On Thu, 1 Mar 2018 14:40:52 -0800 Guenter Roeck <li...@roeck-us.net> wrote:
> When running s390 images with 'compat' processes, the following > BUG is seen repeatedly. > > BUG: non-zero pgtables_bytes on freeing mm: -16384 > > Bisect points to commit b4e98d9ac775 ("mm: account pud page tables"). > Analysis shows that init_new_context() is called with > mm->context.asce_limit set to _REGION3_SIZE. In this situation, > pgtables_bytes remains set to 0 and is not increased. The message is > displayed when the affected process dies and mm_dec_nr_puds() is called. > > Cc: Kirill A. Shutemov <kirill.shute...@linux.intel.com> > Cc: Heiko Carstens <heiko.carst...@de.ibm.com> > Fixes: b4e98d9ac775 ("mm: account pud page tables") > Signed-off-by: Guenter Roeck <li...@roeck-us.net> > --- > RFC since I have no idea if this is correct (or complete, for that matter). > All I can say is that the message is no longer seen after the patch is > applied. > > arch/s390/include/asm/mmu_context.h | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/arch/s390/include/asm/mmu_context.h > b/arch/s390/include/asm/mmu_context.h > index 65154eaa3714..6c8ce15cde7b 100644 > --- a/arch/s390/include/asm/mmu_context.h > +++ b/arch/s390/include/asm/mmu_context.h > @@ -63,6 +63,7 @@ static inline int init_new_context(struct task_struct *tsk, > _ASCE_USER_BITS | _ASCE_TYPE_SEGMENT; > /* pgd_alloc() did not account this pmd */ > mm_inc_nr_pmds(mm); > + mm_inc_nr_puds(mm); > } > crst_table_init((unsigned long *) mm->pgd, pgd_entry_type(mm)); > return 0; The page table folding always makes a knot in my brain. After staring at the code for a while I am now convinced that your patch is correct. On free of a 2 level page table free_pud_range() does a p4d_clear() which is turned into a nop as the page table is folded. But the mm_dec_nr_puds() is done unconditionally and that is where the imbalance comes from. Adding a counter mm_inc_nr_puds to init_new_context fixes it. Applied to s390/linux:fixes. Thanks. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.