From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 47ADCCCFA05 for ; Thu, 6 Nov 2025 10:51:57 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id A63FC8E0014; Thu, 6 Nov 2025 05:51:56 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id A420B8E0002; Thu, 6 Nov 2025 05:51:56 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 978A38E0014; Thu, 6 Nov 2025 05:51:56 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id 83FA88E0002 for ; Thu, 6 Nov 2025 05:51:56 -0500 (EST) Received: from smtpin28.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 2B8C4140647 for ; Thu, 6 Nov 2025 10:51:56 +0000 (UTC) X-FDA: 84079867032.28.6517157 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf03.hostedemail.com (Postfix) with ESMTP id F2D0720005 for ; Thu, 6 Nov 2025 10:51:53 +0000 (UTC) Authentication-Results: imf03.hostedemail.com; dkim=none; spf=pass (imf03.hostedemail.com: domain of kevin.brodsky@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=kevin.brodsky@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1762426314; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=9BUIVBWzSEg5HnaNi8EEWsgRtlWmX6rseREfNx4QEYE=; b=jVxIdO9yJ2MzCnCpgKxSIMrsbOR0BH2YMHWUBnHdPSLuaX37VaoriSxuDW5gXGOV5fqc2C kx0sihWAsM2e45PMycPRVUFzNaiZGS9BAFjrjWlkk5YFaqP8QWd47QlitBnbX0O4cGBNqK 7c30glAjax0GMWXdeNTqPNHKUBO4g9w= ARC-Authentication-Results: i=1; imf03.hostedemail.com; dkim=none; spf=pass (imf03.hostedemail.com: domain of kevin.brodsky@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=kevin.brodsky@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1762426314; a=rsa-sha256; cv=none; b=tdPThe8fPdJl8mvj8sWHYAihV3tOa+1YmfSVib+lDwP3vGmvRDJr8Sd617MxY35RkWggoT WjGwBZQcxcph8Z3S1WMvDCHCqAaVhBCHTsr7lO2/DoJUg9AYa7DdtabTIjZcm2nnmGuwSP vKAaOslowv8zhrGF/xz6qy1InwwkO6I= 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 20A081596; Thu, 6 Nov 2025 02:51:45 -0800 (PST) Received: from [10.1.34.75] (unknown [10.1.34.75]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6866A3F63F; Thu, 6 Nov 2025 02:51:46 -0800 (PST) Message-ID: <48a4ecb5-3412-4d3f-9e43-535f8bee505f@arm.com> Date: Thu, 6 Nov 2025 10:51:43 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 07/12] mm: enable lazy_mmu sections to nest To: Alexander Gordeev , Ritesh Harjani Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andreas Larsson , Andrew Morton , Boris Ostrovsky , Borislav Petkov , Catalin Marinas , Christophe Leroy , Dave Hansen , David Hildenbrand , "David S. Miller" , David Woodhouse , "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 , Yeoreum Yun , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, sparclinux@vger.kernel.org, xen-devel@lists.xenproject.org, x86@kernel.org References: <20251029100909.3381140-1-kevin.brodsky@arm.com> <20251029100909.3381140-8-kevin.brodsky@arm.com> <87ms5050g0.ritesh.list@gmail.com> <50d1b63a-88d7-4484-82c0-3bde96e3207d-agordeev@linux.ibm.com> Content-Language: en-GB From: Kevin Brodsky In-Reply-To: <50d1b63a-88d7-4484-82c0-3bde96e3207d-agordeev@linux.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: F2D0720005 X-Stat-Signature: q8k5u8e3sq3mrmjp7e8e6bpazeqmkujs X-Rspam-User: X-HE-Tag: 1762426313-28509 X-HE-Meta: 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 Ov/fvycs 2kX1HL0x7H/0n8CIhqUUAxNMqxVo6PSagNtOdj2UTyNpyis8mXmpF0l/kigPhS8Ixa77bz3nS2EDIr8PAY6wGeQofAViMGRQjvkB5YAbaBBfCKF5GTeOBYsEBjonEbX4fbPqywWZgcIWfGwt6aVfw/7bV85SwIaWKP6xhz3CLiHNhhuZKGnwGzbcwJUiP2D+yxuCf1IURdObxlVLpPl1eoKhjzgTbViqEy9EeufNfiCBEudFhUW84SVvRVhJ/fxRGWvIPMt2gT4PgDATsgg2vgN9FCpBIzOmBjiHYMIpYOhBBwNEINf2zOvoLjA== 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 05/11/2025 16:12, Alexander Gordeev wrote: > On Wed, Nov 05, 2025 at 02:19:03PM +0530, Ritesh Harjani wrote: >>> + * in_lazy_mmu_mode() can be used to check whether the lazy MMU mode is >>> + * currently enabled. >>> */ >>> #ifdef CONFIG_ARCH_HAS_LAZY_MMU_MODE >>> static inline void lazy_mmu_mode_enable(void) >>> { >>> - arch_enter_lazy_mmu_mode(); >>> + struct lazy_mmu_state *state = ¤t->lazy_mmu_state; >>> + >>> + VM_WARN_ON_ONCE(state->nesting_level == U8_MAX); >>> + /* enable() must not be called while paused */ >>> + VM_WARN_ON(state->nesting_level > 0 && !state->active); >>> + >>> + if (state->nesting_level++ == 0) { >>> + state->active = true; >>> + arch_enter_lazy_mmu_mode(); >>> + } >>> } >> Some architectures disables preemption in their >> arch_enter_lazy_mmu_mode(). So shouldn't the state->active = true should >> happen after arch_enter_lazy_mmu_mode() has disabled preemption()? i.e. > Do you have some scenario in mind that could cause an issue? > IOW, what could go wrong if the process is scheduled to another > CPU before preempt_disable() is called? I'm not sure I understand the issue either. >> static inline void lazy_mmu_mode_enable(void) >> { >> - arch_enter_lazy_mmu_mode(); >> + struct lazy_mmu_state *state = ¤t->lazy_mmu_state; >> + >> + VM_WARN_ON_ONCE(state->nesting_level == U8_MAX); >> + /* enable() must not be called while paused */ >> + VM_WARN_ON(state->nesting_level > 0 && !state->active); >> + >> + if (state->nesting_level++ == 0) { >> + arch_enter_lazy_mmu_mode(); >> + state->active = true; >> + } >> } >> >> ... I think it make more sense to enable the state after the arch_** >> call right. > But then in_lazy_mmu_mode() would return false if called from > arch_enter_lazy_mmu_mode(). Not big problem, but still.. The ordering of nesting_level/active was the way you expected in v3, but the conclusion of the discussion with David H [1] is that it doesn't really matter so I simplified the ordering in v4 - the arch hooks shouldn't call in_lazy_mmu_mode() or inspect lazy_mmu_state. arch_enter()/arch_leave() shouldn't need it anyway since they're called once per outer section (not in nested sections). arch_flush() could potentially do something different when nested, but that seems unlikely. - Kevin [1] https://lore.kernel.org/all/af4414b6-617c-4dc8-bddc-3ea00d1f6f3b@redhat.com/