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 42BDB22D780 for ; Fri, 30 May 2025 14:07:17 +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=1748614040; cv=none; b=oxRrdrvQRqIqtW6Vd+6cVbjdCx5NUKpqEXckbEFOU2AEwlVGPCuBd1IiYBBRMn1H/+ghh/PmLPtA5cH02mIR40e8GlJtF6rcb7RbQwHhPQMlK2Pt0jRGHaBT0cvIeeoDODCw2817jPbFzcnL4KAL/4EKGLefNCQvc3Ou90/kJck= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748614040; c=relaxed/simple; bh=q6JCX5/EEu8B4WyVzDVXYuK540q1eUzF7lbv+37wDCY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=A3JaWpTJHKKpsQ7r4wxjTo3hG/5AGGH6tTHizCi4oYdMUK9ppU1mmkpHUCDGcwBT6v3NC/lEaZaL8q952G8z0QMdhdFmmAx/oLWHX0bdnK8uUh0kbFAjRdKR5cCAKaYvRtXIajZ39xy1AjHv03I6eg/FPbQPtRrT/QvfeCe9P48= 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 2C63D169C; Fri, 30 May 2025 07:07:01 -0700 (PDT) Received: from [10.57.95.14] (unknown [10.57.95.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EDAD73F673; Fri, 30 May 2025 07:07:15 -0700 (PDT) Message-ID: <4934dbba-2a83-427f-bb57-eed25ba2e989@arm.com> Date: Fri, 30 May 2025 15:07:14 +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] arm64: Enable vmalloc-huge with ptdump Content-Language: en-GB To: Will Deacon Cc: Dev Jain , catalin.marinas@arm.com, anshuman.khandual@arm.com, quic_zhenhuah@quicinc.com, kevin.brodsky@arm.com, yangyicong@hisilicon.com, joey.gouly@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, david@redhat.com References: <20250530082021.18182-1-dev.jain@arm.com> <832e84a9-4303-4e21-a88b-94395898fa3e@arm.com> <4202a03d-dacd-429c-91e6-81a5d05726a4@arm.com> <20250530123527.GA30463@willie-the-truck> <20250530133625.GA30622@willie-the-truck> From: Ryan Roberts In-Reply-To: <20250530133625.GA30622@willie-the-truck> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 30/05/2025 14:36, Will Deacon wrote: > On Fri, May 30, 2025 at 02:11:36PM +0100, Ryan Roberts wrote: >> On 30/05/2025 13:35, Will Deacon wrote: >>> On Fri, May 30, 2025 at 12:50:40PM +0100, Ryan Roberts wrote: >>>> On 30/05/2025 10:14, Dev Jain wrote: >>>>> >>>>> On 30/05/25 2:10 pm, Ryan Roberts wrote: >>>>>> On 30/05/2025 09:20, Dev Jain wrote: >>>>>>> arm64 disables vmalloc-huge when kernel page table dumping is enabled, >>>>>>> because an intermediate table may be removed, potentially causing the >>>>>>> ptdump code to dereference an invalid address. We want to be able to >>>>>>> analyze block vs page mappings for kernel mappings with ptdump, so to >>>>>>> enable vmalloc-huge with ptdump, synchronize between page table removal in >>>>>>> pmd_free_pte_page()/pud_free_pmd_page() and ptdump pagetable walking. We >>>>>>> use mmap_read_lock and not write lock because we don't need to synchronize >>>>>>> between two different vm_structs; two vmalloc objects running this same >>>>>>> code path will point to different page tables, hence there is no race. >>>>> >>>>> My "correction" from race->no problem was incorrect after all :) There will >>>>> be no race too since the vm_struct object has exclusive access to whatever >>>>> table it is clearing. >>>>> >>>>>>> >>>>>>> Signed-off-by: Dev Jain >>>>>>> --- >>>>>>>   arch/arm64/include/asm/vmalloc.h | 6 ++---- >>>>>>>   arch/arm64/mm/mmu.c              | 7 +++++++ >>>>>>>   2 files changed, 9 insertions(+), 4 deletions(-) >>>>>>> >>>>>>> diff --git a/arch/arm64/include/asm/vmalloc.h b/arch/arm64/include/asm/vmalloc.h >>>>>>> index 38fafffe699f..28b7173d8693 100644 >>>>>>> --- a/arch/arm64/include/asm/vmalloc.h >>>>>>> +++ b/arch/arm64/include/asm/vmalloc.h >>>>>>> @@ -12,15 +12,13 @@ static inline bool arch_vmap_pud_supported(pgprot_t prot) >>>>>>>       /* >>>>>>>        * SW table walks can't handle removal of intermediate entries. >>>>>>>        */ >>>>>>> -    return pud_sect_supported() && >>>>>>> -           !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS); >>>>>>> +    return pud_sect_supported(); >>>>>>>   } >>>>>>>     #define arch_vmap_pmd_supported arch_vmap_pmd_supported >>>>>>>   static inline bool arch_vmap_pmd_supported(pgprot_t prot) >>>>>>>   { >>>>>>> -    /* See arch_vmap_pud_supported() */ >>>>>>> -    return !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS); >>>>>>> +    return true; >>>>>>>   } >>>>>>>     #endif >>>>>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>>>>>> index ea6695d53fb9..798cebd9e147 100644 >>>>>>> --- a/arch/arm64/mm/mmu.c >>>>>>> +++ b/arch/arm64/mm/mmu.c >>>>>>> @@ -1261,7 +1261,11 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr) >>>>>>>       } >>>>>>>         table = pte_offset_kernel(pmdp, addr); >>>>>>> + >>>>>>> +    /* Synchronize against ptdump_walk_pgd() */ >>>>>>> +    mmap_read_lock(&init_mm); >>>>>>>       pmd_clear(pmdp); >>>>>>> +    mmap_read_unlock(&init_mm); >>>>>> So this works because ptdump_walk_pgd() takes the write_lock (which is mutually >>>>>> exclusive with any read_lock holders) for the duration of the table walk, so it >>>>>> will either consistently see the pgtables before or after this removal. It will >>>>>> never disappear during the walk, correct? >>>>>> >>>>>> I guess there is a risk of this showing up as contention with other init_mm >>>>>> write_lock holders. But I expect that pmd_free_pte_page()/pud_free_pmd_page() >>>>>> are called sufficiently rarely that the risk is very small. Let's fix any perf >>>>>> problem if/when we see it. >>>>> >>>>> We can avoid all of that by my initial approach - to wrap the lock around >>>>> CONFIG_PTDUMP_DEBUGFS. >>>>> I don't have a strong opinion, just putting it out there. >>>> >>>> (I wrote then failed to send earlier): >>>> >>>> It's ugly though. Personally I'd prefer to keep it simple unless we have clear >>>> evidence that its needed. I was just laying out my justification for not needing >>>> to doing the conditional wrapping in this comment. >>> >>> I really don't think we should be adding unconditional locking overhead >>> to core mm routines purely to facilitate a rarely used debug option. >>> >>> Instead, can we either adopt something like the RCU-like walk used by >>> fast GUP or stick the locking behind a static key that's only enabled >>> when a ptdump walk is in progress (a bit like how >>> hugetlb_vmemmap_optimize_folio() manipulates hugetlb_optimize_vmemmap_key)? >> >> My sense is that the static key will be less effort and can be contained fully >> in arm64. I think we would need to enable the key around the call to >> ptdump_walk_pgd() in both ptdump_walk() and ptdump_check_wx(). Then where Dev is >> currently taking the read lock, that would be contingent on the key being >> enabled and the unlock would be contingent on having taken the lock. >> >> Does that sound like an acceptable approach? > > Yup, and I think you'll probably need something like a synchronize_rcu() > when flipping the key to deal with any pre-existing page-table freers. Yeah good point. > > Will