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

[Michal Hocko]

On Tue 27-08-19 15:36:55, Alastair D'Silva wrote:
> From: Alastair D'Silva <alastair@xxxxxxxxxxx>
> 
> By adding offset to memmap before passing it in to clear_hwpoisoned_pages,
> we hide a theoretically null memmap from the null check inside
> clear_hwpoisoned_pages.

Isn't that other way around? Calculating the offset struct page pointer
will actually make the null check effective. Besides that I cannot
really see how pfn_to_page would return NULL. I have to confess that I
cannot really see how offset could lead to a NULL struct page either and
I strongly suspect that the NULL check is not really needed. Maybe it
used to be in the past.

> This patch passes the offset to clear_hwpoisoned_pages instead, allowing
> memmap to successfully perform it's null check.

I do not see any improvement in this patch. It just adds a new argument
unnecessarily.

> Signed-off-by: Alastair D'Silva <alastair@xxxxxxxxxxx>
> ---
>  mm/sparse.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index e41917a7e844..3ff84e627e58 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -882,7 +882,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
>  }
>  
>  #ifdef CONFIG_MEMORY_FAILURE
> -static void clear_hwpoisoned_pages(struct page *memmap, int nr_pages)
> +static void clear_hwpoisoned_pages(struct page *memmap, int start, int count)
>  {
>  	int i;
>  
> @@ -898,7 +898,7 @@ 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])) {
>  			num_poisoned_pages_dec();
>  			ClearPageHWPoison(&memmap[i]);
> @@ -906,7 +906,8 @@ 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, int start,
> +		int count)
>  {
>  }
>  #endif
> @@ -915,7 +916,7 @@ void sparse_remove_section(struct mem_section *ms, unsigned long pfn,
>  		unsigned long nr_pages, unsigned long map_offset,
>  		struct vmem_altmap *altmap)
>  {
> -	clear_hwpoisoned_pages(pfn_to_page(pfn) + map_offset,
> +	clear_hwpoisoned_pages(pfn_to_page(pfn), map_offset,
>  			nr_pages - map_offset);
>  	section_deactivate(pfn, nr_pages, altmap);
>  }
> -- 
> 2.21.0
> 

-- 
Michal Hocko
SUSE Labs