Re: [RFC PATCH 2/7] mm: use rcu_ptr instead of rcu_head

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Feb 06, 2026 at 06:34:05PM +0900, Harry Yoo wrote:
> When slab objects are freed with kfree_rcu() and not call_rcu(),
> using struct rcu_head (16 bytes on 64-bit) is unnecessary and
> struct rcu_ptr (8 bytes on 64-bit) is enough. Save one pointer
> per slab object by using struct rcu_ptr.
> 
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  include/linux/list_lru.h | 2 +-
>  include/linux/shrinker.h | 2 +-
>  mm/vmalloc.c             | 4 ++--
>  3 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
> index fe739d35a864..c79bccb7dafa 100644
> --- a/include/linux/list_lru.h
> +++ b/include/linux/list_lru.h
> @@ -37,7 +37,7 @@ struct list_lru_one {
>  };
>  
>  struct list_lru_memcg {
> -	struct rcu_head		rcu;
> +	struct rcu_ptr		rcu;
>  	/* array of per cgroup per node lists, indexed by node id */
>  	struct list_lru_one	node[];
>  };
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 1a00be90d93a..bad20de2803a 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -19,7 +19,7 @@ struct shrinker_info_unit {
>  };
>  
>  struct shrinker_info {
> -	struct rcu_head rcu;
> +	struct rcu_ptr rcu;
>  	int map_nr_max;
>  	struct shrinker_info_unit *unit[];
>  };
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 41dd01e8430c..89c781dcab58 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2596,7 +2596,7 @@ struct vmap_block {
>  	DECLARE_BITMAP(used_map, VMAP_BBMAP_BITS);
>  	unsigned long dirty_min, dirty_max; /*< dirty range */
>  	struct list_head free_list;
> -	struct rcu_head rcu_head;
> +	struct rcu_ptr rcu;
>  	struct list_head purge;
>  	unsigned int cpu;
>  };
>
Why this change is needed?

If you want to save 8 bytes of vmap_block structure, then i
do not see a big gain here. We do not have so many vmap_block
objects.

Am i missing something here? :)

--
Uladzislau Rezki