Re: [PATCH v2 07/26] userfaultfd: wp: hook userfault handler to write protection fault

[Mike Rapoport]

On Tue, Feb 12, 2019 at 10:56:13AM +0800, Peter Xu wrote:
> From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> 
> There are several cases write protection fault happens. It could be a
> write to zero page, swaped page or userfault write protected
> page. When the fault happens, there is no way to know if userfault
> write protect the page before. Here we just blindly issue a userfault
> notification for vma with VM_UFFD_WP regardless if app write protects
> it yet. Application should be ready to handle such wp fault.
> 
> v1: From: Shaohua Li <shli@xxxxxx>
> 
> v2: Handle the userfault in the common do_wp_page. If we get there a
> pagetable is present and readonly so no need to do further processing
> until we solve the userfault.
> 
> In the swapin case, always swapin as readonly. This will cause false
> positive userfaults. We need to decide later if to eliminate them with
> a flag like soft-dirty in the swap entry (see _PAGE_SWP_SOFT_DIRTY).
> 
> hugetlbfs wouldn't need to worry about swapouts but and tmpfs would
> be handled by a swap entry bit like anonymous memory.
> 
> The main problem with no easy solution to eliminate the false
> positives, will be if/when userfaultfd is extended to real filesystem
> pagecache. When the pagecache is freed by reclaim we can't leave the
> radix tree pinned if the inode and in turn the radix tree is reclaimed
> as well.
> 
> The estimation is that full accuracy and lack of false positives could
> be easily provided only to anonymous memory (as long as there's no
> fork or as long as MADV_DONTFORK is used on the userfaultfd anonymous
> range) tmpfs and hugetlbfs, it's most certainly worth to achieve it
> but in a later incremental patch.
> 
> v3: Add hooking point for THP wrprotect faults.
> 
> CC: Shaohua Li <shli@xxxxxx>
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>

Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>

> ---
>  mm/memory.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index e11ca9dd823f..00781c43407b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2483,6 +2483,11 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
> 
> +	if (userfaultfd_wp(vma)) {
> +		pte_unmap_unlock(vmf->pte, vmf->ptl);
> +		return handle_userfault(vmf, VM_UFFD_WP);
> +	}
> +
>  	vmf->page = vm_normal_page(vma, vmf->address, vmf->orig_pte);
>  	if (!vmf->page) {
>  		/*
> @@ -2800,6 +2805,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
>  	dec_mm_counter_fast(vma->vm_mm, MM_SWAPENTS);
>  	pte = mk_pte(page, vma->vm_page_prot);
> +	if (userfaultfd_wp(vma))
> +		vmf->flags &= ~FAULT_FLAG_WRITE;
>  	if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page, NULL)) {
>  		pte = maybe_mkwrite(pte_mkdirty(pte), vma);
>  		vmf->flags &= ~FAULT_FLAG_WRITE;
> @@ -3684,8 +3691,11 @@ static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
>  /* `inline' is required to avoid gcc 4.1.2 build error */
>  static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf, pmd_t orig_pmd)
>  {
> -	if (vma_is_anonymous(vmf->vma))
> +	if (vma_is_anonymous(vmf->vma)) {
> +		if (userfaultfd_wp(vmf->vma))
> +			return handle_userfault(vmf, VM_UFFD_WP);
>  		return do_huge_pmd_wp_page(vmf, orig_pmd);
> +	}
>  	if (vmf->vma->vm_ops->huge_fault)
>  		return vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);
> 
> -- 
> 2.17.1
> 

-- 
Sincerely yours,
Mike.