From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752260Ab1FNPId (ORCPT ); Tue, 14 Jun 2011 11:08:33 -0400 Received: from casper.infradead.org ([85.118.1.10]:37413 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750780Ab1FNPIb convert rfc822-to-8bit (ORCPT ); Tue, 14 Jun 2011 11:08:31 -0400 Subject: Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement. From: Peter Zijlstra To: Oleg Nesterov Cc: Srikar Dronamraju , Ingo Molnar , Steven Rostedt , Linux-mm , Arnaldo Carvalho de Melo , Linus Torvalds , Masami Hiramatsu , Hugh Dickins , Christoph Hellwig , Andi Kleen , Thomas Gleixner , Jonathan Corbet , Andrew Morton , Jim Keniston , Roland McGrath , Ananth N Mavinakayanahalli , LKML In-Reply-To: <20110614142710.GB5139@redhat.com> References: <20110607125804.28590.92092.sendpatchset@localhost6.localdomain6> <20110607125835.28590.25476.sendpatchset@localhost6.localdomain6> <20110613170020.GA27137@redhat.com> <1308056477.19856.35.camel@twins> <20110614142710.GB5139@redhat.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Tue, 14 Jun 2011 17:07:33 +0200 Message-ID: <1308064053.19856.75.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2011-06-14 at 16:27 +0200, Oleg Nesterov wrote: > On 06/14, Peter Zijlstra wrote: > > > > On Mon, 2011-06-13 at 19:00 +0200, Oleg Nesterov wrote: > > > > > > Also. This is called under down_read(mmap_sem), can't we race with > > > access_process_vm() modifying the same memory? > > > > Shouldn't matter COW and similar things are serialized using the pte > > lock. > > Yes, but afaics this doesn't matter. Suppose that write_opcode() is > called when access_process_vm() does copy_to_user_page(). We can cow > the page before memcpy() completes. access_process_vm() will end up doing a FOLL_WRITE itself when copy_to_user_page() is called since write=1 in that case. At that point we have a COW-race, someone wins, but the other will then return the same page. At this point further PTRACE pokes can indeed race with the memcpy in write_opcode(). A possible fix would be to lock_page() around copy_to_user_page() (its already done in set_page_dirty_lock(), so pulling it out shouldn't matter much). From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with ESMTP id 4D0666B00ED for ; Tue, 14 Jun 2011 11:08:16 -0400 (EDT) Subject: Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement. From: Peter Zijlstra In-Reply-To: <20110614142710.GB5139@redhat.com> References: <20110607125804.28590.92092.sendpatchset@localhost6.localdomain6> <20110607125835.28590.25476.sendpatchset@localhost6.localdomain6> <20110613170020.GA27137@redhat.com> <1308056477.19856.35.camel@twins> <20110614142710.GB5139@redhat.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Date: Tue, 14 Jun 2011 17:07:33 +0200 Message-ID: <1308064053.19856.75.camel@twins> Mime-Version: 1.0 Sender: owner-linux-mm@kvack.org List-ID: To: Oleg Nesterov Cc: Srikar Dronamraju , Ingo Molnar , Steven Rostedt , Linux-mm , Arnaldo Carvalho de Melo , Linus Torvalds , Masami Hiramatsu , Hugh Dickins , Christoph Hellwig , Andi Kleen , Thomas Gleixner , Jonathan Corbet , Andrew Morton , Jim Keniston , Roland McGrath , Ananth N Mavinakayanahalli , LKML On Tue, 2011-06-14 at 16:27 +0200, Oleg Nesterov wrote: > On 06/14, Peter Zijlstra wrote: > > > > On Mon, 2011-06-13 at 19:00 +0200, Oleg Nesterov wrote: > > > > > > Also. This is called under down_read(mmap_sem), can't we race with > > > access_process_vm() modifying the same memory? > > > > Shouldn't matter COW and similar things are serialized using the pte > > lock. >=20 > Yes, but afaics this doesn't matter. Suppose that write_opcode() is > called when access_process_vm() does copy_to_user_page(). We can cow > the page before memcpy() completes. access_process_vm() will end up doing a FOLL_WRITE itself when copy_to_user_page() is called since write=3D1 in that case. At that point we have a COW-race, someone wins, but the other will then return the same page. At this point further PTRACE pokes can indeed race with the memcpy in write_opcode(). A possible fix would be to lock_page() around copy_to_user_page() (its already done in set_page_dirty_lock(), so pulling it out shouldn't matter much). -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org