From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f197.google.com (mail-pf1-f197.google.com [209.85.210.197]) by kanga.kvack.org (Postfix) with ESMTP id 887D06B4682 for ; Tue, 27 Nov 2018 02:34:27 -0500 (EST) Received: by mail-pf1-f197.google.com with SMTP id s71so13364792pfi.22 for ; Mon, 26 Nov 2018 23:34:27 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id y29si3030351pgk.376.2018.11.26.23.34.26 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 26 Nov 2018 23:34:26 -0800 (PST) Received: from pps.filterd (m0098394.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id wAR7YOtg074543 for ; Tue, 27 Nov 2018 02:34:25 -0500 Received: from e06smtp03.uk.ibm.com (e06smtp03.uk.ibm.com [195.75.94.99]) by mx0a-001b2d01.pphosted.com with ESMTP id 2p10kujdxv-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Tue, 27 Nov 2018 02:34:25 -0500 Received: from localhost by e06smtp03.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 27 Nov 2018 07:34:18 -0000 Date: Tue, 27 Nov 2018 08:34:12 +0100 From: Heiko Carstens Subject: Re: [PATCH 3/3] s390/mm: fix mis-accounting of pgtable_bytes References: <1539621759-5967-1-git-send-email-schwidefsky@de.ibm.com> <1539621759-5967-4-git-send-email-schwidefsky@de.ibm.com> <20181031073149.55ddc085@mschwideX1> <20181031100944.GA3546@osiris> <20181031103623.6ykzsjdenrpeth7x@kshutemo-mobl1> MIME-Version: 1.0 In-Reply-To: <20181031103623.6ykzsjdenrpeth7x@kshutemo-mobl1> Message-Id: <20181127073411.GA3625@osiris> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8bit Content-Disposition: inline Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: "Kirill A. Shutemov" , Li Wang , Guenter Roeck , Janosch Frank , linux-kernel , Linux-MM , Martin Schwidefsky On Wed, Oct 31, 2018 at 01:36:23PM +0300, Kirill A. Shutemov wrote: > On Wed, Oct 31, 2018 at 11:09:44AM +0100, Heiko Carstens wrote: > > On Wed, Oct 31, 2018 at 07:31:49AM +0100, Martin Schwidefsky wrote: > > > Thanks for testing. Unfortunately Heiko reported another issue yesterday > > > with the patch applied. This time the other way around: > > > > > > BUG: non-zero pgtables_bytes on freeing mm: -16384 > > > > > > I am trying to understand how this can happen. For now I would like to > > > keep the patch on hold in case they need another change. > > > > FWIW, Kirill: is there a reason why this "BUG:" output is done with > > pr_alert() and not with VM_BUG_ON() or one of the WARN*() variants? > > > > That would to get more information with DEBUG_VM and / or > > panic_on_warn=1 set. At least for automated testing it would be nice > > to have such triggers. > > Stack trace is not helpful there. It will always show the exit path which > is useless. So, even with the updated version of these patches I can flood dmesg and the console with BUG: non-zero pgtables_bytes on freeing mm: 16384 messages with this complex reproducer on s390: echo "void main(void) {}" | gcc -m31 -xc -o compat - && ./compat Besides that this needs to be fixed, I'd really like to see this changed to either a printk_once() or a WARN_ON_ONCE() within check_mm() so that an arbitrary user cannot flood the console. E.g. something like the below. If there aren't any objections, I will provide a proper patch with changelog, etc. diff --git a/kernel/fork.c b/kernel/fork.c index 07cddff89c7b..d7aeec03c57f 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -647,8 +647,8 @@ static void check_mm(struct mm_struct *mm) } if (mm_pgtables_bytes(mm)) - pr_alert("BUG: non-zero pgtables_bytes on freeing mm: %ld\n", - mm_pgtables_bytes(mm)); + printk_once(KERN_ALERT "BUG: non-zero pgtables_bytes on freeing mm: %ld\n", + mm_pgtables_bytes(mm)); #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS VM_BUG_ON_MM(mm->pmd_huge_pte, mm);