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=-17.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 36DABC63697 for ; Mon, 23 Nov 2020 18:36:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CD864221F7 for ; Mon, 23 Nov 2020 18:36:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="rJRxc3FT" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729812AbgKWSgB (ORCPT ); Mon, 23 Nov 2020 13:36:01 -0500 Received: from mail.kernel.org ([198.145.29.99]:39822 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729140AbgKWSgB (ORCPT ); Mon, 23 Nov 2020 13:36:01 -0500 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id D5DA12078E; Mon, 23 Nov 2020 18:35:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1606156560; bh=FZugMw/Hog6jEeQy+QszK3Ewi0gPdbcIiChWPVuppDo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=rJRxc3FThC4AXXQDGd4ySKR1IZkIlI7DzUiFQldSiKAjLI2HAtTPmK5MPZKOrorz3 x1Zh1MdqtAPgOUgE9Ho3vR7dQq6ZnDBqCFbwg77D/fnnUal1k6Max/JTL8kr9GuTdx Ot2iTXuZ80JCwSsItHl9ALlh1HQ7kpo2oU0A8L04= Date: Mon, 23 Nov 2020 18:35:55 +0000 From: Will Deacon To: Yu Zhao Cc: linux-kernel@vger.kernel.org, kernel-team@android.com, Catalin Marinas , Minchan Kim , Peter Zijlstra , Linus Torvalds , Anshuman Khandual , linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH 6/6] mm: proc: Avoid fullmm flush for young/dirty bit toggling Message-ID: <20201123183554.GC11688@willie-the-truck> References: <20201120143557.6715-1-will@kernel.org> <20201120143557.6715-7-will@kernel.org> <20201120204005.GC1303870@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201120204005.GC1303870@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 20, 2020 at 01:40:05PM -0700, Yu Zhao wrote: > On Fri, Nov 20, 2020 at 02:35:57PM +0000, Will Deacon wrote: > > clear_refs_write() uses the 'fullmm' API for invalidating TLBs after > > updating the page-tables for the current mm. However, since the mm is not > > being freed, this can result in stale TLB entries on architectures which > > elide 'fullmm' invalidation. > > > > Ensure that TLB invalidation is performed after updating soft-dirty > > entries via clear_refs_write() by using the non-fullmm API to MMU gather. > > > > Signed-off-by: Will Deacon > > --- > > fs/proc/task_mmu.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > > index a76d339b5754..316af047f1aa 100644 > > --- a/fs/proc/task_mmu.c > > +++ b/fs/proc/task_mmu.c > > @@ -1238,7 +1238,7 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf, > > count = -EINTR; > > goto out_mm; > > } > > - tlb_gather_mmu_fullmm(&tlb, mm); > > + tlb_gather_mmu(&tlb, mm, 0, TASK_SIZE); > > Let's assume my reply to patch 4 is wrong, and therefore we still need > tlb_gather/finish_mmu() here. But then wouldn't this change deprive > architectures other than ARM the opportunity to optimize based on the > fact it's a full-mm flush? Only for the soft-dirty case, but I think TLB invalidation is required there because we are write-protecting the entries and I don't see any mechanism to handle lazy invalidation for that (compared with the aging case, which is handled via pte_accessible()). Furthermore, If we decide that we can relax the TLB invalidation requirements here, then I'd much rather than was done deliberately, rather than as an accidental side-effect of another commit (since I think the current behaviour was a consequence of 7a30df49f63a). > It seems to me ARM's interpretation of tlb->fullmm is a special case, > not the other way around. Although I agree that this is subtle and error-prone (which is why I'm trying to make the API more explicit here), it _is_ documented clearly in asm-generic/tlb.h: * - mmu_gather::fullmm * * A flag set by tlb_gather_mmu() to indicate we're going to free * the entire mm; this allows a number of optimizations. * * - We can ignore tlb_{start,end}_vma(); because we don't * care about ranges. Everything will be shot down. * * - (RISC) architectures that use ASIDs can cycle to a new ASID * and delay the invalidation until ASID space runs out. Will