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=-6.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 099CDC433E2 for ; Thu, 17 Sep 2020 11:26:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B3659208DB for ; Thu, 17 Sep 2020 11:26:32 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=ziepe.ca header.i=@ziepe.ca header.b="amcXrLuQ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726596AbgIQL0O (ORCPT ); Thu, 17 Sep 2020 07:26:14 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58454 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726719AbgIQLZm (ORCPT ); Thu, 17 Sep 2020 07:25:42 -0400 Received: from mail-qk1-x741.google.com (mail-qk1-x741.google.com [IPv6:2607:f8b0:4864:20::741]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A08A2C06174A for ; Thu, 17 Sep 2020 04:25:41 -0700 (PDT) Received: by mail-qk1-x741.google.com with SMTP id w186so1806437qkd.1 for ; Thu, 17 Sep 2020 04:25:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ziepe.ca; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=TmuOAuPqFkLEUVE/VOD7wN6NrUglI5pWEI0/RzoOeAM=; b=amcXrLuQydF5s2mbCGVXNJVE5jL/3upTVVlK/vaqWWqAAWxHo4fWaelHnE4tELsn0B iduI7ukYFh+mfEbYMMDH4FWyOT3yjlOjenKlY9QMCqvvM5nToAops7w513DcVJuI89re VUd+0tjKCbJln+UdJLB96z1IHgmBu8p/+autHL/H/j94cb/i1lCNIw7BcQIB9K2rPTeb GH1fFt7IJuJbztqAk8wj1UmC8gqcGb9QosOSc8M/ezYkBP80R3kJ1JfrZkX8CFYAJbWa xwpz+Lqx2918BxcobFmZQdrR1MZxzfF44xk0Sb3ozAl+J9jze3ptUMaQCYUfoBcJbdbG 3mBw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=TmuOAuPqFkLEUVE/VOD7wN6NrUglI5pWEI0/RzoOeAM=; b=E3v5QQmWcq+Zp61/Vf8DYUfff32B81mCPzWoK6w8M0l3QsWEdc9adQP2rQdtkMaC44 9juIj95dnfQVcgqs5H2lIkfI1ZDxpExY/AeH01h1hXcntis6EMzFA1a31UuEDbI4uAS9 1EA4z5ZEWbXTpqGEBJEqcF4w1aEtgk1t8dFXECboVrfhi9EhligTDEaHM8VgjN676umM 8A/+WwfYwkySmY/xmMJ/7XPC8WI9KSdSo8U5jNMoTGJXJbVPO0U/RawMGzA3Ee+e3Sdn b8dL/Yn/x4O+BOqSuKDpu+IU1c/emWYgZB+qK0ot+OGNmcF4YcMG+U0vCdVzoWgEq/QF JbPg== X-Gm-Message-State: AOAM532A/2xPhVueUoKriswg2SWJPFivAonc2R/TbJvH8UzcN5N+c0IL ozwM6UfMcv7IUwKpv4ZdHJuw8g== X-Google-Smtp-Source: ABdhPJyw+KpoZK0nQOrMdZBZkFTnHb1J6OElAkAlLCNP/cbsBRptilj/xyi8WRz+n/QZKtCz+oUbFQ== X-Received: by 2002:a05:620a:20db:: with SMTP id f27mr28964306qka.11.1600341940464; Thu, 17 Sep 2020 04:25:40 -0700 (PDT) Received: from ziepe.ca (hlfxns017vw-156-34-48-30.dhcp-dynamic.fibreop.ns.bellaliant.net. [156.34.48.30]) by smtp.gmail.com with ESMTPSA id n1sm1367610qte.91.2020.09.17.04.25.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 17 Sep 2020 04:25:39 -0700 (PDT) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1kIs2k-000Q09-QL; Thu, 17 Sep 2020 08:25:38 -0300 Date: Thu, 17 Sep 2020 08:25:38 -0300 From: Jason Gunthorpe To: Peter Xu Cc: John Hubbard , Linus Torvalds , Leon Romanovsky , Linux-MM , Linux Kernel Mailing List , "Maya B . Gokhale" , Yang Shi , Marty Mcfadden , Kirill Shutemov , Oleg Nesterov , Jann Horn , Jan Kara , Kirill Tkhai , Andrea Arcangeli , Christoph Hellwig , Andrew Morton Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification Message-ID: <20200917112538.GD8409@ziepe.ca> References: <20200915145040.GA2949@xz-x1> <20200915160553.GJ1221970@ziepe.ca> <20200915182933.GM1221970@ziepe.ca> <20200915191346.GD2949@xz-x1> <20200915193838.GN1221970@ziepe.ca> <20200915213330.GE2949@xz-x1> <20200915232238.GO1221970@ziepe.ca> <20200916174804.GC8409@ziepe.ca> <20200916184619.GB40154@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200916184619.GB40154@xz-x1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 16, 2020 at 02:46:19PM -0400, Peter Xu wrote: > My understanding is this may only work for the case when the fork()ed child > quitted before we reach here (so we still have mapcount==1 for the > page). Yes > What if not? Then mapcount will be greater than 1, and cow will > still trigger. Is that what we want? That doesn't work today anyhow, so it is fine continuing to be broken. > Another problem is that, aiui, one of the major change previous patch proposed > is to avoid using lock_page() so that we never block in this path. I saw you mention this before, but it looks like the change was to lift some of the atomc_reads out of the lock and avoid the lock if they indicate failure, checking also for page_maybe_dma_pinned() outside the lock just means the rare case of FOLL_PIN we will take the lock again. > Maybe even more complicated, because "correctness" should be even harder > than "best effort reuse" since it can cause data corruption if we didn't do it > right... The only correct way is for the application to avoid write protect on FOLL_PIN pages. The purpose here is to allow applications that hadn't hit "bad luck" and failed to keep working. Another thought is to insert a warning print here as well that the program is working improperly? At least it would give a transition period to evaluate the extent of the problem. We are thinking it is going to be a notable regression. I botched the last version of the patch, here is something a bit better. Does it seem like it could be OK? I know very little about this part of the kernel Thanks, Jason diff --git a/mm/memory.c b/mm/memory.c index 469af373ae76e1..332de777854f8b 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2889,6 +2889,24 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf) return ret; } +static bool cow_needed(struct vm_fault *vmf) +{ + int total_map_swapcount; + + if (!reuse_swap_page(vmf->page, &total_map_swapcount)) + return true; + + if (total_map_swapcount == 1) { + /* + * The page is all ours. Move it to our anon_vma so the rmap + * code will not search our parent or siblings. Protected + * against the rmap code by the page lock. + */ + page_move_anon_rmap(vmf->page, vmf->vma); + } + return false; +} + /* * This routine handles present pages, when users try to write * to a shared page. It is done by copying the page to a new address @@ -2942,13 +2960,27 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf) struct page *page = vmf->page; /* PageKsm() doesn't necessarily raise the page refcount */ - if (PageKsm(page) || page_count(page) != 1) + if (PageKsm(page)) goto copy; + if (page_count(page) != 1) { + /* + * If the page is DMA pinned we can't rely on the + * above to know if there are other CPU references as + * page_count() will be elevated by the + * pin. Needlessly copying the page will cause the DMA + * pin to break, try harder to avoid that. + */ + if (!page_maybe_dma_pinned(page)) + goto copy; + } + if (!trylock_page(page)) goto copy; if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) { - unlock_page(page); - goto copy; + if (cow_needed(vmf)) { + unlock_page(page); + goto copy; + } } /* * Ok, we've got the only map reference, and the only