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

[Li Wang <liwang@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 3287 bytes --]

On Wed, Oct 31, 2018 at 2:31 PM, Martin Schwidefsky <schwidefsky@de.ibm.com>
wrote:

> On Wed, 31 Oct 2018 14:18:33 +0800
> Li Wang <liwang@redhat.com> wrote:
>
> > On Tue, Oct 16, 2018 at 12:42 AM, Martin Schwidefsky <
> schwidefsky@de.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@redhat.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
>

Okay, the problem is still triggered by LTP/cve-2017-17052.c?
I tried this patch on my platform and it works! My test environment as:

# lscpu
Architecture:          s390x
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Big Endian
CPU(s):                2
On-line CPU(s) list:   0,1
Thread(s) per core:    1
Core(s) per socket:    1
Socket(s) per book:    1
Book(s) per drawer:    1
Drawer(s):             2
Vendor ID:             IBM/S390
Machine type:          2827
CPU dynamic MHz:       5504
CPU static MHz:        5504
BogoMIPS:              2913.00
Hypervisor vendor:     vertical
Virtualization type:   full
Dispatching mode:      horizontal
L1d cache:             96K
L1i cache:             64K
L2d cache:             1024K
L2i cache:             1024K
L3 cache:              49152K
L4 cache:              393216K
Flags:                 esan3 zarch stfle msa ldisp eimm dfp edat etf3eh
highgprs te sie


> 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.
>

Sure.

>
> --
> blue skies,
>    Martin.
>
> "Reality continues to ruin my life." - Calvin.
>
>


-- 
Regards,
Li Wang

[-- Attachment #2: Type: text/html, Size: 6510 bytes --]