linux-kernel - Re: [PATCH 1/2] mm/zbud: reuse unbuddied[0] as buddied in zbud

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Date:   Mon, 7 Jun 2021 16:10:10 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <sjenning@...hat.com>, <ddstreet@...e.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] mm/zbud: reuse unbuddied[0] as buddied in zbud_pool

On Sat, 5 Jun 2021 15:51:40 +0800 Miaohe Lin <linmiaohe@...wei.com> wrote:

> Since commit 9d8c5b5284e4 ("mm: zbud: fix condition check on allocation
> size"), zbud_pool.unbuddied[0] is always unused. We can reuse it as buddied
> field to save some possible memory.
> 
> ...
>
> --- a/mm/zbud.c
> +++ b/mm/zbud.c
> @@ -96,7 +96,7 @@
>  struct zbud_pool {
>  	spinlock_t lock;
>  	struct list_head unbuddied[NCHUNKS];
> -	struct list_head buddied;
> +#define buddied unbuddied[0]
>  	struct list_head lru;
>  	u64 pages_nr;
>  	const struct zbud_ops *ops;

That looks a bit hacky.  Can we at least have a comment explaining
what's going on?

Would it be better to implement this with a union, rather than a #define?