linux-kernel - Re: [PATCH 3/3] s390/mm: fix mis-accounting of pgtable

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Message-Id: <20181127153114.0a6193d3@mschwideX1>
Date:   Tue, 27 Nov 2018 15:31:14 +0100
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Heiko Carstens <heiko.carstens@...ibm.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Li Wang <liwang@...hat.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Janosch Frank <frankja@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 3/3] s390/mm: fix mis-accounting of pgtable_bytes

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