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

[Heiko Carstens]

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);