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Date: Wed, 18 Dec 2013 20:28:55 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Jones <davej@redhat.com>, Darren Hart <dvhart@linux.intel.com>,
        Mel Gorman <mgorman@suse.de>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH -mm 7/7] mm: thp: introduce compound_head_put_tail(),
 change get_futex_key() to use it
Message-ID: <20131218192855.GE16438@laptop.programming.kicks-ass.net>
References: <20131218191913.GA6464@redhat.com>
 <20131218192008.GA8343@redhat.com>
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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;
> +}