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 4F9A9C433FE for ; Thu, 9 Dec 2021 13:56:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238268AbhLIN7c (ORCPT ); Thu, 9 Dec 2021 08:59:32 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54146 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229901AbhLIN7b (ORCPT ); Thu, 9 Dec 2021 08:59:31 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C24C8C061746; Thu, 9 Dec 2021 05:55:57 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=1+LEczdi59AY2/papldS92pldhESqdSy5rOO+LXFl/Y=; b=BN5zR4Fap4OgbbW6sVMj4jsSBW ilAnzN36cbHQO/TcIlS7AGgmaJEGWgjklvLL8UYTAqWAvaYX1k6e/+tlghjFURWAcOMIaNCIlTqrq EQxnogm7lkO6cKp4Xe7wWXN6kKowrwd8oLAmDfuxyre3ek1mvbUe7JaT/tgWY3JMhfx//48e82tMw NAfRHXggAMFolsWz/6cYamZIi/SxeNZ65Eg15PASz2d3U5Eor2e0S/cJPJFzZtdVrMiKFu1bmYjMQ aGaDxdoZ+l+UU86+SUjonqqsSlyV0/Gp6jHlCsc8Y9Vuxsq2sYFSP3CiFXaXqc0pkiGLzxGt/3vcy LcRiZktw==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1mvJtf-009Oqj-PV; Thu, 09 Dec 2021 13:55:43 +0000 Date: Thu, 9 Dec 2021 13:55:43 +0000 From: Matthew Wilcox To: Hugh Dickins 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: Re: [PATCH] mm: delete unsafe BUG from page_cache_add_speculative() Message-ID: References: <8b98fc6f-3439-8614-c3f3-945c659a1aba@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8b98fc6f-3439-8614-c3f3-945c659a1aba@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 08, 2021 at 11:19:18PM -0800, Hugh Dickins wrote: > 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. Whoops, that was careless of me. I agree with your reasoning and patch. Reviewed-by: Matthew Wilcox (Oracle) > 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); > } >