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=-5.8 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 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 94F79C47082 for ; Mon, 7 Jun 2021 18:04:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7ACA861001 for ; Mon, 7 Jun 2021 18:04:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230436AbhFGSGK (ORCPT ); Mon, 7 Jun 2021 14:06:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44778 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230233AbhFGSGJ (ORCPT ); Mon, 7 Jun 2021 14:06:09 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 285F4C061766 for ; Mon, 7 Jun 2021 11:04:18 -0700 (PDT) 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=ZTh2RnYV27oLVt4BJj4ZhqnskEuzydcOTMgQsGZpXh4=; b=RxwF/q2Rye4NC95etr0kDbNka+ tROFkFQ+1cDU8FWiO0vAOD4ceReIB59VAIYX3i1CrWedoFgQEr93OxIGkjtMnfxcVNcxP8vaz+bNZ IZ1vpMu9XasdCayeQud6YvPpIXwAYtdtI4dFSeSBWOwuThMJmTQ/76lxj6HOgGrZ3ZgDAAA5A6lpY gXYMN2J+JZrbshybaIkbcQz1KrHScMrbK9bH3SClCJL9Yrp6379zrd01Ox60MSku9JIWi01w2wF4s zlM+UGDmMSyEhNFG4MDnVIXnJZXzs9ISLxhKn3Rr+bXmg8rZz0S1km5AR9f8rAXgvKIN6FJnynitY +bEwad1Q==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lqJat-00G67M-KS; Mon, 07 Jun 2021 18:03:29 +0000 Date: Mon, 7 Jun 2021 19:03:23 +0100 From: Matthew Wilcox To: Jann Horn Cc: Linux-MM , Zi Yan , Peter Xu , "Kirill A. Shutemov" , Konstantin Khlebnikov , Andrew Morton , chinwen.chang@mediatek.com, kernel list , syzkaller-bugs , Vlastimil Babka , Michel Lespinasse , syzbot Subject: Re: split_huge_page_to_list() races with page_mapcount() on migration entry in smaps code? [was: Re: [syzbot] kernel BUG in __page_mapcount] Message-ID: References: <00000000000017977605c395a751@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 07, 2021 at 07:27:23PM +0200, Jann Horn wrote: > === Short summary === > I believe the issue here is a race between /proc/*/smaps and > split_huge_page_to_list(): > > The codepath for /proc/*/smaps walks the pagetables and (e.g. in > smaps_account()) calls page_mapcount() not just on pages from normal > PTEs but also on migration entries (since commit b1d4d9e0cbd0a > "proc/smaps: carefully handle migration entries", from Linux v3.5). > page_mapcount() expects compound pages to be stable. > > The split_huge_page_to_list() path first protects the compound page by > locking it and replacing all its PTEs with migration entries (since > the THP rewrite in v4.5, I think?), then does the actual splitting > using __split_huge_page(). > > So there's a mismatch of expectations here: > The smaps code expects that migration entries point to stable compound > pages, while the THP code expects that it's okay to split a compound > page while it has migration entries. Will it be a colossal performance penalty if we always get the page refcount after looking it up? That will cause split_huge_page() to fail to split the page if it hits this race.