Patch "mm/hugetlb.c: use the right pte val for compare in hugetlb_cow" has been added to the 4.9-stable tree

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

mm/hugetlb.c: use the right pte val for compare in hugetlb_cow

to the 4.9-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:
mm-hugetlb.c-use-the-right-pte-val-for-compare-in-hugetlb_cow.patch
and it can be found in the queue-4.9 subdirectory.

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

From: "Aneesh Kumar K.V" <[email protected]>
Date: Mon, 12 Dec 2016 16:41:56 -0800
Subject: mm/hugetlb.c: use the right pte val for compare in hugetlb_cow

From: Aneesh Kumar K.V <[email protected]>

commit 3999f52e3198e76607446ab1a4610c1ddc406c56 upstream.

We cannot use the pte value used in set_pte_at for pte_same comparison, because archs like ppc64, filter/add new pte flag in set_pte_at. Instead fetch the pte value inside hugetlb_cow. We are comparing pte value to make sure the pte didn't change since we dropped the page table lock. hugetlb_cow get called with page table lock held, and we can take a copy of the pte value before we drop the page table lock.

With hugetlbfs, we optimize the MAP_PRIVATE write fault path with no previous mapping (huge_pte_none entries), by forcing a cow in the fault path. This avoid take an addition fault to covert a read-only mapping to read/write. Here we were comparing a recently instantiated pte (via set_pte_at) to the pte values from linux page table. As explained above on ppc64 such pte_same check returned wrong result, resulting in us taking an additional fault on ppc64.

---
 mm/hugetlb.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

@@ -3450,15 +3450,17 @@ static void unmap_ref_private(struct mm_
  * Keep the pte_same checks anyway to make transition from the mutex easier.
  */
 static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
-			unsigned long address, pte_t *ptep, pte_t pte,
-			struct page *pagecache_page, spinlock_t *ptl)
+		       unsigned long address, pte_t *ptep,
+		       struct page *pagecache_page, spinlock_t *ptl)
 {
+	pte_t pte;
 	struct hstate *h = hstate_vma(vma);
 	struct page *old_page, *new_page;
 	int ret = 0, outside_reserve = 0;
 	unsigned long mmun_start;	/* For mmu_notifiers */
 	unsigned long mmun_end;		/* For mmu_notifiers */
 
+	pte = huge_ptep_get(ptep);
 	old_page = pte_page(pte);
 
 retry_avoidcopy:

@@ -3733,7 +3735,7 @@ retry:
 	hugetlb_count_add(pages_per_huge_page(h), mm);
 	if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
 		/* Optimization, do the COW without a second fault */
-		ret = hugetlb_cow(mm, vma, address, ptep, new_pte, page, ptl);
+		ret = hugetlb_cow(mm, vma, address, ptep, page, ptl);
 	}
 
 	spin_unlock(ptl);

@@ -3888,8 +3890,8 @@ int hugetlb_fault(struct mm_struct *mm,
 
 	if (flags & FAULT_FLAG_WRITE) {
 		if (!huge_pte_write(entry)) {
-			ret = hugetlb_cow(mm, vma, address, ptep, entry,
-					pagecache_page, ptl);
+			ret = hugetlb_cow(mm, vma, address, ptep,
+					  pagecache_page, ptl);
 			goto out_put_page;
 		}
 		entry = huge_pte_mkdirty(entry);

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

queue-4.9/mm-hugetlb.c-use-the-right-pte-val-for-compare-in-hugetlb_cow.patch