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

Christian Borntraeger
2019-09-30T06:30:36Z

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>