linux-kernel - [PATCH 4.9 139/206] mm/hugetlb.c: use the right pte val for compare in hugetlb

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Date:   Tue, 10 Jan 2017 14:37:02 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Jan Stancek <jstancek@...hat.com>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Scott Wood <scottwood@...escale.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.9 139/206] mm/hugetlb.c: use the right pte val for compare in hugetlb_cow

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>

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.

Fixes: 6a119eae942c ("powerpc/mm: Add a _PAGE_PTE bit")
Link: http://lkml.kernel.org/r/20161018154245.18023-1-aneesh.kumar@linux.vnet.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
Reported-by: Jan Stancek <jstancek@...hat.com>
Acked-by: Hillf Danton <hillf.zj@...baba-inc.com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>
Cc: Scott Wood <scottwood@...escale.com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -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);