Re: [PATCH v12 04/20] mm: Allow page fault handlers to perform the COW

[Yigal Korman]

On Sat, Oct 25, 2014 at 12:20 AM, Matthew Wilcox
<matthew.r.wilcox@xxxxxxxxx> wrote:
> Currently COW of an XIP file is done by first bringing in a read-only
> mapping, then retrying the fault and copying the page.  It is much more
> efficient to tell the fault handler that a COW is being attempted (by
> passing in the pre-allocated page in the vm_fault structure), and allow
> the handler to perform the COW operation itself.
>
> The handler cannot insert the page itself if there is already a read-only
> mapping at that address, so allow the handler to return VM_FAULT_LOCKED
> and set the fault_page to be NULL.  This indicates to the MM code that
> the i_mmap_mutex is held instead of the page lock.

I have a question on a related issue (I think).
I've noticed that for pfn-only mappings (VM_FAULT_NOPAGE)
do_shared_fault only maps the pfn with r/o permissions.
So if I use DAX to write the mmap()-ed pfn I get two faults - first
handled by do_shared_fault and then again for making it r/w in
do_wp_page.
Is this simply a missing optimization like was done here with the
cow_page? or am I missing something?

>
> Signed-off-by: Matthew Wilcox <matthew.r.wilcox@xxxxxxxxx>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  include/linux/mm.h |  1 +
>  mm/memory.c        | 33 ++++++++++++++++++++++++---------
>  2 files changed, 25 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 02d11ee..88d1ef4 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -209,6 +209,7 @@ struct vm_fault {
>         pgoff_t pgoff;                  /* Logical page offset based on vma */
>         void __user *virtual_address;   /* Faulting virtual address */
>
> +       struct page *cow_page;          /* Handler may choose to COW */
>         struct page *page;              /* ->fault handlers should return a
>                                          * page here, unless VM_FAULT_NOPAGE
>                                          * is set (which is also implied by
> diff --git a/mm/memory.c b/mm/memory.c
> index 1cc6bfb..6dee424 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2002,6 +2002,7 @@ static int do_page_mkwrite(struct vm_area_struct *vma, struct page *page,
>         vmf.pgoff = page->index;
>         vmf.flags = FAULT_FLAG_WRITE|FAULT_FLAG_MKWRITE;
>         vmf.page = page;
> +       vmf.cow_page = NULL;
>
>         ret = vma->vm_ops->page_mkwrite(vma, &vmf);
>         if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
> @@ -2701,7 +2702,8 @@ oom:
>   * See filemap_fault() and __lock_page_retry().
>   */
>  static int __do_fault(struct vm_area_struct *vma, unsigned long address,
> -               pgoff_t pgoff, unsigned int flags, struct page **page)
> +                       pgoff_t pgoff, unsigned int flags,
> +                       struct page *cow_page, struct page **page)
>  {
>         struct vm_fault vmf;
>         int ret;
> @@ -2710,10 +2712,13 @@ static int __do_fault(struct vm_area_struct *vma, unsigned long address,
>         vmf.pgoff = pgoff;
>         vmf.flags = flags;
>         vmf.page = NULL;
> +       vmf.cow_page = cow_page;
>
>         ret = vma->vm_ops->fault(vma, &vmf);
>         if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
>                 return ret;
> +       if (!vmf.page)
> +               goto out;
>
>         if (unlikely(PageHWPoison(vmf.page))) {
>                 if (ret & VM_FAULT_LOCKED)
> @@ -2727,6 +2732,7 @@ static int __do_fault(struct vm_area_struct *vma, unsigned long address,
>         else
>                 VM_BUG_ON_PAGE(!PageLocked(vmf.page), vmf.page);
>
> + out:
>         *page = vmf.page;
>         return ret;
>  }
> @@ -2900,7 +2906,7 @@ static int do_read_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>                 pte_unmap_unlock(pte, ptl);
>         }
>
> -       ret = __do_fault(vma, address, pgoff, flags, &fault_page);
> +       ret = __do_fault(vma, address, pgoff, flags, NULL, &fault_page);
>         if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
>                 return ret;
>
> @@ -2940,26 +2946,35 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>                 return VM_FAULT_OOM;
>         }
>
> -       ret = __do_fault(vma, address, pgoff, flags, &fault_page);
> +       ret = __do_fault(vma, address, pgoff, flags, new_page, &fault_page);
>         if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
>                 goto uncharge_out;
>
> -       copy_user_highpage(new_page, fault_page, address, vma);
> +       if (fault_page)
> +               copy_user_highpage(new_page, fault_page, address, vma);
>         __SetPageUptodate(new_page);
>
>         pte = pte_offset_map_lock(mm, pmd, address, &ptl);
>         if (unlikely(!pte_same(*pte, orig_pte))) {
>                 pte_unmap_unlock(pte, ptl);
> -               unlock_page(fault_page);
> -               page_cache_release(fault_page);
> +               if (fault_page) {
> +                       unlock_page(fault_page);
> +                       page_cache_release(fault_page);
> +               } else {
> +                       mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
> +               }
>                 goto uncharge_out;
>         }
>         do_set_pte(vma, address, new_page, pte, true, true);
>         mem_cgroup_commit_charge(new_page, memcg, false);
>         lru_cache_add_active_or_unevictable(new_page, vma);
>         pte_unmap_unlock(pte, ptl);
> -       unlock_page(fault_page);
> -       page_cache_release(fault_page);
> +       if (fault_page) {
> +               unlock_page(fault_page);
> +               page_cache_release(fault_page);
> +       } else {
> +               mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
> +       }
>         return ret;
>  uncharge_out:
>         mem_cgroup_cancel_charge(new_page, memcg);
> @@ -2978,7 +2993,7 @@ static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>         int dirtied = 0;
>         int ret, tmp;
>
> -       ret = __do_fault(vma, address, pgoff, flags, &fault_page);
> +       ret = __do_fault(vma, address, pgoff, flags, NULL, &fault_page);
>         if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
>                 return ret;
>
> --
> 2.1.1
>
> --
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