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From: Cong Wang <xiyou.wangcong@gmail.com>
Subject: Re: [RFC PATCH 4/4] mm: change slob's struct page definition to
 accomodate struct page changes
Date: Tue, 3 Jul 2012 10:22:46 +0000 (UTC)
Message-ID: <jsuh5m$fdo$1@dough.gmane.org>
References: <1341287837-7904-1-git-send-email-jiang.liu@huawei.com>
 <1341287837-7904-4-git-send-email-jiang.liu@huawei.com>
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On Tue, 03 Jul 2012 at 03:57 GMT, Jiang Liu <jiang.liu@huawei.com> wrote:
> Changeset fc9bb8c768abe7ae10861c3510e01a95f98d5933 "mm: Rearrange struct page"
> rearranges fields in struct page, so change slob's "struct page" definition
> to accomodate the changes.
>

struct page gets changed too after that commit.

>  	union {
>  		struct {
>  			unsigned long flags;	/* mandatory */
> -			atomic_t _count;	/* mandatory */
> -			slobidx_t units;	/* free units left in page */
> -			unsigned long pad[2];
> +			unsigned long pad1;
>  			slob_t *free;		/* first free slob_t in page */
> +			slobidx_t units;	/* free units left in page */
> +			atomic_t _count;	/* mandatory */
>  			struct list_head list;	/* linked list of free pages */
>  		};
>  		struct page page;

I think we should put two BUILD_ON()'s for this, to prevent future breakage,
something like:

BUILD_BUG_ON(offsetof(struct slob_page, _count) != offsetof(struct
page, _count));
BUILD_BUG_ON(offsetof(struct slob_page, flags) != offsetof(struct
page, flags));


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