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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> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; MISSING_XM_UA(0.00)[]; MIME_TRACE(0.00)[0:+]; RCPT_COUNT_SEVEN(0.00)[10]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FUZZY_BLOCKED(0.00)[rspamd.com]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[localhost.localdomain:mid,suse.de:email,intel.com:email,imap1.dmz-prg2.suse.org:helo] X-Spam-Level: X-Spam-Flag: NO X-Spam-Score: -4.30 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