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Date:   Fri, 26 Oct 2018 11:31:26 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Igor Stoppa <igor.stoppa@gmail.com>
Cc:     Mimi Zohar <zohar@linux.vnet.ibm.com>,
        Kees Cook <keescook@chromium.org>,
        Matthew Wilcox <willy@infradead.org>,
        Dave Chinner <david@fromorbit.com>,
        James Morris <jmorris@namei.org>,
        Michal Hocko <mhocko@kernel.org>,
        kernel-hardening@lists.openwall.com,
        linux-integrity@vger.kernel.org,
        linux-security-module@vger.kernel.org, igor.stoppa@huawei.com,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Laura Abbott <labbott@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Pekka Enberg <penberg@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Lihao Liang <lianglihao@huawei.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 12/17] prmem: linked list: set alignment
Message-ID: <20181026093126.GC3159@worktop.c.hoisthospitality.com>
References: <20181023213504.28905-1-igor.stoppa@huawei.com>
 <20181023213504.28905-13-igor.stoppa@huawei.com>
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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@huawei.com>
> CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> CC: Andrew Morton <akpm@linux-foundation.org>
> CC: Masahiro Yamada <yamada.masahiro@socionext.com>
> CC: Alexey Dobriyan <adobriyan@gmail.com>
> CC: Pekka Enberg <penberg@kernel.org>
> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> CC: Lihao Liang <lianglihao@huawei.com>
> CC: linux-kernel@vger.kernel.org
> ---
>  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).