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=-6.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=no 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 ED99DC43460 for ; Thu, 29 Apr 2021 15:52:53 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 7437D61446 for ; Thu, 29 Apr 2021 15:52:53 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7437D61446 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 C22C46B006C; Thu, 29 Apr 2021 11:52:52 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id BD0E46B006E; Thu, 29 Apr 2021 11:52:52 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A72506B0070; Thu, 29 Apr 2021 11:52:52 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0009.hostedemail.com [216.40.44.9]) by kanga.kvack.org (Postfix) with ESMTP id 889726B006C for ; Thu, 29 Apr 2021 11:52:52 -0400 (EDT) Received: from smtpin01.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 3C86F180AE7E1 for ; Thu, 29 Apr 2021 15:52:52 +0000 (UTC) X-FDA: 78085847784.01.7D2C119 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf24.hostedemail.com (Postfix) with ESMTP id D903CA0009E4 for ; Thu, 29 Apr 2021 15:52:40 +0000 (UTC) Received: by mail.kernel.org (Postfix) with ESMTPSA id A1E9061409; Thu, 29 Apr 2021 15:52:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1619711570; bh=0apMfOT7yVxKteFX5jy7z8m501eJwcSbWxO68VHKSRk=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=LTi6GOSaGUd+ShHp6T14DN+0KcAC/Cf+iJ2wo3JeY4KcVLGrNG7PJX/9lwfGjbBxY xTDUu4H8qbLvYG/bZdRYHjvMpNWvsHCLMLEMJIxagThFeNcyS2oFcoADgUJm66fmy2 pNMUT97gaDLMpoH/LXSAHJgmq92dUwedEYxRW9E5ZrweAl2gVcGUE84KvAewzfqDf3 oVryRx81P1d8gW5CUIUeFOfEVnrodkdPAAcn0ktZzpZvArHntb4DE/cSYIWiEBBrrC xw8Kf8ja7mJLZPAMMRPa4ouCPVy29JrJx3wBy3gM1Mfib7jmRf8aN06yAR8W82z/El oY1ohtDICNDpg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 30F475C00E9; Thu, 29 Apr 2021 08:52:50 -0700 (PDT) Date: Thu, 29 Apr 2021 08:52:50 -0700 From: "Paul E. McKenney" To: Michel Lespinasse Cc: Andy Lutomirski , Linux-MM , Laurent Dufour , Peter Zijlstra , Michal Hocko , Matthew Wilcox , Rik van Riel , Andrew Morton , Suren Baghdasaryan , Joel Fernandes , Rom Lemarchand , Linux-Kernel Subject: Re: [RFC PATCH 13/37] mm: implement speculative handling in __handle_mm_fault(). Message-ID: <20210429155250.GV975577@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210407014502.24091-1-michel@lespinasse.org> <20210407014502.24091-14-michel@lespinasse.org> <20210428145823.GA856@lespinasse.org> <20210428161108.GP975577@paulmck-ThinkPad-P17-Gen-1> <20210429000225.GC10973@lespinasse.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210429000225.GC10973@lespinasse.org> X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: D903CA0009E4 X-Stat-Signature: nokt3q6xqkr6bkigd8oimci7zadr11q5 Received-SPF: none (kernel.org>: No applicable sender policy available) receiver=imf24; identity=mailfrom; envelope-from=""; helo=mail.kernel.org; client-ip=198.145.29.99 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1619711560-206807 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 Wed, Apr 28, 2021 at 05:02:25PM -0700, Michel Lespinasse wrote: > On Wed, Apr 28, 2021 at 09:11:08AM -0700, Paul E. McKenney wrote: > > On Wed, Apr 28, 2021 at 08:13:53AM -0700, Andy Lutomirski wrote: > > > On Wed, Apr 28, 2021 at 8:05 AM Michel Lespinasse wrote: > > > > > > > > On Wed, Apr 07, 2021 at 08:36:01AM -0700, Andy Lutomirski wrote: > > > > > On 4/6/21 6:44 PM, Michel Lespinasse wrote: > > > > > > The page table tree is walked with local irqs disabled, which prevents > > > > > > page table reclamation (similarly to what fast GUP does). The logic is > > > > > > otherwise similar to the non-speculative path, but with additional > > > > > > restrictions: in the speculative path, we do not handle huge pages or > > > > > > wiring new pages tables. > > > > > > > > > > Not on most architectures. Quoting the actual comment in mm/gup.c: > > > > > > > > > > > * Before activating this code, please be aware that the following assumptions > > > > > > * are currently made: > > > > > > * > > > > > > * *) Either MMU_GATHER_RCU_TABLE_FREE is enabled, and tlb_remove_table() is used to > > > > > > * free pages containing page tables or TLB flushing requires IPI broadcast. > > > > > > > > > > On MMU_GATHER_RCU_TABLE_FREE architectures, you cannot make the > > > > > assumption that it is safe to dereference a pointer in a page table just > > > > > because irqs are off. You need RCU protection, too. > > > > > > > > > > You have the same error in the cover letter. > > > > > > > > Hi Andy, > > > > > > > > Thanks for your comment. At first I thought did not matter, because we > > > > only enable ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT on selected > > > > architectures, and I thought MMU_GATHER_RCU_TABLE_FREE is not set on > > > > these. But I was wrong - MMU_GATHER_RCU_TABLE_FREE is enabled on X86 > > > > with paravirt. So I took another look at fast GUP to make sure I > > > > actually understand it. > > > > > > > > This brings a question about lockless_pages_from_mm() - I see it > > > > disabling interrupts, which it explains is necessary for disabling THP > > > > splitting IPIs, but I do not see it taking an RCU read lock as would > > > > be necessary for preventing paga table freeing on > > > > MMU_GATHER_RCU_TABLE_FREE configs. I figure local_irq_save() > > > > indirectly takes an rcu read lock somehow ? I think this is something > > > > I should also mention in my explanation, and I have not seen a good > > > > description of this on the fast GUP side... > > > > > > Sounds like a bug! That being said, based on my extremely limited > > > understanding of how the common RCU modes work, local_irq_save() > > > probably implies an RCU lock in at least some cases. Hi Paul! > > > > In modern kernels, local_irq_save() does have RCU reader semantics, > > meaning that synchronize_rcu() will wait for pre-exiting irq-disabled > > regions. It will also wait for pre-existing bh-disable, preempt-disable, > > and of course rcu_read_lock() sections of code. > > Thanks Paul for confirming / clarifying this. BTW, it would be good to > add this to the rcu header files, just so people have something to > reference to when they depend on such behavior (like fast GUP > currently does). There is this in the synchronize_rcu() header block comment: * synchronize_rcu() was waiting. RCU read-side critical sections are * delimited by rcu_read_lock() and rcu_read_unlock(), and may be nested. * In addition, regions of code across which interrupts, preemption, or * softirqs have been disabled also serve as RCU read-side critical * sections. This includes hardware interrupt handlers, softirq handlers, * and NMI handlers. I have pulled this into a separate paragraph to increase its visibility, and will check out other locations in comments and documentation. Thanx, Paul > Going back to my patch. I don't need to protect against THP splitting > here, as I'm only handling the small page case. So when > MMU_GATHER_RCU_TABLE_FREE is enabled, I *think* I could get away with > using only an rcu read lock, instead of disabling interrupts which > implicitly creates the rcu read lock. I'm not sure which way to go - > fast GUP always disables interrupts regardless of the > MMU_GATHER_RCU_TABLE_FREE setting, and I think there is a case to be > made for following the fast GUP stes rather than trying to be smarter. > > Andy, do you have any opinion on this ? Or anyone else really ? > > Thanks, > > -- > Michel "walken" Lespinasse