From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f174.google.com (mail-ig0-f174.google.com [209.85.213.174]) by kanga.kvack.org (Postfix) with ESMTP id 6B76E6B0257 for ; Thu, 25 Feb 2016 11:08:29 -0500 (EST) Received: by mail-ig0-f174.google.com with SMTP id z8so16655313ige.0 for ; Thu, 25 Feb 2016 08:08:29 -0800 (PST) Received: from mail-ig0-x234.google.com (mail-ig0-x234.google.com. [2607:f8b0:4001:c05::234]) by mx.google.com with ESMTPS id j189si11230250ioj.153.2016.02.25.08.08.28 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 25 Feb 2016 08:08:28 -0800 (PST) Received: by mail-ig0-x234.google.com with SMTP id xg9so16127944igb.1 for ; Thu, 25 Feb 2016 08:08:28 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20160225160111.GB19707@node.shutemov.name> 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> <20160225160111.GB19707@node.shutemov.name> Date: Thu, 25 Feb 2016 16:08:28 +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: "Kirill A. Shutemov" Cc: Will Deacon , Gerald Schaefer , 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 25 February 2016 at 16:01, Kirill A. Shutemov wrote: > On Thu, Feb 25, 2016 at 03:49:33PM +0000, Steve Capper wrote: >> 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. > > If I read code correctly, your pmd_mknotpresent() makes the pmd > pmd_none(), right? If yes, it's a problem. > > It introduces race I've described here: > > https://marc.info/?l=linux-mm&m=144723658100512&w=4 > > Basically, if zap_pmd_range() would see pmd_none() between > pmdp_mknotpresent() and pmd_populate(), we're screwed. > > The race window is small, but it's there. Ahhhh, okay, thank you Kirill. I agree, I'll get a patch out. Cheers, -- Steve -- 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