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X-CSE-ConnectionGUID: fe6KxiM8T+a1QuyhiR6Dsw== X-CSE-MsgGUID: hlP4+EwBSE69LdxsQk20Cg== X-IronPort-AV: E=McAfee;i="6800,10657,11480"; a="70986221" X-IronPort-AV: E=Sophos;i="6.16,278,1744095600"; d="scan'208";a="70986221" Received: from fmviesa004.fm.intel.com ([10.60.135.144]) by orvoesa102.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Jun 2025 13:39:39 -0700 X-CSE-ConnectionGUID: YdRTeGd9QsOM7jud+94s5g== X-CSE-MsgGUID: MQ+jz3/TRVGgdj1ng3tT8w== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.16,278,1744095600"; d="scan'208";a="159073190" Received: from spandruv-mobl4.amr.corp.intel.com (HELO [10.125.109.134]) ([10.125.109.134]) by fmviesa004-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Jun 2025 13:39:38 -0700 Message-ID: <9179e1cd-d635-4e70-931e-4a85c2e6932b@intel.com> Date: Mon, 30 Jun 2025 13:39:37 -0700 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] x86/mm: Disable hugetlb page table sharing on non-PAE 32-bit To: Jann Horn , Dave Hansen , Andy Lutomirski , Peter Zijlstra Cc: Muchun Song , Oscar Salvador , Vitaly Chikunov , linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org References: <20250630-x86-2level-hugetlb-v1-1-077cd53d8255@google.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/30/25 12:07, Jann Horn wrote: > --- a/arch/x86/Kconfig > +++ b/arch/x86/Kconfig > @@ -147,7 +147,7 @@ config X86 > select ARCH_WANTS_DYNAMIC_TASK_STRUCT > select ARCH_WANTS_NO_INSTR > select ARCH_WANT_GENERAL_HUGETLB > - select ARCH_WANT_HUGE_PMD_SHARE > + select ARCH_WANT_HUGE_PMD_SHARE if PGTABLE_LEVELS > 2 > select ARCH_WANT_LD_ORPHAN_WARN > select ARCH_WANT_OPTIMIZE_DAX_VMEMMAP if X86_64 > select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP if X86_64 Does pmd sharing really even work on 32-bit? Just practically, you only ever have 3GB of address space and thus 3 possible PGDs that can be used for sharing (with the 3:1 split configured). You presumably need *some* address space for the binary to even execve(). The vdso and friends go somewhere and we normally don't let anything get mapped at 0x0. I think that leaves _maybe_ one slot. Barring something some specific and compelling actual use case, this should probably just be: select ARCH_WANT_HUGE_PMD_SHARE if X86_64