From nobody Sun Sep 14 08:23:09 2025 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 2BE412FF164; Thu, 4 Sep 2025 12:58:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756990717; cv=none; b=iZmVJbCP6mDVwgDt8wFfTJRsPEmtSPtULv2TaSGQCGLg/VkcESfTAfUJXuLdrTFaEg5lpc0yTnWUutQxfER55CIieylH+ffuttOeFvBtdLM/jf64x520eh5wKcy3QmD4eQavAhL6xxSibEon3yKEZAO+W7QLAhMpNSRxiHH+nEk= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756990717; c=relaxed/simple; bh=hkDsOOLYTwNQnHVD2JN+ks+GUnYIcxo3wNnKCrpM0zU=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=gahKBmvq1KhL/8uu3D2H+89kNPUAWWRoR12gypCw/9tz3SZCudmESDs3rmPNUgR8lzojFDsQfxh2QnEZzLKrCEvHKP59ajBmVDAgthdyBj+UzVgj0tajdnP0exmgDAcoH6iDpgRLyfk/DmktsduazVxhwk2kWN5aVZPITZ/kNOA= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4A9CE2EC6; Thu, 4 Sep 2025 05:58:27 -0700 (PDT) Received: from e123572-lin.arm.com (e123572-lin.cambridge.arm.com [10.1.194.54]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 17AC83F6A8; Thu, 4 Sep 2025 05:58:30 -0700 (PDT) From: Kevin Brodsky To: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Kevin Brodsky , Alexander Gordeev , Andreas Larsson , Andrew Morton , Boris Ostrovsky , Borislav Petkov , Catalin Marinas , Christophe Leroy , Dave Hansen , David Hildenbrand , "David S. Miller" , "H. Peter Anvin" , Ingo Molnar , Jann Horn , Juergen Gross , "Liam R. Howlett" , Lorenzo Stoakes , Madhavan Srinivasan , Michael Ellerman , Michal Hocko , Mike Rapoport , Nicholas Piggin , Peter Zijlstra , Ryan Roberts , Suren Baghdasaryan , Thomas Gleixner , Vlastimil Babka , Will Deacon , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, sparclinux@vger.kernel.org, xen-devel@lists.xenproject.org Subject: [PATCH 3/7] arm64: mm: fully support nested lazy_mmu sections Date: Thu, 4 Sep 2025 13:57:32 +0100 Message-ID: <20250904125736.3918646-4-kevin.brodsky@arm.com> X-Mailer: git-send-email 2.47.0 In-Reply-To: <20250904125736.3918646-1-kevin.brodsky@arm.com> References: <20250904125736.3918646-1-kevin.brodsky@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Despite recent efforts to prevent lazy_mmu sections from nesting, it remains difficult to ensure that it never occurs - and in fact it does occur on arm64 in certain situations (CONFIG_DEBUG_PAGEALLOC). Commit 1ef3095b1405 ("arm64/mm: Permit lazy_mmu_mode to be nested") made nesting tolerable on arm64, but without truly supporting it: the inner leave() call clears TIF_LAZY_MMU, disabling the batching optimisation before the outer section ends. Now that the lazy_mmu API allows enter() to pass through a state to the matching leave() call, we can actually support nesting. If enter() is called inside an active lazy_mmu section, TIF_LAZY_MMU will already be set, and we can then return LAZY_MMU_NESTED to instruct the matching leave() call not to clear TIF_LAZY_MMU. The only effect of this patch is to ensure that TIF_LAZY_MMU (and therefore the batching optimisation) remains set until the outermost lazy_mmu section ends. leave() still emits barriers if needed, regardless of the nesting level, as the caller may expect any page table changes to become visible when leave() returns. Signed-off-by: Kevin Brodsky --- arch/arm64/include/asm/pgtable.h | 19 +++++-------------- 1 file changed, 5 insertions(+), 14 deletions(-) diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgta= ble.h index 816197d08165..602feda97dc4 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -85,24 +85,14 @@ typedef int lazy_mmu_state_t; =20 static inline lazy_mmu_state_t arch_enter_lazy_mmu_mode(void) { - /* - * lazy_mmu_mode is not supposed to permit nesting. But in practice this - * does happen with CONFIG_DEBUG_PAGEALLOC, where a page allocation - * inside a lazy_mmu_mode section (such as zap_pte_range()) will change - * permissions on the linear map with apply_to_page_range(), which - * re-enters lazy_mmu_mode. So we tolerate nesting in our - * implementation. The first call to arch_leave_lazy_mmu_mode() will - * flush and clear the flag such that the remainder of the work in the - * outer nest behaves as if outside of lazy mmu mode. This is safe and - * keeps tracking simple. - */ + int lazy_mmu_nested; =20 if (in_interrupt()) return LAZY_MMU_DEFAULT; =20 - set_thread_flag(TIF_LAZY_MMU); + lazy_mmu_nested =3D test_and_set_thread_flag(TIF_LAZY_MMU); =20 - return LAZY_MMU_DEFAULT; + return lazy_mmu_nested ? LAZY_MMU_NESTED : LAZY_MMU_DEFAULT; } =20 static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state) @@ -113,7 +103,8 @@ static inline void arch_leave_lazy_mmu_mode(lazy_mmu_st= ate_t state) if (test_and_clear_thread_flag(TIF_LAZY_MMU_PENDING)) emit_pte_barriers(); =20 - clear_thread_flag(TIF_LAZY_MMU); + if (state !=3D LAZY_MMU_NESTED) + clear_thread_flag(TIF_LAZY_MMU); } =20 #ifdef CONFIG_TRANSPARENT_HUGEPAGE --=20 2.47.0