Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758247AbaKULrr (ORCPT ); Fri, 21 Nov 2014 06:47:47 -0500 Received: from mta-out1.inet.fi ([62.71.2.195]:53880 "EHLO jenni2.inet.fi" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758223AbaKULrp (ORCPT ); Fri, 21 Nov 2014 06:47:45 -0500 Date: Fri, 21 Nov 2014 13:47:09 +0200 From: "Kirill A. Shutemov" To: "Aneesh Kumar K.V" Cc: Naoya Horiguchi , "Kirill A. Shutemov" , Andrew Morton , Andrea Arcangeli , Dave Hansen , Hugh Dickins , Mel Gorman , Rik van Riel , Vlastimil Babka , Christoph Lameter , Steve Capper , Johannes Weiner , Michal Hocko , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" Subject: Re: [PATCH 06/19] mm: store mapcount for compound page separate Message-ID: <20141121114709.GA16647@node.dhcp.inet.fi> References: <1415198994-15252-1-git-send-email-kirill.shutemov@linux.intel.com> <1415198994-15252-7-git-send-email-kirill.shutemov@linux.intel.com> <20141118084337.GA16714@hori1.linux.bs1.fc.nec.co.jp> <20141118095811.GA21774@node.dhcp.inet.fi> <87egsx6oo1.fsf@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87egsx6oo1.fsf@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 21, 2014 at 12:11:34PM +0530, Aneesh Kumar K.V wrote: > "Kirill A. Shutemov" writes: > > > On Tue, Nov 18, 2014 at 08:43:00AM +0000, Naoya Horiguchi wrote: > >> > @@ -1837,6 +1839,9 @@ static void __split_huge_page_refcount(struct page *page, > >> > atomic_sub(tail_count, &page->_count); > >> > BUG_ON(atomic_read(&page->_count) <= 0); > >> > > >> > + page->_mapcount = *compound_mapcount_ptr(page); > >> > >> Is atomic_set() necessary? > > > > Do you mean > > atomic_set(&page->_mapcount, atomic_read(compound_mapcount_ptr(page))); > > ? > > > > I don't see why we would need this. Simple assignment should work just > > fine. Or we have archs which will break? > > Are you looking at architecture related atomic_set issues, or the fact > that we cannot have parallel _mapcount update and hence the above > assignment should be ok ? If the former, current thp code > use atomic_add instead of even using atomic_set when > updatinge page_tail->_count. > > * from under us on the tail_page. If we used > * atomic_set() below instead of atomic_add(), we > * would then run atomic_set() concurrently with > * get_page_unless_zero(), and atomic_set() is > * implemented in C not using locked ops. spin_unlock > * on x86 sometime uses locked ops because of PPro > * errata 66, 92, so unless somebody can guarantee > * atomic_set() here would be safe on all archs (and > * not only on x86), it's safer to use atomic_add(). > */ > atomic_add(page_mapcount(page) + page_mapcount(page_tail) + 1, > &page_tail->_count); We don't have anything like get_page_unless_zero() for _mapcount as far as I can see. And we have similar assignment there now: page_tail->_mapcount = page->_mapcount; Anyway the assignment goes away by the end of patchset. -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/