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Wed, 2 Jul 2025 08:50:30 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id bgBcBdbyZGi2VAAAD6G6ig (envelope-from ); Wed, 02 Jul 2025 08:50:30 +0000 Date: Wed, 2 Jul 2025 10:50:24 +0200 From: Oscar Salvador To: Jann Horn Cc: Dave Hansen , Andy Lutomirski , Peter Zijlstra , Muchun Song , Vitaly Chikunov , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Dave Hansen , stable@vger.kernel.org Subject: Re: [PATCH v2] x86/mm: Disable hugetlb page table sharing on 32-bit Message-ID: References: <20250702-x86-2level-hugetlb-v2-1-1a98096edf92@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250702-x86-2level-hugetlb-v2-1-1a98096edf92@google.com> X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 50AA880005 X-Stat-Signature: iyypo8ynd31r3whosbgunto6qkc51xd4 X-Rspam-User: X-HE-Tag: 1751446232-33153 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Jul 02, 2025 at 10:32:04AM +0200, Jann Horn wrote: > Only select ARCH_WANT_HUGE_PMD_SHARE on 64-bit x86. > Page table sharing requires at least three levels because it involves > shared references to PMD tables; 32-bit x86 has either two-level paging > (without PAE) or three-level paging (with PAE), but even with > three-level paging, having a dedicated PGD entry for hugetlb is only > barely possible (because the PGD only has four entries), and it seems > unlikely anyone's actually using PMD sharing on 32-bit. > > Having ARCH_WANT_HUGE_PMD_SHARE enabled on non-PAE 32-bit X86 (which > has 2-level paging) became particularly problematic after commit > 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count"), > since that changes `struct ptdesc` such that the `pt_mm` (for PGDs) and > the `pt_share_count` (for PMDs) share the same union storage - and with > 2-level paging, PMDs are PGDs. > > (For comparison, arm64 also gates ARCH_WANT_HUGE_PMD_SHARE on the > configuration of page tables such that it is never enabled with 2-level > paging.) > > Reported-by: Vitaly Chikunov > Closes: https://lore.kernel.org/r/srhpjxlqfna67blvma5frmy3aa@altlinux.org > Suggested-by: Dave Hansen > Tested-by: Vitaly Chikunov > Fixes: cfe28c5d63d8 ("x86: mm: Remove x86 version of huge_pmd_share.") > Cc: stable@vger.kernel.org > Signed-off-by: Jann Horn Acked-by: Oscar Salvador Thanks! -- Oscar Salvador SUSE Labs