From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758123AbcBXIWX (ORCPT ); Wed, 24 Feb 2016 03:22:23 -0500 Received: from e06smtp08.uk.ibm.com ([195.75.94.104]:35604 "EHLO e06smtp08.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758101AbcBXIWS (ORCPT ); Wed, 24 Feb 2016 03:22:18 -0500 X-IBM-Helo: d06dlp03.portsmouth.uk.ibm.com X-IBM-MailFrom: schwidefsky@de.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org;linux-s390@vger.kernel.org Date: Wed, 24 Feb 2016 09:22:08 +0100 From: Martin Schwidefsky To: Gerald Schaefer Cc: "Kirill A. Shutemov" , Christian Borntraeger , "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, 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: <20160224092208.49e013ff@mschwide> In-Reply-To: <20160223191907.25719a4d@thinkpad> References: <20160211192223.4b517057@thinkpad> <20160211190942.GA10244@node.shutemov.name> <20160211205702.24f0d17a@thinkpad> <20160212154116.GA15142@node.shutemov.name> <56BE00E7.1010303@de.ibm.com> <20160212181640.4eabb85f@thinkpad> <20160223103221.GA1418@node.shutemov.name> <20160223191907.25719a4d@thinkpad> X-Mailer: Claws Mail 3.9.3 (GTK+ 2.24.23; x86_64-pc-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: 16022408-0033-0000-0000-000005E27309 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 23 Feb 2016 19:19:07 +0100 Gerald Schaefer wrote: > On Tue, 23 Feb 2016 13:32:21 +0300 > "Kirill A. Shutemov" wrote: > > > On Fri, Feb 12, 2016 at 06:16:40PM +0100, Gerald Schaefer wrote: > > > On Fri, 12 Feb 2016 16:57:27 +0100 > > > Christian Borntraeger wrote: > > > > > > > > I'm also confused by pmd_none() is equal to !pmd_present() on s390. Hm? > > > > > > > > Don't know, Gerald or Martin? > > > > > > The implementation frequently changes depending on how many new bits Martin > > > needs to squeeze out :-) > > > We don't have a _PAGE_PRESENT bit for pmds, so pmd_present() just checks if the > > > entry is not empty. pmd_none() of course does the opposite, it checks if it is > > > empty. > > > > I still worry about pmd_present(). It looks wrong to me. I wounder if > > patch below makes a difference. > > > > The theory is that the splitting bit effetely masked bogus pmd_present(): > > we had pmd_trans_splitting() in all code path and that prevented mm from > > touching the pmd. Once pmd_trans_splitting() has gone, mm proceed with the > > pmd where it shouldn't and here's a boom. > > Well, I don't think pmd_present() == true is bogus for a trans_huge pmd under > splitting, after all there is a page behind the the pmd. Also, if it was > bogus, and it would need to be false, why should it be marked !pmd_present() > only at the pmdp_invalidate() step before the pmd_populate()? It clearly > is pmd_present() before that, on all architectures, and if there was any > problem/race with that, setting it to !pmd_present() at this stage would > only (marginally) reduce the race window. > > BTW, PowerPC and Sparc seem to do the same thing in pmdp_invalidate(), > i.e. they do not set pmd_present() == false, only mark it so that it would > not generate a new TLB entry, just like on s390. After all, the function > is called pmdp_invalidate(), and I think the comment in mm/huge_memory.c > before that call is just a little ambiguous in its wording. When it says > "mark the pmd notpresent" it probably means "mark it so that it will not > generate a new TLB entry", which is also what the comment is really about: > prevent huge and small entries in the TLB for the same page at the same > time. If I am not mistaken this is true for x86 as well. The generic implementation for pmdp_invalidate sets a new pmd that has been modified with pmd_mknotpresent. For x86 this function removes the _PAGE_PRESENT and _PAGE_PROTNONE bits from the entry. The _PAGE_PSE bit stays set and that makes pmd_present return true. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.