Re: [PATCH] mm/hugetlb: Use the right pte val for compare in hugetlb_cow

["Hillf Danton" <hillf.zj@alibaba-inc.com>]

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 <jstancek@redhat.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> ---

Acked-by: Hillf Danton <hillf.zj@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

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