From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751259AbcBKT5M (ORCPT ); Thu, 11 Feb 2016 14:57:12 -0500 Received: from e06smtp11.uk.ibm.com ([195.75.94.107]:38449 "EHLO e06smtp11.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750752AbcBKT5K (ORCPT ); Thu, 11 Feb 2016 14:57:10 -0500 X-IBM-Helo: d06dlp03.portsmouth.uk.ibm.com X-IBM-MailFrom: gerald.schaefer@de.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org;linux-s390@vger.kernel.org Date: Thu, 11 Feb 2016 20:57:02 +0100 From: Gerald Schaefer To: "Kirill A. Shutemov" Cc: "Kirill A. Shutemov" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Aneesh Kumar K.V" , Andrew Morton , Linus Torvalds , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , linuxppc-dev@lists.ozlabs.org, Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, Martin Schwidefsky , Heiko Carstens , linux-s390@vger.kernel.org, Sebastian Ott Subject: Re: [BUG] random kernel crashes after THP rework on s390 (maybe also on PowerPC and ARM) Message-ID: <20160211205702.24f0d17a@thinkpad> In-Reply-To: <20160211190942.GA10244@node.shutemov.name> References: <20160211192223.4b517057@thinkpad> <20160211190942.GA10244@node.shutemov.name> Organization: IBM Deutschland Research & Development GmbH / Vorsitzende des Aufsichtsrats: Martina Koederitz / Geschaeftsfuehrung: Dirk Wittkopp / Sitz der Gesellschaft: Boeblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294 X-Mailer: Claws Mail 3.9.0 (GTK+ 2.24.23; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16021119-0041-0000-0000-000007797A04 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 11 Feb 2016 21:09:42 +0200 "Kirill A. Shutemov" wrote: > On Thu, Feb 11, 2016 at 07:22:23PM +0100, Gerald Schaefer wrote: > > Hi, > > > > Sebastian Ott reported random kernel crashes beginning with v4.5-rc1 and > > he also bisected this to commit 61f5d698 "mm: re-enable THP". Further > > review of the THP rework patches, which cannot be bisected, revealed > > commit fecffad "s390, thp: remove infrastructure for handling splitting PMDs" > > (and also similar commits for other archs). > > > > This commit removes the THP splitting bit and also the architecture > > implementation of pmdp_splitting_flush(), which took care of the IPI for > > fast_gup serialization. The commit message says > > > > pmdp_splitting_flush() is not needed too: on splitting PMD we will do > > pmdp_clear_flush() + set_pte_at(). pmdp_clear_flush() will do IPI as > > needed for fast_gup > > > > The assumption that a TLB flush will also produce an IPI is wrong on s390, > > and maybe also on other architectures, and I thought that this was actually > > the main reason for having an arch-specific pmdp_splitting_flush(). > > > > At least PowerPC and ARM also had an individual implementation of > > pmdp_splitting_flush() that used kick_all_cpus_sync() instead of a TLB > > flush to send the IPI, and those were also removed. Putting the arch > > maintainers and mailing lists on cc to verify. > > > > On s390 this will break the IPI serialization against fast_gup, which > > would certainly explain the random kernel crashes, please revert or fix > > the pmdp_splitting_flush() removal. > > Sorry for that. > > I believe, the problem was already addressed for PowerPC: > > http://lkml.kernel.org/g/454980831-16631-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com > > I think kick_all_cpus_sync() in arch-specific pmdp_invalidate() would do > the trick, right? Hmm, not sure about that. After pmdp_invalidate(), a pmd_none() check in fast_gup will still return false, because the pmd is not empty (at least on s390). So I don't see spontaneously how it will help fast_gup to break out to the slow path in case of THP splitting. > > If yes, I'll prepare patch tomorrow (some sleep required). > We'll check if adding kick_all_cpus_sync() to pmdp_invalidate() helps. It would also be good if Martin has a look at this, he'll return on Monday.