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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3BAD1C433FE for ; Thu, 9 Dec 2021 07:19:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233634AbhLIHW4 (ORCPT ); Thu, 9 Dec 2021 02:22:56 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46176 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233467AbhLIHWz (ORCPT ); Thu, 9 Dec 2021 02:22:55 -0500 Received: from mail-qt1-x82a.google.com (mail-qt1-x82a.google.com [IPv6:2607:f8b0:4864:20::82a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 927A2C061746 for ; Wed, 8 Dec 2021 23:19:22 -0800 (PST) Received: by mail-qt1-x82a.google.com with SMTP id a2so4467951qtx.11 for ; Wed, 08 Dec 2021 23:19:22 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:from:to:cc:subject:message-id:mime-version; bh=wp7bsIiEGq1v0UzZDarToFJILpj4ru1SGwyQaMb3p4E=; b=n+ugZQ1C7dMkoFU9ez645n+AJqXmFaYJgGVuTfIepCmJyDDnCohuhijGr9jGafo/wy ke5ok+6E3zz7kA5TnbpKDDfzpnH1mUlESdrcsPQX3rQ/5EWkuzqbXbOcU7kKOUckIgth O3HHcGIovw7KB4FOWO0jiUkVl5qccjHYAntQnRbxWffMfENTLHdEbWcJPbTowO9D36lh 64hdvosY2W7EEQR7dNWhu7EJiIi92GFTNn8C83HMjh4dkXwoXA66KkTp40tVE3kupDAq JMqxYPzhj9IyRHZAsGhQpuTwy2HT4CE1oPWh6CbJnqiZk+a9EqxqpKmrpdtx6oHOyez0 QR3A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:mime-version; bh=wp7bsIiEGq1v0UzZDarToFJILpj4ru1SGwyQaMb3p4E=; b=VWvcY+O68j9ggpQC/Vx25hAThtB5V3c8Nq4PopE4i6ast3wR5XdXg6wEYyvKaMTsfi lSNXZxBXUqNEUrg/iiTBrRP7DKWEfysmy136yIUaIca5NAtdTf0gx2lS76IeoS9oiIb1 faSu8VY/Z/DdHcme5l8Es+shtjVdsrNjO0Bc834+5/bAiS0Qls9SEG2xh/HUHQYzEboR Q6ZWOg7+CPSxvrO/hWq8Zy3coxwL2U3z5uzOLXTmEEwKrH54x0tTzrftB2bZnNjQnzoJ dcOyVjbDJA7iO/pJhEZ8QmCVRuB5RN17+iGNBubtBi9KD5PbImt/4/HzknP3lSZw6+5P DzwA== X-Gm-Message-State: AOAM532ke75gGrH5geHZUBqEgQkgi2hqrkCMo7IQjxIZE3o0EO1yByXS KzRsh4c9NYYXfn5Vh7ztNnkKycEfXZeCCg== X-Google-Smtp-Source: ABdhPJyDk5gwcYkseNoR8GrR8PAhZjoMS9Xyrz93qdo0YVs5eDJPx98/mF7EChyYF/5No6CJchsA8g== X-Received: by 2002:ac8:580b:: with SMTP id g11mr14831442qtg.268.1639034361429; Wed, 08 Dec 2021 23:19:21 -0800 (PST) Received: from ripple.attlocal.net (172-10-233-147.lightspeed.sntcca.sbcglobal.net. [172.10.233.147]) by smtp.gmail.com with ESMTPSA id z4sm3382336qtj.42.2021.12.08.23.19.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 08 Dec 2021 23:19:20 -0800 (PST) Date: Wed, 8 Dec 2021 23:19:18 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Matthew Wilcox cc: Andrew Morton , Vlastimil Babka , William Kucharski , Christoph Hellwig , "Kirill A. Shutemov" , Mike Rapoport , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH] mm: delete unsafe BUG from page_cache_add_speculative() Message-ID: <8b98fc6f-3439-8614-c3f3-945c659a1aba@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org It is not easily reproducible, but on 5.16-rc I have several times hit the VM_BUG_ON_PAGE(PageTail(page), page) in page_cache_add_speculative(): usually from filemap_get_read_batch() for an ext4 read, yesterday from next_uptodate_page() from filemap_map_pages() for a shmem fault. That BUG used to be placed where page_ref_add_unless() had succeeded, but now it is placed before folio_ref_add_unless() is attempted: that is not safe, since it is only the acquired reference which makes the page safe from racing THP collapse or split. We could keep the BUG, checking PageTail only when folio_ref_try_add_rcu() has succeeded; but I don't think it adds much value - just delete it. Fixes: 020853b6f5ea ("mm: Add folio_try_get_rcu()") Signed-off-by: Hugh Dickins --- include/linux/pagemap.h | 1 - 1 file changed, 1 deletion(-) --- 5.16-rc4/include/linux/pagemap.h +++ linux/include/linux/pagemap.h @@ -285,7 +285,6 @@ static inline struct inode *folio_inode( static inline bool page_cache_add_speculative(struct page *page, int count) { - VM_BUG_ON_PAGE(PageTail(page), page); return folio_ref_try_add_rcu((struct folio *)page, count); }