Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757911AbaKUGmP (ORCPT ); Fri, 21 Nov 2014 01:42:15 -0500 Received: from e23smtp09.au.ibm.com ([202.81.31.142]:35841 "EHLO e23smtp09.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750922AbaKUGmO (ORCPT ); Fri, 21 Nov 2014 01:42:14 -0500 From: "Aneesh Kumar K.V" To: "Kirill A. Shutemov" , Naoya Horiguchi Cc: "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 In-Reply-To: <20141118095811.GA21774@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> User-Agent: Notmuch/0.18.2+162~g34d2c62 (http://notmuchmail.org) Emacs/25.0.50.1 (x86_64-pc-linux-gnu) Date: Fri, 21 Nov 2014 12:11:34 +0530 Message-ID: <87egsx6oo1.fsf@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14112106-0033-0000-0000-000000901A1A Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org "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); -aneesh -- 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/