On Tuesday, October 18, 2016 11:43 PM Aneesh Kumar K.V wrote:
>
> 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")
>
> Reported-by: Jan Stancek <jstan...@redhat.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com>
> ---
Acked-by: Hillf Danton <hillf...@alibaba-inc.com>
> mm/hugetlb.c | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ec49d9ef1eef..da8fbd02b92e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3386,15 +3386,17 @@ 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,
> - 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:
> @@ -3668,7 +3670,7 @@ static int hugetlb_no_page(struct mm_struct *mm, struct
> vm_area_struct *vma,
> 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);
> @@ -3822,8 +3824,8 @@ 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,
> - pagecache_page, ptl);
> + ret = hugetlb_cow(mm, vma, address, ptep,
> + pagecache_page, ptl);
> goto out_put_page;
> }
> entry = huge_pte_mkdirty(entry);
> --
> 2.10.1
