Re: [patch v2] mm, page_alloc: make first_page visible before PageTail

[Michal Hocko <mhocko@suse.cz>]

On Tue 04-02-14 16:22:53, David Rientjes wrote:
> Commit bf6bddf1924e ("mm: introduce compaction and migration for ballooned
> pages") introduces page_count(page) into memory compaction which
> dereferences page->first_page if PageTail(page).
> 
> Introduce a store memory barrier to ensure page->first_page is properly
> initialized so that code that does page_count(page) on pages off the lru
> always have a valid p->first_page.
> 
> Reported-by: Holger Kiehl <Holger.Kiehl@dwd.de>
> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  v2: with commentary, per checkpatch
> 
>  mm/page_alloc.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -369,9 +369,11 @@ void prep_compound_page(struct page *page, unsigned long order)
>  	__SetPageHead(page);
>  	for (i = 1; i < nr_pages; i++) {
>  		struct page *p = page + i;
> -		__SetPageTail(p);
>  		set_page_count(p, 0);
>  		p->first_page = page;
> +		/* Make sure p->first_page is always valid for PageTail() */
> +		smp_wmb();
> +		__SetPageTail(p);

Where is the pairing smp_rmb? I would expect it in comound_head.

>  	}
>  }
>  

-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>