From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760863AbcBYPtg (ORCPT ); Thu, 25 Feb 2016 10:49:36 -0500 Received: from mail-ig0-f170.google.com ([209.85.213.170]:38780 "EHLO mail-ig0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759764AbcBYPte (ORCPT ); Thu, 25 Feb 2016 10:49:34 -0500 MIME-Version: 1.0 In-Reply-To: <20160223184658.GA27281@arm.com> 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> <20160223184658.GA27281@arm.com> Date: Thu, 25 Feb 2016 15:49:33 +0000 Message-ID: Subject: Re: [BUG] random kernel crashes after THP rework on s390 (maybe also on PowerPC and ARM) From: Steve Capper To: Will Deacon Cc: Gerald Schaefer , "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 , "linux-arm-kernel@lists.infradead.org" , Martin Schwidefsky , Heiko Carstens , linux-s390@vger.kernel.org, Sebastian Ott , Steve Capper Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 23 February 2016 at 18:47, Will Deacon wrote: > [adding Steve, since he worked on THP for 32-bit ARM] Apologies for my late reply... > > On Tue, Feb 23, 2016 at 07:19:07PM +0100, Gerald Schaefer wrote: >> On Tue, 23 Feb 2016 13:32:21 +0300 >> "Kirill A. Shutemov" wrote: >> > 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. >> >> FWIW, and since the ARM arch-list is already on cc, I think there is >> an issue with pmdp_invalidate() on ARM, since it also seems to clear >> the trans_huge (and formerly trans_splitting) bit, which actually makes >> the pmd !pmd_present(), but it violates the other requirement from the >> comment: >> "the pmd_trans_huge and pmd_trans_splitting must remain set at all times >> on the pmd until the split is complete for this pmd" > > I've only been testing this for arm64 (where I'm yet to see a problem), > but we use the generic pmdp_invalidate implementation from > mm/pgtable-generic.c there. On arm64, pmd_trans_huge will return true > after pmd_mknotpresent. On arm, it does look to be buggy, since it nukes > the entire entry... Steve? pmd_mknotpresent on arm looks inconsistent with the other architectures and can be changed. Having had a look at the usage, I can't see it causing an immediate problem (that needs to be addressed by an emergency patch). We don't have a notion of splitting pmds (so there is no splitting information to lose), and the only usage I could see of pmd_mknotpresent was: pmdp_invalidate(vma, haddr, pmd); pmd_populate(mm, pmd, pgtable); In mm/huge_memory.c, around line 3588. So we invalidate the entry (which puts down a faulting entry from pmd_mknotpresent and invalidates tlb), then immediately put down a table entry with pmd_populate. I have run a 32-bit ARM test kernel and exacerbated THP splits (that's what took me time), and I didn't notice any problems with 4.5-rc5. Cheers, -- Steve > > Will > > -- > 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/ . > Don't email: email@kvack.org From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f172.google.com (mail-io0-f172.google.com [209.85.223.172]) by kanga.kvack.org (Postfix) with ESMTP id D1CA86B0254 for ; Thu, 25 Feb 2016 10:49:34 -0500 (EST) Received: by mail-io0-f172.google.com with SMTP id l127so91987709iof.3 for ; Thu, 25 Feb 2016 07:49:34 -0800 (PST) Received: from mail-ig0-x229.google.com (mail-ig0-x229.google.com. [2607:f8b0:4001:c05::229]) by mx.google.com with ESMTPS id s85si11119289ioe.89.2016.02.25.07.49.33 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 25 Feb 2016 07:49:33 -0800 (PST) Received: by mail-ig0-x229.google.com with SMTP id xg9so15705312igb.1 for ; Thu, 25 Feb 2016 07:49:33 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20160223184658.GA27281@arm.com> 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> <20160223184658.GA27281@arm.com> Date: Thu, 25 Feb 2016 15:49:33 +0000 Message-ID: Subject: Re: [BUG] random kernel crashes after THP rework on s390 (maybe also on PowerPC and ARM) From: Steve Capper Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Will Deacon Cc: Gerald Schaefer , "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 , "linux-arm-kernel@lists.infradead.org" , Martin Schwidefsky , Heiko Carstens , linux-s390@vger.kernel.org, Sebastian Ott , Steve Capper On 23 February 2016 at 18:47, Will Deacon wrote: > [adding Steve, since he worked on THP for 32-bit ARM] Apologies for my late reply... > > On Tue, Feb 23, 2016 at 07:19:07PM +0100, Gerald Schaefer wrote: >> On Tue, 23 Feb 2016 13:32:21 +0300 >> "Kirill A. Shutemov" wrote: >> > 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. >> >> FWIW, and since the ARM arch-list is already on cc, I think there is >> an issue with pmdp_invalidate() on ARM, since it also seems to clear >> the trans_huge (and formerly trans_splitting) bit, which actually makes >> the pmd !pmd_present(), but it violates the other requirement from the >> comment: >> "the pmd_trans_huge and pmd_trans_splitting must remain set at all times >> on the pmd until the split is complete for this pmd" > > I've only been testing this for arm64 (where I'm yet to see a problem), > but we use the generic pmdp_invalidate implementation from > mm/pgtable-generic.c there. On arm64, pmd_trans_huge will return true > after pmd_mknotpresent. On arm, it does look to be buggy, since it nukes > the entire entry... Steve? pmd_mknotpresent on arm looks inconsistent with the other architectures and can be changed. Having had a look at the usage, I can't see it causing an immediate problem (that needs to be addressed by an emergency patch). We don't have a notion of splitting pmds (so there is no splitting information to lose), and the only usage I could see of pmd_mknotpresent was: pmdp_invalidate(vma, haddr, pmd); pmd_populate(mm, pmd, pgtable); In mm/huge_memory.c, around line 3588. So we invalidate the entry (which puts down a faulting entry from pmd_mknotpresent and invalidates tlb), then immediately put down a table entry with pmd_populate. I have run a 32-bit ARM test kernel and exacerbated THP splits (that's what took me time), and I didn't notice any problems with 4.5-rc5. Cheers, -- Steve > > Will > > -- > 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/ . > Don't email: email@kvack.org -- 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/ . Don't email: email@kvack.org From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-x22f.google.com (mail-ig0-x22f.google.com [IPv6:2607:f8b0:4001:c05::22f]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id AA1341A0342 for ; Fri, 26 Feb 2016 02:49:36 +1100 (AEDT) Received: by mail-ig0-x22f.google.com with SMTP id y8so17120413igp.0 for ; Thu, 25 Feb 2016 07:49:36 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20160223184658.GA27281@arm.com> 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> <20160223184658.GA27281@arm.com> Date: Thu, 25 Feb 2016 15:49:33 +0000 Message-ID: Subject: Re: [BUG] random kernel crashes after THP rework on s390 (maybe also on PowerPC and ARM) From: Steve Capper To: Will Deacon Cc: Gerald Schaefer , "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 , "linux-arm-kernel@lists.infradead.org" , Martin Schwidefsky , Heiko Carstens , linux-s390@vger.kernel.org, Sebastian Ott , Steve Capper Content-Type: text/plain; charset=UTF-8 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 23 February 2016 at 18:47, Will Deacon wrote: > [adding Steve, since he worked on THP for 32-bit ARM] Apologies for my late reply... > > On Tue, Feb 23, 2016 at 07:19:07PM +0100, Gerald Schaefer wrote: >> On Tue, 23 Feb 2016 13:32:21 +0300 >> "Kirill A. Shutemov" wrote: >> > 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. >> >> FWIW, and since the ARM arch-list is already on cc, I think there is >> an issue with pmdp_invalidate() on ARM, since it also seems to clear >> the trans_huge (and formerly trans_splitting) bit, which actually makes >> the pmd !pmd_present(), but it violates the other requirement from the >> comment: >> "the pmd_trans_huge and pmd_trans_splitting must remain set at all times >> on the pmd until the split is complete for this pmd" > > I've only been testing this for arm64 (where I'm yet to see a problem), > but we use the generic pmdp_invalidate implementation from > mm/pgtable-generic.c there. On arm64, pmd_trans_huge will return true > after pmd_mknotpresent. On arm, it does look to be buggy, since it nukes > the entire entry... Steve? pmd_mknotpresent on arm looks inconsistent with the other architectures and can be changed. Having had a look at the usage, I can't see it causing an immediate problem (that needs to be addressed by an emergency patch). We don't have a notion of splitting pmds (so there is no splitting information to lose), and the only usage I could see of pmd_mknotpresent was: pmdp_invalidate(vma, haddr, pmd); pmd_populate(mm, pmd, pgtable); In mm/huge_memory.c, around line 3588. So we invalidate the entry (which puts down a faulting entry from pmd_mknotpresent and invalidates tlb), then immediately put down a table entry with pmd_populate. I have run a 32-bit ARM test kernel and exacerbated THP splits (that's what took me time), and I didn't notice any problems with 4.5-rc5. Cheers, -- Steve > > Will > > -- > 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/ . > Don't email: email@kvack.org From mboxrd@z Thu Jan 1 00:00:00 1970 From: steve.capper@linaro.org (Steve Capper) Date: Thu, 25 Feb 2016 15:49:33 +0000 Subject: [BUG] random kernel crashes after THP rework on s390 (maybe also on PowerPC and ARM) In-Reply-To: <20160223184658.GA27281@arm.com> 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> <20160223184658.GA27281@arm.com> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 23 February 2016 at 18:47, Will Deacon wrote: > [adding Steve, since he worked on THP for 32-bit ARM] Apologies for my late reply... > > On Tue, Feb 23, 2016 at 07:19:07PM +0100, Gerald Schaefer wrote: >> On Tue, 23 Feb 2016 13:32:21 +0300 >> "Kirill A. Shutemov" wrote: >> > 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. >> >> FWIW, and since the ARM arch-list is already on cc, I think there is >> an issue with pmdp_invalidate() on ARM, since it also seems to clear >> the trans_huge (and formerly trans_splitting) bit, which actually makes >> the pmd !pmd_present(), but it violates the other requirement from the >> comment: >> "the pmd_trans_huge and pmd_trans_splitting must remain set at all times >> on the pmd until the split is complete for this pmd" > > I've only been testing this for arm64 (where I'm yet to see a problem), > but we use the generic pmdp_invalidate implementation from > mm/pgtable-generic.c there. On arm64, pmd_trans_huge will return true > after pmd_mknotpresent. On arm, it does look to be buggy, since it nukes > the entire entry... Steve? pmd_mknotpresent on arm looks inconsistent with the other architectures and can be changed. Having had a look at the usage, I can't see it causing an immediate problem (that needs to be addressed by an emergency patch). We don't have a notion of splitting pmds (so there is no splitting information to lose), and the only usage I could see of pmd_mknotpresent was: pmdp_invalidate(vma, haddr, pmd); pmd_populate(mm, pmd, pgtable); In mm/huge_memory.c, around line 3588. So we invalidate the entry (which puts down a faulting entry from pmd_mknotpresent and invalidates tlb), then immediately put down a table entry with pmd_populate. I have run a 32-bit ARM test kernel and exacerbated THP splits (that's what took me time), and I didn't notice any problems with 4.5-rc5. Cheers, -- Steve > > Will > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo at kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email at kvack.org