Re: [PATCH v4 2/2] mm/vmalloc.c: Modify struct vmap_area to reduce its size

[Matthew Wilcox]

On Fri, Jul 12, 2019 at 08:02:13PM +0800, Pengfei Li wrote:
> +++ b/include/linux/vmalloc.h
> @@ -51,15 +51,37 @@ struct vmap_area {
>       unsigned long va_start;
>       unsigned long va_end;
>  
> -     /*
> -      * Largest available free size in subtree.
> -      */
> -     unsigned long subtree_max_size;
> -     unsigned long flags;
> -     struct rb_node rb_node;         /* address sorted rbtree */
> -     struct list_head list;          /* address sorted list */
> -     struct llist_node purge_list;    /* "lazy purge" list */
> -     struct vm_struct *vm;
> +     union {
> +             /* In rbtree and list sorted by address */
> +             struct {
> +                     union {
> +                             /*
> +                              * In "busy" rbtree and list.
> +                              * rbtree root: vmap_area_root
> +                              * list head:   vmap_area_list
> +                              */
> +                             struct vm_struct *vm;
> +
> +                             /*
> +                              * In "free" rbtree and list.
> +                              * rbtree root: free_vmap_area_root
> +                              * list head:   free_vmap_area_list
> +                              */
> +                             unsigned long subtree_max_size;
> +                     };
> +
> +                     struct rb_node rb_node;
> +                     struct list_head list;
> +             };
> +
> +             /*
> +              * In "lazy purge" list.
> +              * llist head: vmap_purge_list
> +              */
> +             struct {
> +                     struct llist_node purge_list;
> +             };

I don't think you need struct union struct union.  Because llist_node
is just a pointer, you can get the same savings with just:

        union {
                struct llist_node purge_list;
                struct vm_struct *vm;
                unsigned long subtree_max_size;
        };