linux-kernel - Re: [PATCHv4 14/39] thp, mm: rewrite delete_from_page

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Date:	Fri, 07 Jun 2013 08:56:10 -0700
From:	Dave Hansen <dave@...1.net>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 14/39] thp, mm: rewrite delete_from_page_cache() to
 support huge pages

On 06/07/2013 08:10 AM, Kirill A. Shutemov wrote:
> +	/*
> +	 * When we add a huge page to page cache we take only reference to head
> +	 * page, but on split we need to take addition reference to all tail
> +	 * pages since they are still in page cache after splitting.
> +	 */
> +	init_tail_refcount = PageAnon(page) ? 0 : 1;

What's the "init" for in the name?

In add_to_page_cache_locked() in patch 12/39, you do
> +       spin_lock_irq(&mapping->tree_lock);
> +       for (i = 0; i < nr; i++) {
> +               page_cache_get(page + i);

That looks to me to be taking references to the tail pages.  What gives? :)

>  	for (i = HPAGE_PMD_NR - 1; i >= 1; i--) {
>  		struct page *page_tail = page + i;
>  
> @@ -1587,8 +1595,9 @@ static void __split_huge_page_refcount(struct page *page,
>  		 * atomic_set() here would be safe on all archs (and
>  		 * not only on x86), it's safer to use atomic_add().
>  		 */
> -		atomic_add(page_mapcount(page) + page_mapcount(page_tail) + 1,
> -			   &page_tail->_count);
> +		atomic_add(init_tail_refcount + page_mapcount(page) +
> +				page_mapcount(page_tail) + 1,
> +				&page_tail->_count);
>  
>  		/* after clearing PageTail the gup refcount can be released */
>  		smp_mb();

This does look much better in general, though.

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