Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751609AbbHLVQr (ORCPT ); Wed, 12 Aug 2015 17:16:47 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:48747 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751078AbbHLVQq (ORCPT ); Wed, 12 Aug 2015 17:16:46 -0400 Date: Wed, 12 Aug 2015 14:16:44 -0700 From: Andrew Morton To: "Kirill A. Shutemov" Cc: Hugh Dickins , David Rientjes , Vlastimil Babka , "Kirill A. Shutemov" , Andrea Arcangeli , Dave Hansen , Mel Gorman , Rik van Riel , Christoph Lameter , Naoya Horiguchi , Steve Capper , "Aneesh Kumar K.V" , Johannes Weiner , Michal Hocko , Jerome Marchand , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: page-flags behavior on compound pages: a worry Message-Id: <20150812141644.ceb541e5b52d76049339a243@linux-foundation.org> In-Reply-To: <20150812143509.GA12320@node.dhcp.inet.fi> References: <1426784902-125149-1-git-send-email-kirill.shutemov@linux.intel.com> <1426784902-125149-5-git-send-email-kirill.shutemov@linux.intel.com> <20150806153259.GA2834@node.dhcp.inet.fi> <20150812143509.GA12320@node.dhcp.inet.fi> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2424 Lines: 65 On Wed, 12 Aug 2015 17:35:09 +0300 "Kirill A. Shutemov" wrote: > On Thu, Aug 06, 2015 at 12:24:22PM -0700, Hugh Dickins wrote: > > > IIUC, the only potentially problematic callsites left are physical memory > > > scanners. This code requires audit. I'll do that. > > > > Please. > > I haven't finished the exercise yet. But here's an issue I believe present > in current *Linus* tree: > > >From e78eec7d7a8c4cba8b5952a997973f7741e704f4 Mon Sep 17 00:00:00 2001 > From: "Kirill A. Shutemov" > Date: Wed, 12 Aug 2015 17:09:16 +0300 > Subject: [PATCH] mm: fix potential race in isolate_migratepages_block() > > Hugh has pointed that compound_head() call can be unsafe in some context. > There's one example: > > CPU0 CPU1 > > isolate_migratepages_block() > page_count() > compound_head() > !!PageTail() == true > put_page() > tail->first_page = NULL > head = tail->first_page > alloc_pages(__GFP_COMP) > prep_compound_page() > tail->first_page = head > __SetPageTail(p); > !!PageTail() == true > > > The race is pure theoretical. I don't it's possible to trigger it in > practice. But who knows. > > This can be fixed by avoiding compound_head() in unsafe context. This is nuts :( page_count() should Just Work without us having to worry about bizarre races against splitting. Sigh. > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -787,7 +787,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, > * admittedly racy check. > */ > if (!page_mapping(page) && > - page_count(page) > page_mapcount(page)) > + atomic_read(&page->_count) > page_mapcount(page)) > continue; If we're going to do this sort of thing, can we please do it in a more transparent manner? Let's not sprinkle unexplained and incomprehensible direct accesses to ->_count all over the place. Create a formal function to do this, with an appropriate name and with documentation which fully explains what's going on. Then use that here, and in has_unmovable_pages() (at least). -- 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/