On 06/17/2016 05:25 AM, Kirill A. Shutemov wrote: > > From fd22922e7b4664e83653a84331f0a95b985bff0c Mon Sep 17 00:00:00 2001 > From: "Kirill A. Shutemov" <[email protected]> > Date: Fri, 17 Jun 2016 15:07:03 +0300 > Subject: [PATCH] hugetlb: fix nr_pmds accounting with shared page tables > > We account HugeTLB's shared page table to all processes who share it. > The accounting happens during huge_pmd_share(). > > If somebody populates pud entry under us, we should decrease pagetable's > refcount and decrease nr_pmds of the process. > > By mistake, I increase nr_pmds again in this case. :-/ > It will lead to "BUG: non-zero nr_pmds on freeing mm: 2" on process' > exit. > > Let's fix this by increasing nr_pmds only when we're sure that the page > table will be used. > > Signed-off-by: Kirill A. Shutemov <[email protected]>
Nice, Reviewed-by: Mike Kravetz <[email protected]> I agree that we do not necessarily need a back port. I have not seen reports of people experiencing this race and seeing the BUG (on mm tear-down). zhongjiang, did someone actually hit the BUG? Or, did you find it by code examination? -- Mike Kravetz > Reported-by: zhongjiang <[email protected]> > Fixes: dc6c9a35b66b ("mm: account pmd page tables to the process") > Cc: <[email protected]> [4.0+] > --- > mm/hugetlb.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index e197cd7080e6..ed6a537f0878 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -4216,7 +4216,6 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned > long addr, pud_t *pud) > if (saddr) { > spte = huge_pte_offset(svma->vm_mm, saddr); > if (spte) { > - mm_inc_nr_pmds(mm); > get_page(virt_to_page(spte)); > break; > } > @@ -4231,9 +4230,9 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned > long addr, pud_t *pud) > if (pud_none(*pud)) { > pud_populate(mm, pud, > (pmd_t *)((unsigned long)spte & PAGE_MASK)); > + mm_inc_nr_pmds(mm); > } else { > put_page(virt_to_page(spte)); > - mm_inc_nr_pmds(mm); > } > spin_unlock(ptl); > out: >
