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

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Date:	Mon, 10 Sep 2012 16:48:07 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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, 2012-09-10 at 17:44 +0300, Kirill A. Shutemov wrote:
> 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 you have one online cpu, and the second thread is real time or
something like that, it wont give cpu back to preempted thread.



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