linux-kernel - Re: [PATCH v2 10/10] thp: implement refcounting for huge zero page

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Date:	Mon, 10 Sep 2012 17:44:38 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v2 10/10] thp: implement refcounting for huge zero page

On Mon, Sep 10, 2012 at 04:02:39PM +0200, Eric Dumazet wrote:
> On Mon, 2012-09-10 at 16:13 +0300, Kirill A. Shutemov wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.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.

I fail to see why the second thread might loop forever. Long time yes, but
forever?

Yes, disabling preemption before alloc_pages() and enabling after
atomic_set() looks reasonable. Thanks.

> 
> If preemption already disabled, a comment would be nice.
> 
> 
> > +
> > +	/* We take additional reference here. It will be put back by shinker */
> 
> typo : shrinker

Thx.

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

-- 
 Kirill A. Shutemov

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