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

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Date:   Wed, 31 Oct 2018 07:31:49 +0100
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Li Wang <liwang@...hat.com>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Janosch Frank <frankja@...ux.vnet.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Heiko Carstens <heiko.carstens@...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 Wed, 31 Oct 2018 14:18:33 +0800
Li Wang <liwang@...hat.com> wrote:

> On Tue, Oct 16, 2018 at 12:42 AM, Martin Schwidefsky <schwidefsky@...ibm.com
> > wrote:  
> 
> > In case a fork or a clone system fails in copy_process and the error
> > handling does the mmput() at the bad_fork_cleanup_mm label, the
> > following warning messages will appear on the console:
> >
> >   BUG: non-zero pgtables_bytes on freeing mm: 16384
> >
> > The reason for that is the tricks we play with mm_inc_nr_puds() and
> > mm_inc_nr_pmds() in init_new_context().
> >
> > A normal 64-bit process has 3 levels of page table, the p4d level and
> > the pud level are folded. On process termination the free_pud_range()
> > function in mm/memory.c will subtract 16KB from pgtable_bytes with a
> > mm_dec_nr_puds() call, but there actually is not really a pud table.
> >
> > One issue with this is the fact that pgtable_bytes is usually off
> > by a few kilobytes, but the more severe problem is that for a failed
> > fork or clone the free_pgtables() function is not called. In this case
> > there is no mm_dec_nr_puds() or mm_dec_nr_pmds() that go together with
> > the mm_inc_nr_puds() and mm_inc_nr_pmds in init_new_context().
> > The pgtable_bytes will be off by 16384 or 32768 bytes and we get the
> > BUG message. The message itself is purely cosmetic, but annoying.
> >
> > To fix this override the mm_pmd_folded, mm_pud_folded and mm_p4d_folded
> > function to check for the true size of the address space.
> >  
> 
> I can confirm that it works to the problem, the warning message is gone
> after applying this patch on s390x. And I also done ltp syscalls/cve test
> for the patch set on x86_64 arch, there has no new regression.
> 
> Tested-by: Li Wang <liwang@...hat.com>

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.

-- 
blue skies,
   Martin.

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