linux-kernel - Re: [PATCH v27 3/4] mm/page_poison: expose page_poisoning

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Date:   Wed, 7 Feb 2018 20:34:16 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mhocko@...nel.org, akpm@...ux-foundation.org,
        pbonzini@...hat.com, liliang.opensource@...il.com,
        yang.zhang.wz@...il.com, quan.xu0@...il.com, nilal@...hat.com,
        riel@...hat.com, huangzhichao@...wei.com
Subject: Re: [PATCH v27 3/4] mm/page_poison: expose page_poisoning_enabled to
 kernel modules

On Wed, Feb 07, 2018 at 02:54:30PM +0800, Wei Wang wrote:
> In some usages, e.g. virtio-balloon, a kernel module needs to know if
> page poisoning is in use. This patch exposes the page_poisoning_enabled
> function to kernel modules.
> 
> Signed-off-by: Wei Wang <wei.w.wang@...el.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Michael S. Tsirkin <mst@...hat.com>
> ---
>  mm/page_poison.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/page_poison.c b/mm/page_poison.c
> index e83fd44..c08d02a 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -30,6 +30,11 @@ bool page_poisoning_enabled(void)
>  		debug_pagealloc_enabled()));
>  }
>  
> +/**
> + * page_poisoning_enabled - check if page poisoning is enabled
> + *
> + * Return true if page poisoning is enabled, or false if not.
> + */
>  static void poison_page(struct page *page)
>  {
>  	void *addr = kmap_atomic(page);
> @@ -37,6 +42,7 @@ static void poison_page(struct page *page)
>  	memset(addr, PAGE_POISON, PAGE_SIZE);
>  	kunmap_atomic(addr);
>  }
> +EXPORT_SYMBOL_GPL(page_poisoning_enabled);
>  
>  static void poison_pages(struct page *page, int n)
>  {

Looks like both the comment and the export are in the wrong place.
I'm a bit concerned that callers also in fact poke at the
PAGE_POISON - exporting that seems to be more of an accident
as it's only used without page_poisoning.c - it might be
better to have page_poisoning_enabled get u8 * and set it.

> -- 
> 2.7.4