Re: [PATCH 2/2] thp: avoid race on multiple parallel page faults to the same page

[Bob Liu <lliubbo@gmail.com>]

On Fri, Nov 30, 2012 at 11:03 PM, Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>
> pmd value is stable only with mm->page_table_lock taken. After taking
> the lock we need to check that nobody modified the pmd before change it.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Reviewed-by: Bob Liu <lliubbo@gmail.com>

> ---
>  mm/huge_memory.c | 29 ++++++++++++++++++++++++-----
>  1 file changed, 24 insertions(+), 5 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9d6f521..51cb8fe 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -770,17 +770,20 @@ static inline struct page *alloc_hugepage(int defrag)
>  }
>  #endif
>
> -static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
> +static bool set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
>                 struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd,
>                 unsigned long zero_pfn)
>  {
>         pmd_t entry;
> +       if (!pmd_none(*pmd))
> +               return false;
>         entry = pfn_pmd(zero_pfn, vma->vm_page_prot);
>         entry = pmd_wrprotect(entry);
>         entry = pmd_mkhuge(entry);
>         set_pmd_at(mm, haddr, pmd, entry);
>         pgtable_trans_huge_deposit(mm, pgtable);
>         mm->nr_ptes++;
> +       return true;
>  }
>
>  int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
> @@ -800,6 +803,7 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
>                                 transparent_hugepage_use_zero_page()) {
>                         pgtable_t pgtable;
>                         unsigned long zero_pfn;
> +                       bool set;
>                         pgtable = pte_alloc_one(mm, haddr);
>                         if (unlikely(!pgtable))
>                                 return VM_FAULT_OOM;
> @@ -810,9 +814,13 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
>                                 goto out;
>                         }
>                         spin_lock(&mm->page_table_lock);
> -                       set_huge_zero_page(pgtable, mm, vma, haddr, pmd,
> +                       set = set_huge_zero_page(pgtable, mm, vma, haddr, pmd,
>                                         zero_pfn);
>                         spin_unlock(&mm->page_table_lock);
> +                       if (!set) {
> +                               pte_free(mm, pgtable);
> +                               put_huge_zero_page();
> +                       }
>                         return 0;
>                 }
>                 page = alloc_hugepage_vma(transparent_hugepage_defrag(vma),
> @@ -1046,14 +1054,16 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>          */
>         if (is_huge_zero_pmd(pmd)) {
>                 unsigned long zero_pfn;
> +               bool set;
>                 /*
>                  * get_huge_zero_page() will never allocate a new page here,
>                  * since we already have a zero page to copy. It just takes a
>                  * reference.
>                  */
>                 zero_pfn = get_huge_zero_page();
> -               set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
> +               set = set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
>                                 zero_pfn);
> +               BUG_ON(!set); /* unexpected !pmd_none(dst_pmd) */
>                 ret = 0;
>                 goto out_unlock;
>         }
> @@ -1110,7 +1120,7 @@ unlock:
>
>  static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
>                 struct vm_area_struct *vma, unsigned long address,
> -               pmd_t *pmd, unsigned long haddr)
> +               pmd_t *pmd, pmd_t orig_pmd, unsigned long haddr)
>  {
>         pgtable_t pgtable;
>         pmd_t _pmd;
> @@ -1139,6 +1149,9 @@ static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
>         mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
>
>         spin_lock(&mm->page_table_lock);
> +       if (unlikely(!pmd_same(*pmd, orig_pmd)))
> +               goto out_free_page;
> +
>         pmdp_clear_flush(vma, haddr, pmd);
>         /* leave pmd empty until pte is filled */
>
> @@ -1171,6 +1184,12 @@ static int do_huge_pmd_wp_zero_page_fallback(struct mm_struct *mm,
>         ret |= VM_FAULT_WRITE;
>  out:
>         return ret;
> +out_free_page:
> +       spin_unlock(&mm->page_table_lock);
> +       mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
> +       mem_cgroup_uncharge_page(page);
> +       put_page(page);
> +       goto out;
>  }
>
>  static int do_huge_pmd_wp_page_fallback(struct mm_struct *mm,
> @@ -1317,7 +1336,7 @@ alloc:
>                 count_vm_event(THP_FAULT_FALLBACK);
>                 if (is_huge_zero_pmd(orig_pmd)) {
>                         ret = do_huge_pmd_wp_zero_page_fallback(mm, vma,
> -                                       address, pmd, haddr);
> +                                       address, pmd, orig_pmd, haddr);
>                 } else {
>                         ret = do_huge_pmd_wp_page_fallback(mm, vma, address,
>                                         pmd, orig_pmd, page, haddr);
> --
> 1.7.11.7
>



-- 
Regards,
--Bob

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