linux-kernel - Re: [PATCH v4 2/2] mm/vmalloc.c: Modify struct vmap

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Date:   Fri, 12 Jul 2019 06:49:55 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Pengfei Li <lpf.vector@...il.com>
Cc:     akpm@...ux-foundation.org, urezki@...il.com, rpenyaev@...e.de,
        peterz@...radead.org, guro@...com, rick.p.edgecombe@...el.com,
        rppt@...ux.ibm.com, aryabinin@...tuozzo.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] mm/vmalloc.c: Modify struct vmap_area to reduce
 its size

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;
	};