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.

Link: http://lkml.kernel.org/r/20160617122506.GC6534@node.shutemov.name
Fixes: dc6c9a35b66b ("mm: account pmd page tables to the process")
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: zhongjiang <zhongjiang@huawei.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 388c2bb9b55c..ee0302169959 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4228,7 +4228,6 @@ 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;
                         }
@@ -4243,9 +4242,9 @@ 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: