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 X-Spam-Level: X-Spam-Status: No, score=-2.1 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 31E31C04ABB for ; Thu, 13 Sep 2018 10:57:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C7E1A20861 for ; Thu, 13 Sep 2018 10:57:53 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="JvYDEY/U" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C7E1A20861 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727127AbeIMQGr (ORCPT ); Thu, 13 Sep 2018 12:06:47 -0400 Received: from merlin.infradead.org ([205.233.59.134]:55884 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726741AbeIMQGr (ORCPT ); Thu, 13 Sep 2018 12:06:47 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; 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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=58pngwe/Y8svAz545CEisaXl6sbjbE0b75SZatOv5RQ=; b=JvYDEY/UtSjR6PIRgxMYZ/2to JyL4G2C+daw+P0mb9+ShFWt1k24UFN7mHkHuCaK5WVTEQFggiCqjtI04qFrDzwCaD9wtqb4E2tLZQ wC5yDPh0GQMGgkdzrDZOXWoB1ur7ZlutL8CXbic5kgsm/Kfny1AOux2lGqgsF/+eLg2rGryoTIZig Py+XQFP4o2oCzwnO4O/3MuRBrJZudNhE6OIXceReYgQ1DXqwOLP5hwww2xSFXfsq5nLo93SEv5Npn /9UyEB6QBnpqGtb90RBjWu21oiv/dnm9W10xAd+jMkb4m3qtE4wabivaK4Jx/i02wDKi9dxDIGI6n A8m/oaE3Q==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1g0PJd-0001se-7J; Thu, 13 Sep 2018 10:57:41 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 44516202C1A2D; Thu, 13 Sep 2018 12:57:38 +0200 (CEST) Date: Thu, 13 Sep 2018 12:57:38 +0200 From: Peter Zijlstra To: Martin Schwidefsky Cc: will.deacon@arm.com, aneesh.kumar@linux.vnet.ibm.com, akpm@linux-foundation.org, npiggin@gmail.com, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux@armlinux.org.uk, heiko.carstens@de.ibm.com Subject: Re: [RFC][PATCH 01/11] asm-generic/tlb: Provide a comment Message-ID: <20180913105738.GW24124@hirez.programming.kicks-ass.net> References: <20180913092110.817204997@infradead.org> <20180913092811.894806629@infradead.org> <20180913123014.0d9321b8@mschwideX1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180913123014.0d9321b8@mschwideX1> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 13, 2018 at 12:30:14PM +0200, Martin Schwidefsky wrote: > > + * The mmu_gather data structure is used by the mm code to implement the > > + * correct and efficient ordering of freeing pages and TLB invalidations. > > + * > > + * This correct ordering is: > > + * > > + * 1) unhook page > > + * 2) TLB invalidate page > > + * 3) free page > > + * > > + * That is, we must never free a page before we have ensured there are no live > > + * translations left to it. Otherwise it might be possible to observe (or > > + * worse, change) the page content after it has been reused. > > + * > > This first comment already includes the reason why s390 is probably better off > with its own mmu-gather implementation. It depends on the situation if we have > > 1) unhook the page and do a TLB flush at the same time > 2) free page > > or > > 1) unhook page > 2) free page > 3) final TLB flush of the whole mm that's the fullmm case, right? > A variant of the second order we had in the past is to do the mm TLB flush first, > then the unhooks and frees of the individual pages. The are some tricky corners > switching between the two variants, see finish_arch_post_lock_switch. > > The point is: we *never* have the order 1) unhook, 2) TLB invalidate, 3) free. > If there is concurrency due to a multi-threaded application we have to do the > unhook of the page-table entry and the TLB flush with a single instruction. You can still get the thing you want if for !fullmm you have a no-op tlb_flush() implementation, assuming your arch page-table frobbing thing has the required TLB flush in. Note that that's not utterly unlike how the PowerPC/Sparc hash things work, they clear and invalidate entries different from others and don't use the mmu_gather tlb-flush.