Re: [PATCH 2/5] mm: don't hide potentially null memmap pointer in sparse_remove_one_section

[David Hildenbrand]

On 17.06.19 06:36, Alastair D'Silva wrote:
> From: Alastair D'Silva <alast...@d-silva.org>
> 
> By adding offset to memmap before passing it in to clear_hwpoisoned_pages,
> is hides 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.
> 
> Signed-off-by: Alastair D'Silva <alast...@d-silva.org>
> ---
>  mm/sparse.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 104a79fedd00..66a99da9b11b 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -746,12 +746,14 @@ int __meminit sparse_add_one_section(int nid, unsigned 
> long start_pfn,
>               kfree(usemap);
>               __kfree_section_memmap(memmap, altmap);
>       }
> +
>       return ret;
>  }
>  
>  #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 map_offset, int nr_pages)
>  {
>       int i;
>  
> @@ -767,7 +769,7 @@ 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 = map_offset; i < nr_pages; i++) {
>               if (PageHWPoison(&memmap[i])) {
>                       atomic_long_sub(1, &num_poisoned_pages);
>                       ClearPageHWPoison(&memmap[i]);
> @@ -775,7 +777,8 @@ 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 map_offset, int nr_pages)

I somewhat dislike that map_offset modifies nr_pages internally.

I would prefer decoupling both and passing the actual number of pages to
clear instead:

clear_hwpoisoned_pages(memmap, map_offset,
                       PAGES_PER_SECTION - map_offset);


>  {
>  }
>  #endif
> @@ -822,8 +825,7 @@ void sparse_remove_one_section(struct zone *zone, struct 
> mem_section *ms,
>               ms->pageblock_flags = NULL;
>       }
>  
> -     clear_hwpoisoned_pages(memmap + map_offset,
> -                     PAGES_PER_SECTION - map_offset);
> +     clear_hwpoisoned_pages(memmap, map_offset, PAGES_PER_SECTION);
>       free_section_usemap(memmap, usemap, altmap);
>  }
>  #endif /* CONFIG_MEMORY_HOTREMOVE */
> 


-- 

Thanks,

David / dhildenb