linux-kernel - Re: [PATCH -mm 7/7] mm: thp: introduce compound_head_put_tail(), change get_futex

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Date:	Wed, 18 Dec 2013 20:28:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm 7/7] mm: thp: introduce compound_head_put_tail(),
 change get_futex_key() to use it

On Wed, Dec 18, 2013 at 08:20:08PM +0100, Oleg Nesterov wrote:
> +struct page *compound_head_put_tail(struct page *page)
> +{
> +	struct page *page_head;
> +	unsigned long flags;
> +
> +	if (!PageTail(page))
> +		return page;
> +
> +	page_head = compound_trans_head(page);
> +
> +	if (!__compound_tail_refcounted(page_head)) {

This barrier is missing a comment describing the ordering and the
pairing.

> +		smp_rmb();
> +		if (likely(PageTail(page)))
> +			return page_head;
> +		else
> +			return page;
> +	}
> +
> +	if (likely(get_lock_thp_head(page_head, page, &flags))) {
> +		if (put_page_testzero(page_head))
> +			VM_BUG_ON(1);
> +		atomic_dec(&page->_mapcount);
> +		compound_unlock_irqrestore(page_head, flags);
> +
> +		return page_head;
> +	}
> +
> +	return page;
> +}
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