Re: [PATCH 3/4] hugetlb, rmap: fix confusing page locking inhugetlb_cow()

[Linus Torvalds]

On Fri, 10 Sep 2010, Naoya Horiguchi wrote:
>  
> -	if (!pagecache_page) {
> -		page = pte_page(entry);
> +	/*
> +	 * hugetlb_cow() requires page locks of pte_page(entry) and
> +	 * pagecache_page, so here we need take the former one
> +	 * when page != pagecache_page or !pagecache_page.
> +	 */
> +	page = pte_page(entry);
> +	if (page != pagecache_page)
>  		lock_page(page);

Why isn't this a potential deadlock? You have two pages, and lock them 
both. Is there some ordering guarantee that says that 'pagecache_page' and 
'page' will always be in a certain relationship so that you cannot get 
A->B and B->A lock ordering?

Please document that ordering rule if so.

			Linus
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