From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id EED08C48260 for ; Tue, 13 Feb 2024 13:07:35 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4TZ1ny4jh2z3cQj for ; Wed, 14 Feb 2024 00:07:34 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=arm.com (client-ip=217.140.110.172; helo=foss.arm.com; envelope-from=ryan.roberts@arm.com; receiver=lists.ozlabs.org) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lists.ozlabs.org (Postfix) with ESMTP id 4TZ1nW3fRNz2xQL for ; Wed, 14 Feb 2024 00:07:10 +1100 (AEDT) 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 652A7DA7; Tue, 13 Feb 2024 05:07:20 -0800 (PST) Received: from [10.1.36.184] (XHFQ2J9959.cambridge.arm.com [10.1.36.184]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 93AE23F762; Tue, 13 Feb 2024 05:06:35 -0800 (PST) Message-ID: <1d302d7a-50ab-4ab4-b049-75ed4a71a87d@arm.com> Date: Tue, 13 Feb 2024 13:06:34 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings Content-Language: en-GB To: David Hildenbrand , Mark Rutland References: <20240202080756.1453939-1-ryan.roberts@arm.com> <20240202080756.1453939-20-ryan.roberts@arm.com> <64395ae4-3a7d-45dd-8f1d-ea6b232829c5@arm.com> <41499621-482f-455b-9f68-b43ea8052557@redhat.com> From: Ryan Roberts In-Reply-To: <41499621-482f-455b-9f68-b43ea8052557@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kefeng Wang , x86@kernel.org, Catalin Marinas , Yang Shi , Dave Hansen , linux-mm@kvack.org, Andrey Ryabinin , "H. Peter Anvin" , Will Deacon , Ard Biesheuvel , Marc Zyngier , Alistair Popple , Barry Song <21cnbao@gmail.com>, Matthew Wilcox , "Aneesh Kumar K.V" , Ingo Molnar , Zi Yan , "Naveen N. Rao" , John Hubbard , Nicholas Piggin , Borislav Petkov , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, James Morse , Andrew Morton , linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 13/02/2024 12:19, David Hildenbrand wrote: > On 13.02.24 13:06, Ryan Roberts wrote: >> On 12/02/2024 20:38, Ryan Roberts wrote: >>> [...] >>> >>>>>>> +static inline bool mm_is_user(struct mm_struct *mm) >>>>>>> +{ >>>>>>> +    /* >>>>>>> +     * Don't attempt to apply the contig bit to kernel mappings, because >>>>>>> +     * dynamically adding/removing the contig bit can cause page faults. >>>>>>> +     * These racing faults are ok for user space, since they get serialized >>>>>>> +     * on the PTL. But kernel mappings can't tolerate faults. >>>>>>> +     */ >>>>>>> +    return mm != &init_mm; >>>>>>> +} >>>>>> >>>>>> We also have the efi_mm as a non-user mm, though I don't think we manipulate >>>>>> that while it is live, and I'm not sure if that needs any special handling. >>>>> >>>>> Well we never need this function in the hot (order-0 folio) path, so I think I >>>>> could add a check for efi_mm here with performance implication. It's probably >>>>> safest to explicitly exclude it? What do you think? >>>> >>>> Oops: This should have read "I think I could add a check for efi_mm here >>>> *without* performance implication" >>> >>> It turns out that efi_mm is only defined when CONFIG_EFI is enabled. I can do >>> this: >>> >>> return mm != &init_mm && (!IS_ENABLED(CONFIG_EFI) || mm != &efi_mm); >>> >>> Is that acceptable? This is my preference, but nothing else outside of efi >>> references this symbol currently. >>> >>> Or perhaps I can convince myself that its safe to treat efi_mm like userspace. >>> There are a couple of things that need to be garanteed for it to be safe: >>> >>>    - The PFNs of present ptes either need to have an associated struct page or >>>      need to have the PTE_SPECIAL bit set (either pte_mkspecial() or >>>      pte_mkdevmap()) >>> >>>    - Live mappings must either be static (no changes that could cause >>> fold/unfold >>>      while live) or the system must be able to tolerate a temporary fault >>> >>> Mark suggests efi_mm is not manipulated while live, so that meets the latter >>> requirement, but I'm not sure about the former? >> >> I've gone through all the efi code, and conclude that, as Mark suggests, the >> mappings are indeed static. And additionally, the ptes are populated using only >> the _private_ ptep API, so there is no issue here. As just discussed with Mark, >> my prefereence is to not make any changes to code, and just add a comment >> describing why efi_mm is safe. >> >> Details: >> >> * Registered with ptdump >>      * ptep_get_lockless() >> * efi_create_mapping -> create_pgd_mapping … -> init_pte: >>      * __ptep_get() >>      * __set_pte() >> * efi_memattr_apply_permissions -> efi_set_mapping_permissions … -> >> set_permissions >>      * __ptep_get() >>      * __set_pte() > > Sound good. We could add some VM_WARN_ON if we ever get the efi_mm via the > "official" APIs. We could, but that would lead to the same linkage issue, which I'm trying to avoid in the first place: VM_WARN_ON(IS_ENABLED(CONFIG_EFI) && mm == efi_mm); This creates new source code dependencies, which I would rather avoid if possible.