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Subject: Re: [PATCH] mm/page_alloc: fix a crash in free_pages_prepare()
To:     Andrew Morton <akpm@linux-foundation.org>, Qian Cai <cai@lca.pw>
Cc:     heiko.carstens@de.ibm.com, gor@linux.ibm.com,
        linux-s390@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <1569613623-16820-1-git-send-email-cai@lca.pw>
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 <20190927145945.846a3f3405d3af066827d3f5@linux-foundation.org>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
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Date:   Mon, 30 Sep 2019 08:30:25 +0200
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On 27.09.19 23:59, Andrew Morton wrote:
> On Fri, 27 Sep 2019 17:28:06 -0400 Qian Cai <cai@lca.pw> wrote:
> 
>>>
>>> So I think you've moved the arch_free_page() to be after the final
>>> thing which can access page contents, yes?  If so, we should have a
>>> comment in free_pages_prepare() to attmept to prevent this problem from
>>> reoccurring as the code evolves?
>>
>> Right, something like this above arch_free_page() there?
>>
>> /*
>>  * It needs to be just above kernel_map_pages(), as s390 could mark those
>>  * pages unused and then trigger a fault when accessing.
>>  */
> 
> I did this.
> 
> --- a/mm/page_alloc.c~mm-page_alloc-fix-a-crash-in-free_pages_prepare-fix
> +++ a/mm/page_alloc.c
> @@ -1179,7 +1179,13 @@ static __always_inline bool free_pages_p
>  		kernel_init_free_pages(page, 1 << order);
>  
>  	kernel_poison_pages(page, 1 << order, 0);
> +	/*
> +	 * arch_free_page() can make the page's contents inaccessible.  s390
> +	 * does this.  So nothing which can access the page's contents should
> +	 * happen after this.
> +	 */
>  	arch_free_page(page, order);
> +
>  	if (debug_pagealloc_enabled())
>  		kernel_map_pages(page, 1 << order, 0);


With that Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>