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

    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.

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