Patch "hugetlb: fix nr_pmds accounting with shared page tables" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

hugetlb: fix nr_pmds accounting with shared page tables

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
hugetlb-fix-nr_pmds-accounting-with-shared-page-tables.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Kirill A. Shutemov" <[email protected]>
Date: Fri, 24 Jun 2016 14:49:51 -0700
Subject: hugetlb: fix nr_pmds accounting with shared page tables

From: Kirill A. Shutemov <[email protected]>

commit c17b1f42594eb71b8d3eb5a6dfc907a7eb88a51d upstream.

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.

---
 mm/hugetlb.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

@@ -4213,7 +4213,6 @@ pte_t *huge_pmd_share(struct mm_struct *
 		if (saddr) {
 			spte = huge_pte_offset(svma->vm_mm, saddr);
 			if (spte) {
-				mm_inc_nr_pmds(mm);
 				get_page(virt_to_page(spte));
 				break;
 			}

@@ -4228,9 +4227,9 @@ pte_t *huge_pmd_share(struct mm_struct *
 	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:

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/hugetlb-fix-nr_pmds-accounting-with-shared-page-tables.patch