Commit 3999f52e authored Dec 12, 2016 by Aneesh Kumar K.V Committed by Linus Torvalds Dec 12, 2016

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

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: 6a119eae ("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@linux.vnet.ibm.com>
Reported-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 771ab430

mm/hugetlb.c

+7 −5

Original line number	Diff line number	Diff line
		@@ -3450,15 +3450,17 @@ static void unmap_ref_private(struct mm_struct mm, struct vm_area_struct vma,
		* 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,
		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);
		@@ -3888,7 +3890,7 @@ int hugetlb_fault(struct mm_struct mm, struct vm_area_struct vma,

		if (flags & FAULT_FLAG_WRITE) {
		if (!huge_pte_write(entry)) {
		ret = hugetlb_cow(mm, vma, address, ptep, entry,
		ret = hugetlb_cow(mm, vma, address, ptep,
		pagecache_page, ptl);
		goto out_put_page;
		}

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