindings::kvfree_rcu_head>,
>>  }
>>  
>>  // SAFETY: PollCondVar is Send
> 
>