Re: [PATCH] mm/page_alloc: fix a crash in free_pages_prepare()

[Christian Borntraeger]

On 27.09.19 23:59, Andrew Morton wrote:
> On Fri, 27 Sep 2019 17:28:06 -0400 Qian Cai <cai@xxxxxx> 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 aboveÂkernel_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@xxxxxxxxxx>