Re: [PATCH stable-4.4 1/2] mm: check the return value of lookup_page_ext for all call sites

[Ben Hutchings <ben.hutchings@codethink.co.uk>]

On Wed, 2017-11-22 at 13:09 +0100, Michal Hocko wrote:
> From: Yang Shi <yang.shi@linaro.org>
> 
> commit f86e4271978bd93db466d6a95dad4b0fdcdb04f6 upstream.
> 
> Per the discussion with Joonsoo Kim [1], we need check the return value
> of lookup_page_ext() for all call sites since it might return NULL in
> some cases, although it is unlikely, i.e.  memory hotplug.
> 
> Tested with ltp with "page_owner=0".
[...]
> --- a/mm/debug-pagealloc.c
> +++ b/mm/debug-pagealloc.c
> @@ -34,6 +34,8 @@ static inline void set_page_poison(struct page
> *page)
>  	struct page_ext *page_ext;
>  
>  	page_ext = lookup_page_ext(page);
> +	if (page_ext)
> +		return;

This, and the other checks added to debug-pagealloc.c, are reversed. 
(This is specific to the 4.4 backport - in the upstream version these
functions are in mm/page_poison.c and were patched correctly.)

Ben.

>  	__set_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
>  }
>  
> @@ -42,6 +44,8 @@ static inline void clear_page_poison(struct page
> *page)
>  	struct page_ext *page_ext;
>  
>  	page_ext = lookup_page_ext(page);
> +	if (page_ext)
> +		return;
>  	__clear_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
>  }
>  
> @@ -50,6 +54,8 @@ static inline bool page_poison(struct page *page)
>  	struct page_ext *page_ext;
>  
>  	page_ext = lookup_page_ext(page);
> +	if (page_ext)
> +		return false;
>  	return test_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
>  }
>  
[...]

-- 
Ben Hutchings
Software Developer, Codethink Ltd.