From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx162.postini.com [74.125.245.162]) by kanga.kvack.org (Postfix) with SMTP id A08A46B005D for ; Mon, 10 Sep 2012 10:48:13 -0400 (EDT) Received: by eeke49 with SMTP id e49so1400476eek.14 for ; Mon, 10 Sep 2012 07:48:12 -0700 (PDT) Subject: Re: [PATCH v2 10/10] thp: implement refcounting for huge zero page From: Eric Dumazet In-Reply-To: <20120910144438.GA31697@otc-wbsnb-06> References: <1347282813-21935-1-git-send-email-kirill.shutemov@linux.intel.com> <1347282813-21935-11-git-send-email-kirill.shutemov@linux.intel.com> <1347285759.1234.1645.camel@edumazet-glaptop> <20120910144438.GA31697@otc-wbsnb-06> Content-Type: text/plain; charset="UTF-8" Date: Mon, 10 Sep 2012 16:48:07 +0200 Message-ID: <1347288487.1234.1692.camel@edumazet-glaptop> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Andrew Morton , Andrea Arcangeli , linux-mm@kvack.org, Andi Kleen , "H. Peter Anvin" , linux-kernel@vger.kernel.org, "Kirill A. Shutemov" 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" > > > > > > 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org