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