From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 18FC764CC0 for ; Mon, 29 Apr 2024 15:34:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714404900; cv=none; b=QjSLgRWOf0O3R5kFwZCrttufZA10kJSppYm6O1YsBcKotfpd/0HRGIONuLNZOLLe/ffFnngxggAK76uWYyLe0vkncnxmk9nVTiRfxY5aoIKlTPWgHneV2RB/V6+OIWWcc4Y3kKTLH6iA2iO55IWOpYXBf/4CUY1W/iAIHjt/y3U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714404900; c=relaxed/simple; bh=1+k1quiOkszeSfEDn/ofRri5lvYBYUzIqrPAxFaeW2E=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=OmSAYwShl3L5X//xBrtLaOm+s2/+d69QtNpg2pp2yVj1DXS1CMJIfe+YnYVGWXu86y20x7L2Jg/3/oRqqysT1wQiqmIN3bosKYoN0BpExA0PmHazP4oaZA1eMZfieos0NgpDZUPCEXlBQz/EVtKmoUZu1urQ5Mm+6ou7i/dtq+I= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 29D292F4; Mon, 29 Apr 2024 08:35:25 -0700 (PDT) Received: from [10.57.65.53] (unknown [10.57.65.53]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 623AE3F793; Mon, 29 Apr 2024 08:34:57 -0700 (PDT) Message-ID: <99384a25-9ff5-43c9-b09d-5a048c456d02@arm.com> Date: Mon, 29 Apr 2024 16:34:56 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] mm: Fix race between __split_huge_pmd_locked() and GUP-fast Content-Language: en-GB To: Zi Yan Cc: John Hubbard , Andrew Morton , Zi Yan , "Aneesh Kumar K.V" , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240425170704.3379492-1-ryan.roberts@arm.com> <17956e0f-1101-42d7-9cba-87e196312484@nvidia.com> <2C9D897B-0E96-42F0-B3C4-9926E6DF5F4B@nvidia.com> <328b2b69-e4ab-4a00-9137-1f7c3dc2d195@nvidia.com> <23fbca83-a175-4d5a-8cf7-ed54c1830d37@arm.com> <64037B35-3E80-4367-BA0B-334356373A78@nvidia.com> From: Ryan Roberts In-Reply-To: <64037B35-3E80-4367-BA0B-334356373A78@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 29/04/2024 15:45, Zi Yan wrote: > On 29 Apr 2024, at 5:29, Ryan Roberts wrote: > >> On 27/04/2024 20:11, John Hubbard wrote: >>> On 4/27/24 8:14 AM, Zi Yan wrote: >>>> On 27 Apr 2024, at 0:41, John Hubbard wrote: >>>>> On 4/25/24 10:07 AM, Ryan Roberts wrote: >>>>>> __split_huge_pmd_locked() can be called for a present THP, devmap or >>>>>> (non-present) migration entry. It calls pmdp_invalidate() >>>>>> unconditionally on the pmdp and only determines if it is present or not >>>>>> based on the returned old pmd. This is a problem for the migration entry >>>>>> case because pmd_mkinvalid(), called by pmdp_invalidate() must only be >>>>>> called for a present pmd. >>>>>> >>>>>> On arm64 at least, pmd_mkinvalid() will mark the pmd such that any >>>>>> future call to pmd_present() will return true. And therefore any >>>>>> lockless pgtable walker could see the migration entry pmd in this state >>>>>> and start interpretting the fields as if it were present, leading to >>>>>> BadThings (TM). GUP-fast appears to be one such lockless pgtable walker. >>>>>> I suspect the same is possible on other architectures. >>>>>> >>>>>> Fix this by only calling pmdp_invalidate() for a present pmd. And for >>>>> >>>>> Yes, this seems like a good design decision (after reading through the >>>>> discussion that you all had in the other threads). >>>> >>>> This will only be good for arm64 and does not prevent other arch developers >>>> to write code breaking arm64, since only arm64's pmd_mkinvalid() can turn >>>> a swap entry to a pmd_present() entry. >>> >>> Well, let's characterize it in a bit more detail, then: >> >> Hi All, >> >> Thanks for all the feedback! I had thought that this patch would be entirely >> uncontraversial - obviously I was wrong :) >> >> I've read all the emails, and trying to summarize a way forward here... >> >>> >>> 1) This patch will make things better for arm64. That's important! >>> >>> 2) Equally important, this patch does not make anything worse for >>>    other CPU arches. >>> >>> 3) This patch represents a new design constraint on the CPU arch >>>    layer, and thus requires documentation and whatever enforcement >>>    we can provide, in order to keep future code out of trouble. >> >> I know its only semantics, but I don't view this as a new design constraint. I >> see it as an existing constraint that was previously being violated, and this >> patch aims to fix that. The generic version of pmdp_invalidate() unconditionally >> does a tlb invalidation on the address range covered by the pmd. That makes no >> sense unless the pmd was previously present. So my conclusion is that the >> function only expects to be called for present pmds. >> >> Additionally Documentation/mm/arch_pgtable_helpers.rst already says this: >> >> " >> | pmd_mkinvalid | Invalidates a mapped PMD [1] | >> " >> >> I read "mapped" to be a synonym for "present". So I think its already >> documented. Happy to explcitly change "mapped" to "present" though, if it helps? >> >> Finally, [1] which is linked from Documentation/mm/arch_pgtable_helpers.rst, >> also implies this constraint, although it doesn't explicitly say it. >> >> [1] https://lore.kernel.org/linux-mm/20181017020930.GN30832@redhat.com/ >> >>> >>> 3.a) See the VM_WARN_ON() hunks below. >> >> It sounds like everybody would be happy if I sprinkle these into the arches that >> override pmdp_invalidate[_ad]()? There are 3 arches that have their own version >> of pmdp_invalidate(); powerpc, s390 and sparc. And 1 that has its own version of >> pmdp_invalidate_ad(); x86. I'll add them in all of those. >> >> I'll use VM_WARN_ON_ONCE() as suggested by John. >> >> I'd rather not put it directly into pmd_mkinvalid() since that would set a >> precedent for adding them absolutely everywhere. (e.g. pte_mkdirty(), ...). > > I understand your concern here. I assume you also understand the potential issue > with this, namely it does not prevent one from using pmd_mkinvalid() improperly > and causing a bug and the bug might only appear on arm64. Are you saying that arm64 is the *only* arch where pmd_mkinvalid() can turn a swap pte into a present pte? I hadn't appreciated that; in your first reply to this patch you said "I notice that x86, risc-v, mips behave the same" - I thought you were saying they behaved the same as arm64, but on re-reading, I think I've taken that out of context. But in spite of that, it still remains my view that making arm64's pmd_mkinvalid() robust to non-present ptes is not the right fix - or at least not sufficient on its own. That change on its own would still result in issuing a TLBI for the non-present pte from pmdp_invalidate(). That's not a correctness issue, but certainly could be a performance concern. I think its much better to have the design constraint that pmd_mkinvalid(), pmdp_invalidate() and pmdp_invalidate_ad() can only be called for present ptes. And I think the combination of WARNs and docs that we've discussed should be enough to allay your concerns about introduction of arm64-specific bugs. > >> >>> >>> 3.b) I like the new design constraint, because it is reasonable and >>>      clearly understandable: don't invalidate a non-present page >>>      table entry. >>> >>> I do wonder if there is somewhere else that this should be documented? >> >> If I change: >> >> " >> | pmd_mkinvalid | Invalidates a mapped PMD [1] | >> " >> >> To: >> >> " >> | pmd_mkinvalid | Invalidates a present PMD; do not call for | >> | non-present pmd [1] | >> " >> >> Is that sufficient? (I'll do the same for pud_mkinvalid() too. > > Sounds good to me. > > Also, if you move pmdp_invalidate(), please move the big comment with it to > avoid confusion. Thanks. Yes good spot, I'll move it. > > -- > Best Regards, > Yan, Zi