Re: [PATCH 12/17] prmem: linked list: set alignment

[Peter Zijlstra]

On Wed, Oct 24, 2018 at 12:34:59AM +0300, Igor Stoppa wrote:
> As preparation to using write rare on the nodes of various types of
> lists, specify that the fields in the basic data structures must be
> aligned to sizeof(void *)
> 
> It is meant to ensure that any static allocation will not cross a page
> boundary, to allow pointers to be updated in one step.
> 
> Signed-off-by: Igor Stoppa <igor.stoppa@xxxxxxxxxx>
> CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> CC: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> CC: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> CC: Pekka Enberg <penberg@xxxxxxxxxx>
> CC: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> CC: Lihao Liang <lianglihao@xxxxxxxxxx>
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  include/linux/types.h | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/types.h b/include/linux/types.h
> index 9834e90aa010..53609bbdcf0f 100644
> --- a/include/linux/types.h
> +++ b/include/linux/types.h
> @@ -183,17 +183,29 @@ typedef struct {
>  } atomic64_t;
>  #endif
>  
> +#ifdef CONFIG_PRMEM
>  struct list_head {
> -	struct list_head *next, *prev;
> -};
> +	struct list_head *next __aligned(sizeof(void *));
> +	struct list_head *prev __aligned(sizeof(void *));
> +} __aligned(sizeof(void *));
>  
> -struct hlist_head {
> -	struct hlist_node *first;
> +struct hlist_node {
> +	struct hlist_node *next __aligned(sizeof(void *));
> +	struct hlist_node **pprev __aligned(sizeof(void *));
> +} __aligned(sizeof(void *));

Argh.. are we really supporting platforms that do not naturally align
this? If so, which and can't we fix those?

Also, if you force alignment on a member, the structure as a whole
inherits the largest member alignment.

Also, you made something that was simple an unreadable mess without
proper justification (ie. you fail to show need).