Re: rust compile failure in next-20260730

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

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 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