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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 89D6FC433F5 for ; Thu, 2 Dec 2021 03:52:37 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id DB95E6B0072; Wed, 1 Dec 2021 22:52:26 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id D41EF6B0073; Wed, 1 Dec 2021 22:52:26 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BE2756B0074; Wed, 1 Dec 2021 22:52:26 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0063.hostedemail.com [216.40.44.63]) by kanga.kvack.org (Postfix) with ESMTP id ABEA66B0072 for ; Wed, 1 Dec 2021 22:52:26 -0500 (EST) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 619F38911B for ; Thu, 2 Dec 2021 03:52:16 +0000 (UTC) X-FDA: 78871481472.30.1F92959 Received: from mail-lf1-f41.google.com (mail-lf1-f41.google.com [209.85.167.41]) by imf11.hostedemail.com (Postfix) with ESMTP id 12446F0000A1 for ; Thu, 2 Dec 2021 03:52:15 +0000 (UTC) Received: by mail-lf1-f41.google.com with SMTP id b1so68050453lfs.13 for ; Wed, 01 Dec 2021 19:52:15 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=drQK4psebq0QJ6AjkRP87K0UP25BX0tUwNPI2Mxwh1Y=; b=DO1aq2LgHHFFhGcOWvX2pTGyIdhqCHQwSBeoglKvAhlR2yUspI1rAptPS2yqvbx3OA cmjFmtDdOlYtai8nQhY+DfSQECbgNeoCtLylICuye7Ym5APn5f2FN1Xxp2jcEZqb4fWa ePAIi+MqssKlG9EgdHomV615N2VTdu+xzL+IZl2XpyCg+9r3w/B4J6AG4ap7YA5QDxg1 nIbJUOl+H1puHtu1dLJtZCGHSL45g75Un+AjaUCnP6zBY+V+RP4s8kA9ocvmyn5XLeCe uX6p+iIw5OiLlFYBBr4qvQ6MVDeQLo99UAplja6WXiJXjySa64JDy2lj2VOhhW315xSa okbA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=drQK4psebq0QJ6AjkRP87K0UP25BX0tUwNPI2Mxwh1Y=; b=YhIzFSSrS1nSylP+uUqS0vrlGnUMxRqXgW2R1XigVqqrTBEouDazPnt/f1C8MB6bNv pRm/HCdrdTqFEVKKe6mWfwnOSuEeJBCAQY1rQNKDFC6KHjrXZDfdlUg83qoiI1tuPm6l lMrP553gjmxn1JYyzk7DciLoNbBORJkiIfqGCQ3TDcQlpDlQhJsP9/r/yJTx56Xd+0+s Hgke75wPW8nCy0dUMGvLHyBePKMrd6NNSIKx99qWJCgZDhTwxlDuBaSdPIm/X4zjc3sS ELQErjY/N7RhRV1lJ5kHbPaGWwqb29JMLlav7Gu3j6ie7hYmpJRi/A+NZ0Ex07uhQADp DEtw== X-Gm-Message-State: AOAM530oMA70aMTAciWJz2CkeGZ9g9mJ2gfQqaeGAdXTwV3h0BdhBWtg 4siuaLbm6c5BKiIIONBYWLH0ll1qjuZjFuXV5uD0NQ== X-Google-Smtp-Source: ABdhPJwq8a6alRqcdlos5TuK1YQUxzZdMEuWGyWRQof/6My/8vp03KglHtkzDhbFfaNlU3kgVQMGKRo2giHcFZLNleM= X-Received: by 2002:ac2:418f:: with SMTP id z15mr8929431lfh.213.1638417134025; Wed, 01 Dec 2021 19:52:14 -0800 (PST) MIME-Version: 1.0 References: <20211201231757.332199-1-willy@infradead.org> In-Reply-To: <20211201231757.332199-1-willy@infradead.org> From: Jann Horn Date: Thu, 2 Dec 2021 04:51:47 +0100 Message-ID: Subject: Re: [5.4 PATCH] mm/gup: Do not force a COW break on file-backed memory To: "Matthew Wilcox (Oracle)" Cc: Linus Torvalds , Jan Kara , Kirill Shutemov , Oleg Nesterov , Christoph Hellwig , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mike Kravetz Content-Type: text/plain; charset="UTF-8" X-Stat-Signature: ce4e7yd1wwwqb89j4zjwcutmy1od4i69 X-Rspamd-Queue-Id: 12446F0000A1 X-Rspamd-Server: rspam07 Authentication-Results: imf11.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=DO1aq2Lg; spf=pass (imf11.hostedemail.com: domain of jannh@google.com designates 209.85.167.41 as permitted sender) smtp.mailfrom=jannh@google.com; dmarc=pass (policy=reject) header.from=google.com X-HE-Tag: 1638417135-346374 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 Thu, Dec 2, 2021 at 12:18 AM Matthew Wilcox (Oracle) wrote: > Commit 17839856fd58 ("gup: document and work around "COW can break either > way" issue") forces a COW break, even for read-only GUP. This interacts > badly with CONFIG_READ_ONLY_THP_FOR_FS as it tries to write to a read-only > PMD and follow_trans_huge_pmd() returns NULL which induces an endless > loop as __get_user_pages() interprets that as page-not-present, tries > to fault it in and retries the follow_page_mask(). > > The issues fixed by 17839856fd58 don't apply to files. We know which way > the COW breaks; the page cache keeps the original and any modifications > are private to that process. There's no optimisation that allows a > process to reuse a file-backed MAP_PRIVATE page. So we can skip the > breaking of the COW for file-backed mappings. > > This problem only exists in v5.4.y; other stable kernels either predate > CONFIG_READ_ONLY_THP_FOR_FS or they include commit a308c71bf1e6 ("mm/gup: > Remove enfornced COW mechanism"). > > Signed-off-by: Matthew Wilcox (Oracle) > --- > mm/gup.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 3ef769529548..d55e02411010 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -176,7 +176,8 @@ static inline bool can_follow_write_pte(pte_t pte, unsigned int flags) > */ > static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags) > { > - return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET); > + return is_cow_mapping(vma->vm_flags) && vma_is_anonymous(vma) && > + (flags & FOLL_GET); > } To be fully correct, the check would have to check for PageAnon(), not whether the mapping is anonymous, right? Since a private file mapping can still contain anonymous pages from a prior CoW?