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 D14BAC388F9 for ; Mon, 23 Nov 2020 21:18:00 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 24232206FB for ; Mon, 23 Nov 2020 21:18:00 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="XY4IMJzj" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 24232206FB Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 66A0F6B005D; Mon, 23 Nov 2020 16:17:59 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 6417D6B006E; Mon, 23 Nov 2020 16:17:59 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 508FD6B0070; Mon, 23 Nov 2020 16:17:59 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0233.hostedemail.com [216.40.44.233]) by kanga.kvack.org (Postfix) with ESMTP id 2470C6B005D for ; Mon, 23 Nov 2020 16:17:59 -0500 (EST) Received: from smtpin01.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id C198A1EF1 for ; Mon, 23 Nov 2020 21:17:58 +0000 (UTC) X-FDA: 77516945436.01.debt89_1d0138527368 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin01.hostedemail.com (Postfix) with ESMTP id A393E10045356 for ; Mon, 23 Nov 2020 21:17:58 +0000 (UTC) X-HE-Tag: debt89_1d0138527368 X-Filterd-Recvd-Size: 5576 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf29.hostedemail.com (Postfix) with ESMTP for ; Mon, 23 Nov 2020 21:17:58 +0000 (UTC) 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 1C4BE206B5; Mon, 23 Nov 2020 21:17:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1606166276; bh=zbTKH20VVZucoaew5ZXWATj6gCGP2JnDyZG1DT3kGTI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=XY4IMJzjKhC2/RYHNdBmKxgJiiRAean7wQCa7lewSt3Xm56dKXrzNeJcNRGYORfpG YqWohL+HmE81nh3gpZWmE/tS9caSMs5+NeN2M6LFWR5uWAUH60m0S+dbgAPeckdjqG CGmFjwEV6GKCUIpGsgTkQzcW4hHbCAoSvVGmS5ms= Date: Mon, 23 Nov 2020 21:17:51 +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: <20201123211750.GA12069@willie-the-truck> References: <20201120143557.6715-1-will@kernel.org> <20201120143557.6715-7-will@kernel.org> <20201120204005.GC1303870@google.com> <20201123183554.GC11688@willie-the-truck> <20201123200403.GA3888699@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201123200403.GA3888699@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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: On Mon, Nov 23, 2020 at 01:04:03PM -0700, Yu Zhao wrote: > On Mon, Nov 23, 2020 at 06:35:55PM +0000, Will Deacon wrote: > > 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? > > I double checked my conclusion on patch 4, and aside from a couple > of typos, it still seems correct after the weekend. I still need to digest that, but I would prefer that we restore the invalidation first, and then have a subsequent commit to relax it. I find it hard to believe that the behaviour in mainline at the moment is deliberate. That is, I'm not against optimising this, but I'd rather get it "obviously correct" first and the current code is definitely not that. > > 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()). > > The lazy invalidation for that is done when we write-protect a page, > not an individual PTE. When we do so, our decision is based on both > the dirty bit and the writable bit on each PTE mapping this page. So > we only need to make sure we don't lose both on a PTE. And we don't > here. Sorry, I don't follow what you're getting at here (page vs pte). Please can you point me to the code you're referring to? The case I'm worried about is code that holds sufficient locks (e.g. mmap_sem + ptl) finding an entry where !pte_write() and assuming (despite pte_dirty()) that there can't be any concurrent modifications to the mapped page. Granted, I haven't found anything doing that, but I could not convince myself that it would be a bug to write such code, either. > > 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). > > Nope. tlb_gather/finish_mmu() should be added by b3a81d0841a9 > ("mm: fix KSM data corruption") in the first place. Sure, but if you check out b3a81d0841a9 then you have a fullmm TLB invalidation in tlb_finish_mmu(). 7a30df49f63a is what removed that, no? Will