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

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Tue, 27 Nov 2018 08:34:12 +0100
Heiko Carstens <heiko.carstens@de.ibm.com> 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

Forgot a hunk in the fix.. I claim not enough coffee :-/
Patch is queued and I will send a please pull by the end of the week.
--
From c0499f2aa853939984ecaf0d393012486e56c7ce Mon Sep 17 00:00:00 2001
From: Martin Schwidefsky <schwidefsky@de.ibm.com>
Date: Tue, 27 Nov 2018 14:04:04 +0100
Subject: [PATCH] s390/mm: correct pgtable_bytes on page table downgrade

The downgrade of a page table from 3 levels to 2 levels for a 31-bit compat
process removes a pmd table which has to be counted against pgtable_bytes.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---
 arch/s390/mm/pgalloc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c
index 814f26520aa2..6791562779ee 100644
--- a/arch/s390/mm/pgalloc.c
+++ b/arch/s390/mm/pgalloc.c
@@ -131,6 +131,7 @@ void crst_table_downgrade(struct mm_struct *mm)
 	}
 
 	pgd = mm->pgd;
+	mm_dec_nr_pmds(mm);
 	mm->pgd = (pgd_t *) (pgd_val(*pgd) & _REGION_ENTRY_ORIGIN);
 	mm->context.asce_limit = _REGION3_SIZE;
 	mm->context.asce = __pa(mm->pgd) | _ASCE_TABLE_LENGTH |
-- 
2.16.4
-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.