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=-7.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 45B57C433B4 for ; Wed, 28 Apr 2021 15:00:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0CD2961433 for ; Wed, 28 Apr 2021 15:00:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240397AbhD1PBN (ORCPT ); Wed, 28 Apr 2021 11:01:13 -0400 Received: from server.lespinasse.org ([63.205.204.226]:57569 "EHLO server.lespinasse.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240496AbhD1O7K (ORCPT ); Wed, 28 Apr 2021 10:59:10 -0400 DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=lespinasse.org; i=@lespinasse.org; q=dns/txt; s=srv-13-ed; t=1619621903; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to : from; bh=QydOVvpj2OAyZRQLFyThi3nH5+pcZsNgV/K3JPyDtNY=; b=v0pzT2ftLbgMGbGTh60i1QNqjD03znuBtiCVjyDA+AlHGKjxRg095hn5nSntHwdJwW5G2 6ieNuTl11n2gIsIBg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=lespinasse.org; i=@lespinasse.org; q=dns/txt; s=srv-13-rsa; t=1619621903; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to : from; bh=QydOVvpj2OAyZRQLFyThi3nH5+pcZsNgV/K3JPyDtNY=; b=x/TTWv4tQOnLutEIDbG9cc6Wu1R2lL73vSbCsE8Edvz0ZzakWl8TXCdqzHPoB4q7KtApc PS63BgYRDrK2I9sJbFt8w5Wb3W+YiKEv+O7IVJm/IPQEJcRmH4oiDN7wOBl8+KmMEdOzMEQ Bcqg/QcQ7qk+RP6dkeEYE8bVyfEwJQb/LLVgCmYr3sTVHLG47D8GgPAA+ceHeEVOMaND8ZV qyRoeiMdhcU7ZcbsIrXL3tRexMT8znCDwT/Zv+I8e3V07y3OlHEWb2uXB4NrwjqkimLqeku dwPEpKGb8yQQBd/CPngXo6zar+9JaHutiDx2DHNR1JpZkjMwl9RU4K4kL2ag== Received: by server.lespinasse.org (Postfix, from userid 1000) id B1B4816026E; Wed, 28 Apr 2021 07:58:23 -0700 (PDT) Date: Wed, 28 Apr 2021 07:58:23 -0700 From: Michel Lespinasse To: Andy Lutomirski , Paul McKenney Cc: Michel Lespinasse , 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: <20210428145823.GA856@lespinasse.org> References: <20210407014502.24091-1-michel@lespinasse.org> <20210407014502.24091-14-michel@lespinasse.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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... Thanks, -- Michel "walken" Lespinasse