From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752785Ab0CQEPT (ORCPT ); Wed, 17 Mar 2010 00:15:19 -0400 Received: from mail-pw0-f46.google.com ([209.85.160.46]:62800 "EHLO mail-pw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752396Ab0CQEPQ convert rfc822-to-8bit (ORCPT ); Wed, 17 Mar 2010 00:15:16 -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=j8eANTWtc3oRWPZAZx1RfgMoJPbrwLD1wQRC+NNeLqniO1B/rJnvdVr+BWnVYczB15 Ntj5+MhVPzzdwhK8v1562Axd+jr/RQsIJbGVSSzULhPxJq/uWCOuBRWMqKiSYtwYODwf R/L9aSq6k7dL0H/k+3qUgVuM8zaI0TbTdeZCM= MIME-Version: 1.0 In-Reply-To: <20100317121551.b619f55b.kamezawa.hiroyu@jp.fujitsu.com> References: <1268412087-13536-1-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> <28c262361003162000w34cc13ecnbd32840a0df80f95@mail.gmail.com> <20100317121551.b619f55b.kamezawa.hiroyu@jp.fujitsu.com> Date: Wed, 17 Mar 2010 13:15:14 +0900 Message-ID: <28c262361003162115k79e3d40fka6e1def6472823ef@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 12:15 PM, KAMEZAWA Hiroyuki wrote: > On Wed, 17 Mar 2010 12:00:15 +0900 > Minchan Kim wrote: > >> 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? >> > yes. > >> 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? >> > There are 4 cases. >        A) anon_vma->list is not empty because anon_vma is not freed. >        B) anon_vma->list is empty because it's freed. >        C) anon_vma->list is empty but it's reused. >        D) anon_vma->list is not empty but it's reused. E) anon_vma is used for other object. That's because we don't hold rcu_read_lock. I think Mel met this E) situation. AFAIU, even slab page of SLAB_BY_RCU can be freed after grace period. Do I miss something? > >> but such case is handled by page_check_address, vma_address, I think. >> > yes. Then, this corrupt nothing, as I wrote. We just modify anon_vma->lock > and it's safe because of SLAB_DESTROY_BY_RCU. > > >> >        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? >> > > Ah, my point is "how use-after-free is detected ?" > > If use-after-free is detected by free_pages() (DEBUG_PGALLOC), it seems > strange because DESTROY_BY_RCU guarantee that never happens. > > So, I assume use-after-free is detected in SLAB layer. If so, > in above B), C), D) case, it seems there is use-after free in slab's point > of view but it works as expected, no corruption. > > Then, my question is > "Does use-after-free check for SLAB_DESTROY_BY_RCU work correctly ?" > I am not sure Mel found that by DEBUG_PGALLOC. But, E) case can be founded by DEBUG_PGALLOC. > and implies we need this patch ? > (But this will prevent unnecessary page copy etc. by easy check.) > > Thanks, > -Kame > > > > -- Kind regards, Minchan Kim