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Subject: Re: [PATCH v2 10/10] thp: implement refcounting for huge zero page
From: Eric Dumazet <eric.dumazet@gmail.com>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>, linux-mm@kvack.org,
        Andi Kleen <ak@linux.intel.com>,
        "H. Peter Anvin" <hpa@linux.intel.com>, linux-kernel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill@shutemov.name>
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Date: Mon, 10 Sep 2012 16:02:39 +0200
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On Mon, 2012-09-10 at 16:13 +0300, Kirill A. Shutemov wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> 
> H. Peter Anvin doesn't like huge zero page which sticks in memory forever
> after the first allocation. Here's implementation of lockless refcounting
> for huge zero page.
> 
...

> +static unsigned long get_huge_zero_page(void)
> +{
> +	struct page *zero_page;
> +retry:
> +	if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
> +		return ACCESS_ONCE(huge_zero_pfn);
> +
> +	zero_page = alloc_pages(GFP_TRANSHUGE | __GFP_ZERO, HPAGE_PMD_ORDER);
> +	if (!zero_page)
> +		return 0;
> +	if (cmpxchg(&huge_zero_pfn, 0, page_to_pfn(zero_page))) {
> +		__free_page(zero_page);
> +		goto retry;
> +	}

This might break if preemption can happen here ?

The second thread might loop forever because huge_zero_refcount is 0,
and huge_zero_pfn not zero.

If preemption already disabled, a comment would be nice.


> +
> +	/* We take additional reference here. It will be put back by shinker */

typo : shrinker

> +	atomic_set(&huge_zero_refcount, 2);
> +	return ACCESS_ONCE(huge_zero_pfn);
> +}
> +