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Date:   Tue, 27 Nov 2018 11:05:15 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Heiko Carstens <heiko.carstens@de.ibm.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Li Wang <liwang@redhat.com>,
        Guenter Roeck <linux@roeck-us.net>,
        Janosch Frank <frankja@linux.vnet.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [PATCH 3/3] s390/mm: fix mis-accounting of pgtable_bytes
Message-ID: <20181127080515.py6naga4gsi2yad2@kshutemo-mobl1>
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On Tue, Nov 27, 2018 at 08:34:12AM +0100, Heiko Carstens wrote:
> 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));

You can be the first user of pr_alert_once(). Don't miss a chance! ;)

-- 
 Kirill A. Shutemov