From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751313Ab0CQDAc (ORCPT ); Tue, 16 Mar 2010 23:00:32 -0400 Received: from mail-pw0-f46.google.com ([209.85.160.46]:63317 "EHLO mail-pw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750810Ab0CQDAa convert rfc822-to-8bit (ORCPT ); Tue, 16 Mar 2010 23:00:30 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=W7B4UwUn8hAaLi5QR2RREXOn96H+yd+oAnhu6SH2TUy25u1BcKBmtotDOMLirgs6Rw V58NtnR3v5OBEKY+c+35JV3oxJGWFYg/EU7jrESay7I7kd7HGBeS+5GUkRkE7GuYQ01a lAA17RT+B7SZ4yipKO3/DChwzN0r+R//A6bPo= MIME-Version: 1.0 In-Reply-To: <20100317111234.d224f3fd.kamezawa.hiroyu@jp.fujitsu.com> References: <1268412087-13536-1-git-send-email-mel@csn.ul.ie> <1268412087-13536-3-git-send-email-mel@csn.ul.ie> <28c262361003141728g4aa40901hb040144c5a4aeeed@mail.gmail.com> <20100315143420.6ec3bdf9.kamezawa.hiroyu@jp.fujitsu.com> <20100315112829.GI18274@csn.ul.ie> <1268657329.1889.4.camel@barrios-desktop> <20100315142124.GL18274@csn.ul.ie> <20100316084934.3798576c.kamezawa.hiroyu@jp.fujitsu.com> <20100317111234.d224f3fd.kamezawa.hiroyu@jp.fujitsu.com> Date: Wed, 17 Mar 2010 12:00:15 +0900 Message-ID: <28c262361003162000w34cc13ecnbd32840a0df80f95@mail.gmail.com> Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages From: Minchan Kim To: KAMEZAWA Hiroyuki Cc: Mel Gorman , Andrew Morton , Andrea Arcangeli , Christoph Lameter , Adam Litke , Avi Kivity , David Rientjes , KOSAKI Motohiro , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 17, 2010 at 11:12 AM, KAMEZAWA Hiroyuki > BTW, I doubt freeing anon_vma can happen even when we check mapcount. > > "unmap" is 2-stage operation. >        1. unmap_vmas() => modify ptes, free pages, etc. >        2. free_pgtables() => free pgtables, unlink vma and free it. > > Then, if migration is enough slow. > >        Migration():                            Exit(): >        check mapcount >        rcu_read_lock >        pte_lock >        replace pte with migration pte >        pte_unlock >                                                pte_lock >        copy page etc...                        zap pte (clear pte) >                                                pte_unlock >                                                free_pgtables >                                                ->free vma >                                                ->free anon_vma >        pte_lock >        remap pte with new pfn(fail) >        pte_unlock > >        lock anon_vma->lock             # modification after free. >        check list is empty check list is empty? Do you mean anon_vma->head? If it is, is it possible that that list isn't empty since anon_vma is used by others due to SLAB_DESTROY_BY_RCU? but such case is handled by page_check_address, vma_address, I think. >        unlock anon_vma->lock >        free anon_vma >        rcu_read_unlock > > > Hmm. IIUC, anon_vma is allocated as SLAB_DESTROY_BY_RCU. Then, while > rcu_read_lock() is taken, anon_vma is anon_vma even if freed. But it > may reused as anon_vma for someone else. > (IOW, it may be reused but never pushed back to general purpose memory >  until RCU grace period.) > Then, touching anon_vma->lock never cause any corruption. > > Does use-after-free check for SLAB_DESTROY_BY_RCU correct behavior ? Could you elaborate your point? > Above case is not use-after-free. It's safe and expected sequence. > > Thanks, > -Kame > > > >> > --- >> >  mm/migrate.c |   13 +++++++++++++ >> >  1 files changed, 13 insertions(+), 0 deletions(-) >> > >> > diff --git a/mm/migrate.c b/mm/migrate.c >> > index 98eaaf2..6eb1efe 100644 >> > --- a/mm/migrate.c >> > +++ b/mm/migrate.c >> > @@ -603,6 +603,19 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private, >> >      */ >> >     if (PageAnon(page)) { >> >             rcu_read_lock(); >> > + >> > +           /* >> > +            * If the page has no mappings any more, just bail. An >> > +            * unmapped anon page is likely to be freed soon but worse, >> > +            * it's possible its anon_vma disappeared between when >> > +            * the page was isolated and when we reached here while >> > +            * the RCU lock was not held >> > +            */ >> > +           if (!page_mapcount(page)) { >> > +                   rcu_read_unlock(); >> > +                   goto uncharge; >> > +           } >> > + >> >             rcu_locked = 1; >> >             anon_vma = page_anon_vma(page); >> >             atomic_inc(&anon_vma->migrate_refcount); >> > >> >> -- >> 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 >> > > -- Kind regards, Minchan Kim