Re: [PATCH v2 2/3] mm: don't hide potentially null memmap pointer in sparse_remove_one_section

[Michal Hocko <mhocko@kernel.org>]

On Wed 26-06-19 16:11:22, Alastair D'Silva wrote:
> From: Alastair D'Silva <alastair@d-silva.org>
> 
> By adding offset to memmap before passing it in to clear_hwpoisoned_pages,
> we hide a potentially null memmap from the null check inside
> clear_hwpoisoned_pages.
> 
> This patch passes the offset to clear_hwpoisoned_pages instead, allowing
> memmap to successfully peform it's null check.

Same issue with the changelog as the previous patch (missing WHY).

> 
> Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> ---
>  mm/sparse.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 57a1a3d9c1cf..1ec32aef5590 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -753,7 +753,8 @@ int __meminit sparse_add_one_section(int nid, unsigned long start_pfn,
>  
>  #ifdef CONFIG_MEMORY_HOTREMOVE
>  #ifdef CONFIG_MEMORY_FAILURE
> -static void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
> +static void clear_hwpoisoned_pages(struct page *memmap,
> +		unsigned long start, unsigned long count)
>  {
>  	int i;
>  
> @@ -769,7 +770,7 @@ static void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
>  	if (atomic_long_read(&num_poisoned_pages) == 0)
>  		return;
>  
> -	for (i = 0; i < nr_pages; i++) {
> +	for (i = start; i < start + count; i++) {
>  		if (PageHWPoison(&memmap[i])) {
>  			atomic_long_sub(1, &num_poisoned_pages);
>  			ClearPageHWPoison(&memmap[i]);
> @@ -777,7 +778,8 @@ static void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
>  	}
>  }
>  #else
> -static inline void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
> +static inline void clear_hwpoisoned_pages(struct page *memmap,
> +		unsigned long start, unsigned long count)
>  {
>  }
>  #endif
> @@ -824,7 +826,7 @@ void sparse_remove_one_section(struct zone *zone, struct mem_section *ms,
>  		ms->pageblock_flags = NULL;
>  	}
>  
> -	clear_hwpoisoned_pages(memmap + map_offset,
> +	clear_hwpoisoned_pages(memmap, map_offset,
>  			PAGES_PER_SECTION - map_offset);
>  	free_section_usemap(memmap, usemap, altmap);
>  }
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs