Re: [PATCH] mm/memory-failure: fix race with compound page split/merge

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

# CCed Andrew,

On Mon, Apr 18, 2016 at 02:43:45PM +0300, Konstantin Khlebnikov wrote:
> Get_hwpoison_page() must recheck relation between head and tail pages.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

Looks good to me. Without this recheck, the race causes kernel to pin
an irrelevant page, and finally makes kernel crash for refcount mismcach...

Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

> ---
>  mm/memory-failure.c |   10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 78f5f2641b91..ca5acee53b7a 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -888,7 +888,15 @@ int get_hwpoison_page(struct page *page)
>  		}
>  	}
>  
> -	return get_page_unless_zero(head);
> +	if (get_page_unless_zero(head)) {
> +		if (head == compound_head(page))
> +			return 1;
> +
> +		pr_info("MCE: %#lx cannot catch tail\n", page_to_pfn(page));

Recently Chen Yucong replaced the label "MCE:" with "Memory failure:",
but the resolution is trivial, I think.

Thanks,
Naoya Horiguchi

> +		put_page(head);
> +	}
> +
> +	return 0;
>  }
>  EXPORT_SYMBOL_GPL(get_hwpoison_page);
>  
>