Re: [patch] mm, hugetlb: close race when setting PageTail for gigantic pages

[Hillf Danton]

> Now that gigantic pages are dynamically allocatable, care must be taken
> to ensure that p->first_page is valid before setting PageTail.
> 
> If this isn't done, then it is possible to race and have compound_head()
> return NULL.
> 
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>

>  mm/hugetlb.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -917,7 +917,6 @@ static void prep_compound_gigantic_page(struct page *page, unsigned long order)
>  	__SetPageHead(page);
>  	__ClearPageReserved(page);
>  	for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
> -		__SetPageTail(p);
>  		/*
>  		 * For gigantic hugepages allocated through bootmem at
>  		 * boot, it's safer to be consistent with the not-gigantic
> @@ -933,6 +932,9 @@ static void prep_compound_gigantic_page(struct page *page, unsigned long order)
>  		__ClearPageReserved(p);
>  		set_page_count(p, 0);
>  		p->first_page = page;
> +		/* Make sure p->first_page is always valid for PageTail() */
> +		smp_wmb();
> +		__SetPageTail(p);
>  	}
>  }
> 
> --
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