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=-12.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_PASS autolearn=ham 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 64C33C04EB9 for ; Mon, 3 Dec 2018 10:24:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 354BC2087F for ; Mon, 3 Dec 2018 10:24:09 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 354BC2087F Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726287AbeLCKYd (ORCPT ); Mon, 3 Dec 2018 05:24:33 -0500 Received: from mx1.redhat.com ([209.132.183.28]:35170 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726052AbeLCKYc (ORCPT ); Mon, 3 Dec 2018 05:24:32 -0500 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id E3F4A30832C9; Mon, 3 Dec 2018 10:24:06 +0000 (UTC) Received: from lacos-laptop-7.usersys.redhat.com (ovpn-120-61.rdu2.redhat.com [10.10.120.61]) by smtp.corp.redhat.com (Postfix) with ESMTP id 39ADE604CC; Mon, 3 Dec 2018 10:23:59 +0000 (UTC) Subject: Re: [PATCH v2] mm: page_mapped: don't assume compound page is huge or THP To: Jan Stancek , linux-mm@kvack.org, alex.williamson@redhat.com, aarcange@redhat.com, rientjes@google.com, kirill@shutemov.name, mgorman@techsingularity.net, mhocko@suse.com Cc: linux-kernel@vger.kernel.org References: From: Laszlo Ersek Message-ID: <35a664c0-6dab-bb32-811e-65250200d195@redhat.com> Date: Mon, 3 Dec 2018 11:23:58 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Mon, 03 Dec 2018 10:24:07 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/30/18 13:06, Jan Stancek wrote: > LTP proc01 testcase has been observed to rarely trigger crashes > on arm64: > page_mapped+0x78/0xb4 > stable_page_flags+0x27c/0x338 > kpageflags_read+0xfc/0x164 > proc_reg_read+0x7c/0xb8 > __vfs_read+0x58/0x178 > vfs_read+0x90/0x14c > SyS_read+0x60/0xc0 > > Issue is that page_mapped() assumes that if compound page is not > huge, then it must be THP. But if this is 'normal' compound page > (COMPOUND_PAGE_DTOR), then following loop can keep running > (for HPAGE_PMD_NR iterations) until it tries to read from memory > that isn't mapped and triggers a panic: > for (i = 0; i < hpage_nr_pages(page); i++) { > if (atomic_read(&page[i]._mapcount) >= 0) > return true; > } > > I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only > with a custom kernel module [1] which: > - allocates compound page (PAGEC) of order 1 > - allocates 2 normal pages (COPY), which are initialized to 0xff > (to satisfy _mapcount >= 0) > - 2 PAGEC page structs are copied to address of first COPY page > - second page of COPY is marked as not present > - call to page_mapped(COPY) now triggers fault on access to 2nd > COPY page at offset 0x30 (_mapcount) > > [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c > > Fix the loop to iterate for "1 << compound_order" pages. > > Debugged-by: Laszlo Ersek > Suggested-by: "Kirill A. Shutemov" > Signed-off-by: Jan Stancek > --- > mm/util.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > Changes in v2: > - change the loop instead so we check also mapcount of subpages > > diff --git a/mm/util.c b/mm/util.c > index 8bf08b5b5760..5c9c7359ee8a 100644 > --- a/mm/util.c > +++ b/mm/util.c > @@ -478,7 +478,7 @@ bool page_mapped(struct page *page) > return true; > if (PageHuge(page)) > return false; > - for (i = 0; i < hpage_nr_pages(page); i++) { > + for (i = 0; i < (1 << compound_order(page)); i++) { > if (atomic_read(&page[i]._mapcount) >= 0) > return true; > } > Totally uninformed side-question: how large can the return value of compound_order() be? MAX_ORDER? Apparently, MAX_ORDER can be defined as CONFIG_FORCE_MAX_ZONEORDER. "config FORCE_MAX_ZONEORDER" is listed in a number of Kconfig files. Among those, "arch/mips/Kconfig" permits "ranges" (?) that extend up to 64. Same applies to "arch/powerpc/Kconfig" and "arch/sh/mm/Kconfig". If we left-shift "1" -- a signed int, which I assume in practice will always have two's complement representation, 1 sign bit, 31 value bits, and 0 padding bits --, by 31 or more bit positions, we get undefined behavior (as part of the left-shift operation). Is this a practical concern? Thanks, Laszlo