Re: [PATCH 3/3] s390/mm: fix mis-accounting of pgtable_bytes

[Guenter Roeck]

On 11/26/18 11:34 PM, 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));
>   

pr_alert_once ?

Guenter

>   #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
>   	VM_BUG_ON_MM(mm->pmd_huge_pte, mm);