linux-kernel - Re: [PATCH v2] x86/mm: Disable hugetlb page table sharing on 32-bit

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Message-ID: <aGTy0GPyc5o_uDgq@localhost.localdomain>
Date: Wed, 2 Jul 2025 10:50:24 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Jann Horn <jannh@...gle.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Vitaly Chikunov <vt@...linux.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Dave Hansen <dave.hansen@...el.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH v2] x86/mm: Disable hugetlb page table sharing on 32-bit

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 <vt@...linux.org>
> Closes: https://lore.kernel.org/r/srhpjxlqfna67blvma5frmy3aa@altlinux.org
> Suggested-by: Dave Hansen <dave.hansen@...el.com>
> Tested-by: Vitaly Chikunov <vt@...linux.org>
> Fixes: cfe28c5d63d8 ("x86: mm: Remove x86 version of huge_pmd_share.")
> Cc: stable@...r.kernel.org
> Signed-off-by: Jann Horn <jannh@...gle.com>

Acked-by: Oscar Salvador <osalvador@...e.de>

Thanks!

-- 
Oscar Salvador
SUSE Labs