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Subject: Re: [PATCH 2/5] mm: don't hide potentially null memmap pointer in
 sparse_remove_one_section
To:     Alastair D'Silva <alastair@au1.ibm.com>, alastair@d-silva.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.com>,
        Michal Hocko <mhocko@suse.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Wei Yang <richard.weiyang@gmail.com>,
        Juergen Gross <jgross@suse.com>, Qian Cai <cai@lca.pw>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Jiri Kosina <jkosina@suse.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Mukesh Ojha <mojha@codeaurora.org>,
        Arun KS <arunks@codeaurora.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Baoquan He <bhe@redhat.com>,
        Logan Gunthorpe <logang@deltatee.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20190617043635.13201-1-alastair@au1.ibm.com>
 <20190617043635.13201-3-alastair@au1.ibm.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <e3dd8031-2091-4d65-7c76-0ec7283f92f5@redhat.com>
Date:   Mon, 17 Jun 2019 09:26:07 +0200
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On 17.06.19 06:36, 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,
> 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 <alastair@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