From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4CA3A31AF3B; Wed, 17 Jun 2026 11:07:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781694440; cv=none; b=qbeUBcWUcEZdE9L0WeyQZDqLEUE3jQPsy1k8EwKgrhgi1A9Oht4FiZGuxlcpcv3lqaxsS7O6uc3q6V0WUDuHLJhnUMKjGnETdsc1qPEex2k+QrJYG0NfOXgH5Zbq7fxKyaQyIy6dJl4/UK6q8n3dHFMR4L/Dit7OfS8+05Jdswg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781694440; c=relaxed/simple; bh=3xkPKprCMmR8MGpm2WCjli9q3iD8ESXoj4jzw94j/F4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Qp7UGQJ2aQ/YAM6VE2t2puiJJVeiGFDoMm/zbd5SORe5NfHh6V30+1Qa31hTIJjqxV/3O5uuK+6039FKlgtB5sgYRogUjbw2N7zIMggbJhJb+JI7Uc1cQhhDzuM2Xcc+fbPdD98fHcLiLx7q2iyNrmLfGmQpbHligM/r5xF0J+o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Xd4qAxDs; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Xd4qAxDs" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BA6D91F000E9; Wed, 17 Jun 2026 11:07:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781694439; bh=h6dXIG9Ix1iq8lF+scJp0tVtUJekSbTJUAEd9UVcqC4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Xd4qAxDsspeL4VPFXtGf6ezzzzy3sFoOLNA/rgK9CqhbNBV78Bx5Jw9dOc/7ORvpi Ux6t84B976GEVfgr0WeTR1nEJ4qY/eFWkymBFbNit34duGhAjlcOeDiCyyvKOFCeZ7 iopv/+rFJH6HABE6inEDqWJGZBipr3UEcuXPwKTpHamr8JCV7OXlRSYl5fZJpdE07x bFLbJDMrkeb3VkwSO5fiQh+RG2+I/pwnfFeKCl29NnmcqlR32zvjlYlmrc+0D3TNTV icpze+hF5EEQvSMoi9xpBxk/Xszdn+0LXCVKGC+BAH9YfIVZ+IAHHn5eSjH3CdgP23 HFV3qaRNMhiVQ== Message-ID: Date: Wed, 17 Jun 2026 13:07:14 +0200 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] mm/page_table_check: do not track special (PFN-mapped) PTEs To: Andrey Smirnov , pasha.tatashin@soleen.com, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, syzbot+2b5fe617654be3d8848b@syzkaller.appspotmail.com, Thomas Gleixner , =?UTF-8?Q?Thomas_Wei=C3=9Fschuh?= , Andrei Vagin , Andy Lutomirski , Vincenzo Frascino , stable@vger.kernel.org References: <20260608155758.1220420-1-andrey.smirnov@siderolabs.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/8/26 17:57, Andrey Smirnov wrote: > The vDSO data store ("[vvar]") special mapping is created as a VM_PFNMAP > mapping and its pages are installed into userspace with vmf_insert_pfn(), > which produces special PTEs (pte_special()). On x86 and arm64 (and riscv) > pte_user_accessible_page() only tests the PRESENT/USER bits and does not > exclude special PTEs, so page_table_check accounts these PFN mappings in > the per-page anon/file map counters even though they are not rmap-managed > pages (vm_normal_page() returns NULL for them). > > Most of these data pages live in the kernel image and are never freed, so > the stray accounting is invisible. The time-namespace VVAR page is the > exception: it is a real alloc_page() page that is released with > __free_page() in free_time_ns() when the last task of a time namespace > exits. Across the map / unmap / vdso_join_timens() zap transitions the > special-PTE accounting is not balanced for this page, so a non-zero > file_map_count survives to the free path and trips: > > kernel BUG at mm/page_table_check.c:143! > __page_table_check_zero+0xfb/0x130 > __free_frozen_pages+0x52f/0x650 > free_time_ns+0x85/0xc0 > free_nsproxy+0x7f/0x130 > do_exit+0x313/0xa60 > do_group_exit+0x77/0x90 > > This is reliably reproducible on x86_64 and arm64 under heavy container/CI > churn that rapidly creates and destroys time namespaces (CLONE_NEWTIME via > runc / docker-init / tini), and was independently reported by syzbot on > riscv. It only manifests when CONFIG_PAGE_TABLE_CHECK is active. > > Special PTEs have no struct-page rmap semantics and must never have been > tracked by page table check. Skip them in both the set and clear paths so > the counters stay balanced (always zero) for PFN-mapped pages, regardless > of how the architecture defines pte_user_accessible_page(). pte_special() > is available generically (it is a no-op returning false on architectures > without ARCH_HAS_PTE_SPECIAL), so this is a single, arch-independent fix. Using pte_special() is usually a sign that something is likely shaky, as it misses architectures that don't support CONFIG_ARCH_HAS_PTE_SPECIAL. I assume relevant architectures (loongarch32?) do not support CONFIG_PAGE_TABLE_CHECK. arch/arm64/Kconfig: select ARCH_SUPPORTS_PAGE_TABLE_CHECK arch/powerpc/Kconfig: select ARCH_SUPPORTS_PAGE_TABLE_CHECK if !HUGETLB_PAGE arch/riscv/Kconfig: select ARCH_SUPPORTS_PAGE_TABLE_CHECK if MMU arch/s390/Kconfig: select ARCH_SUPPORTS_PAGE_TABLE_CHECK arch/x86/Kconfig: select ARCH_SUPPORTS_PAGE_TABLE_CHECK if X86_64 mm/Kconfig.debug: depends on ARCH_SUPPORTS_PAGE_TABLE_CHECK Can we enforce somehow that we expect CONFIG_ARCH_HAS_PTE_SPECIAL, so anybody unlocking ARCH_SUPPORTS_PAGE_TABLE_CHECK is aware of this? For example, through a BUILD_BUG_ON? -- Cheers, David