From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753197AbdHOQjv (ORCPT ); Tue, 15 Aug 2017 12:39:51 -0400 Received: from mail-pg0-f66.google.com ([74.125.83.66]:33800 "EHLO mail-pg0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752617AbdHOQjt (ORCPT ); Tue, 15 Aug 2017 12:39:49 -0400 Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 10.3 \(3273\)) Subject: Re: [PATCH 4.4 18/58] mm, mprotect: flush TLB if potentially racing with a parallel reclaim leaving stale TLB entries From: Nadav Amit In-Reply-To: <1502804197.2047.53.camel@codethink.co.uk> Date: Tue, 15 Aug 2017 09:39:45 -0700 Cc: Linux Kernel Mailing List , stable@vger.kernel.org, Andy Lutomirski , Andrew Morton , Linus Torvalds , Greg Kroah-Hartman Message-Id: <6478C2F6-1841-4C54-AD1A-A2CFC134D31F@gmail.com> References: <20170809194146.501519882@linuxfoundation.org> <20170809194147.234463750@linuxfoundation.org> <1502473549.2047.36.camel@codethink.co.uk> <1502804197.2047.53.camel@codethink.co.uk> To: Ben Hutchings , Mel Gorman X-Mailer: Apple Mail (2.3273) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by nfs id v7FGdr2p001822 Ben Hutchings wrote: > On Sat, 2017-08-12 at 23:27 -0700, Nadav Amit wrote: >> Ben Hutchings wrote: >> >>> On Wed, 2017-08-09 at 12:41 -0700, Greg Kroah-Hartman wrote: >>>> 4.4-stable review patch. If anyone has any objections, please let me know. >>>> >>>> ------------------ >>>> >>>> From: Mel Gorman >>>> >>>> commit 3ea277194daaeaa84ce75180ec7c7a2075027a68 upstream. >>> [...] >>>> +/* >>>> + * Reclaim unmaps pages under the PTL but do not flush the TLB prior to >>>> + * releasing the PTL if TLB flushes are batched. It's possible for a parallel >>>> + * operation such as mprotect or munmap to race between reclaim unmapping >>>> + * the page and flushing the page. If this race occurs, it potentially allows >>>> + * access to data via a stale TLB entry. Tracking all mm's that have TLB >>>> + * batching in flight would be expensive during reclaim so instead track >>>> + * whether TLB batching occurred in the past and if so then do a flush here >>>> + * if required. This will cost one additional flush per reclaim cycle paid >>>> + * by the first operation at risk such as mprotect and mumap. >>>> + * >>>> + * This must be called under the PTL so that an access to tlb_flush_batched >>>> + * that is potentially a "reclaim vs mprotect/munmap/etc" race will synchronise >>>> + * via the PTL. >>> >>> What about USE_SPLIT_PTE_PTLOCKS? I don't see how you can use "the PTL" >>> to synchronise access to a per-mm flag. >> >> Although it is a per-mm flag, the only situations we care about it are those >> in which “the PTL” (i.e. the same PTL) is accessed by both the reclaimer >> (which batches the flushes) and mprotect/munmap/etc. > > Is there anything that presents this sequence? > > P0 P1 P2 > -- -- -- > > change_pte_range() [ptl=X] > -> flush_tlb_batch_pending() > -> flush_tlb_mm() > try_to_unmap_one() [ptl=Y] > -> set_tlb_ubc_flush_pending() > -> tlb_flush_batched = true > -> tlb_flush_batched = false > > change_pte_range() [ptl=Y] > -> > flush_tlb_batch_pending() > (nop) I think (but not sure) that you regard a similar concern I raised before. Mel gave an answer [1], but I cannot say I feel very comfortable with it. [1] http://www.spinics.net/lists/linux-mm/msg131265.html Nadav