linux-kernel - Re: [PATCH v2 2/3] mm: don't hide potentially null memmap pointer in sparse_remove_one

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Date:   Wed, 26 Jun 2019 08:23:44 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Alastair D'Silva <alastair@....ibm.com>
Cc:     alastair@...ilva.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Oscar Salvador <osalvador@...e.de>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Baoquan He <bhe@...hat.com>, Qian Cai <cai@....pw>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 2/3] mm: don't hide potentially null memmap pointer in
 sparse_remove_one_section

On Wed 26-06-19 16:11:22, Alastair D'Silva wrote:
> From: Alastair D'Silva <alastair@...ilva.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@...ilva.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