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 A4B96E7315D for ; Mon, 2 Feb 2026 12:52:01 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 018A26B00AF; Mon, 2 Feb 2026 07:52:01 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id EEE016B00B2; Mon, 2 Feb 2026 07:52:00 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DFE556B00B3; Mon, 2 Feb 2026 07:52:00 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id CE4C96B00AF for ; Mon, 2 Feb 2026 07:52:00 -0500 (EST) Received: from smtpin27.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 9C39FB1FCD for ; Mon, 2 Feb 2026 12:52:00 +0000 (UTC) X-FDA: 84399504000.27.0295132 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) by imf05.hostedemail.com (Postfix) with ESMTP id B5BF4100003 for ; Mon, 2 Feb 2026 12:51:58 +0000 (UTC) Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=infradead.org header.s=desiato.20200630 header.b=fByy43pZ; spf=none (imf05.hostedemail.com: domain of peterz@infradead.org has no SPF policy when checking 90.155.92.199) smtp.mailfrom=peterz@infradead.org; dmarc=pass (policy=none) header.from=infradead.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1770036719; 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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=yY07cg0hwKCQwR25CLGXxidJIQ1squuIk4ZcGlGC93E=; b=Uf8zMQXr+epwXmlkX9ftCY9GaIT92JAuBdGmMrIFJeeUiquvIgYvfwH1jgtSY7Txhm23zW Gk86OBdIlEwxDPoRdqthe2Rt+aHmEhvk2P/GzuH9N9tHizSfK48NmGc5sd5A1v7MG4z2KV UEhhHgXi/3EIzOwSFb3WQV/THB/lIaM= ARC-Authentication-Results: i=1; imf05.hostedemail.com; dkim=pass header.d=infradead.org header.s=desiato.20200630 header.b=fByy43pZ; spf=none (imf05.hostedemail.com: domain of peterz@infradead.org has no SPF policy when checking 90.155.92.199) smtp.mailfrom=peterz@infradead.org; dmarc=pass (policy=none) header.from=infradead.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1770036719; a=rsa-sha256; cv=none; b=fFmh4dVC1WwFxJi3kSr9lxh0VznakqyqGmx5TuANnh28c8Z9a2M7vDajpaFg6aUxyx54ex lpxAC86bcrjgQ/UNLgDOlx8MaB6MdgPPcoKDasdyeZKsZNA2lbWIPUXdR0TZ5sHBMHTWjP kmwy22C43UM43aCsFlVWaC78t3Izpjc= DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=yY07cg0hwKCQwR25CLGXxidJIQ1squuIk4ZcGlGC93E=; b=fByy43pZRv6IaSc7ayvOokHhhm 5nHBXvsWjBjdd/xvRrfB8JohvFI92kqsc+T8fxnQS8DspTZJBokZ+++dD2QJ673IPcJR7hMFjZagM 5kEV22tYADCLkQLgG8je7md7HqxtQGwQC3REA1XhKC0+SbEKiFX7am6PwbUYOt7kZh5H/x57al9U0 SvCvl0kh8/DdAI2aqGOlxbk6ddSWY79OoHHR7Vx6h4gkngqasafqwe28K7Mef5K6oZp8j+OxWfo8W hH4pWxQqtb/6uPMkNeFaRG4IkkrYkI9OytHqnVhhXbns4/mDpnHL/1qVI6GopYwxVNLXMp3h/E3ny uvrKmnWA==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vmtPA-0000000ETbd-13uw; Mon, 02 Feb 2026 12:51:48 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id E0777300208; Mon, 02 Feb 2026 13:51:46 +0100 (CET) Date: Mon, 2 Feb 2026 13:51:46 +0100 From: Peter Zijlstra To: Lance Yang Cc: akpm@linux-foundation.org, david@kernel.org, dave.hansen@intel.com, dave.hansen@linux.intel.com, ypodemsk@redhat.com, hughd@google.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, arnd@arndb.de, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, shy828301@gmail.com, riel@surriel.com, jannh@google.com, jgross@suse.com, seanjc@google.com, pbonzini@redhat.com, boris.ostrovsky@oracle.com, virtualization@lists.linux.dev, kvm@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ioworker0@gmail.com Subject: Re: [PATCH v4 1/3] mm: use targeted IPIs for TLB sync with lockless page table walkers Message-ID: <20260202125146.GC1395266@noisy.programming.kicks-ass.net> References: <20260202074557.16544-1-lance.yang@linux.dev> <20260202074557.16544-2-lance.yang@linux.dev> <20260202094245.GD2995752@noisy.programming.kicks-ass.net> <0f44dfb7-fce3-44c1-ab25-b013ba18a59b@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0f44dfb7-fce3-44c1-ab25-b013ba18a59b@linux.dev> X-Rspam-User: X-Rspamd-Queue-Id: B5BF4100003 X-Rspamd-Server: rspam07 X-Stat-Signature: be7uyrz6ef5i5i1ui8fi3tk7x4z458h8 X-HE-Tag: 1770036718-555057 X-HE-Meta: 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 ewy53H8B Jprb2xhrHEfGXcj9PT+67LMvQB2kjc+b3eNcjOOK0r7Wb3MfAGyjLFlgsryFMpTvkU8hA1CDImtBCq5UiNku9dGP96qtt9jVY8vr8ojKBbSOIDoHIWojjj0gp4Vc9/MQoRUdJyPRWZidYmTCOHYWE2/l07t40tneC72EMdPCZfVAksPtqg2QDGxw9go80H9sa8iGy4dJqGLB2TSH3MkXxD7ZNTw== 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 Mon, Feb 02, 2026 at 08:14:32PM +0800, Lance Yang wrote: > > > + /* Pairs with smp_mb() in pt_walk_lockless_start(). */ > > > > Pairs how? The start thing does something like: > > > > [W] active_lockless_pt_walk_mm = mm > > MB > > [L] page-tables > > > > So this is: > > > > [L] page-tables > > RMB > > [L] active_lockless_pt_walk_mm > > > > ? > > On the walker side (pt_walk_lockless_start): > > [W] active_lockless_pt_walk_mm = mm > MB > [L] page-tables (walker reads page tables) > > So the walker publishes "I'm walking this mm" before reading page tables. > > On the sync side we don't read page-tables. We do: > > RMB > [L] active_lockless_pt_walk_mm (we read the per-CPU pointer below) > > We need to observe the walker's store of active_lockless_pt_walk_mm before > we decide which CPUs to IPI. > > So on the sync side we do smp_rmb(), then read active_lockless_pt_walk_mm. > > That pairs with the full barrier in pt_walk_lockless_start(). No it doesn't; this is not how memory barriers work.