From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 26D4E54774; Mon, 2 Feb 2026 12:52:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770036728; cv=none; b=GReDQtDD2v0L9ZYTUSQNTXejJbbv3j2QWb/136zWjnNGRTCV7BSYLiALAmEgjWVjv4r+kDCjV4DvBoNZlprX803hEuMRrREApWrtGd2kfGS+FQHxFPFyLBw5VgSzGKLau5sbSg0NYnMhn+wRgkQZWu9C/Zb4PQXdSssQofe5OWI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770036728; c=relaxed/simple; bh=lru2yyaiBhkqPmn0DEoUzd8wpXzMSS28OTVqgMEXZ8Y=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=aRgGZUqsyFaOJxSfQX1HJNWIRkaI81o/IzQ04uzEKO5jygXMgdstHdxXgM+JNmBUGis64LJhQ4Njuq0LnoLI8HcBVRFTyZf3boCVJ3m064cBahYyh8NMux9Mf2GLrik4J0oJcZO6lowKic1r226+aQPH0KKnEEl2oC4l8xby4fo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=fByy43pZ; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="fByy43pZ" 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> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0f44dfb7-fce3-44c1-ab25-b013ba18a59b@linux.dev> 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.