Re: [PATCH 16/21] mm: Provide helper for finishing mkwrite faults

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Nov 04, 2016 at 05:25:12AM +0100, Jan Kara wrote:
> Provide a helper function for finishing write faults due to PTE being
> read-only. The helper will be used by DAX to avoid the need of
> complicating generic MM code with DAX locking specifics.
> 
> Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  include/linux/mm.h |  1 +
>  mm/memory.c        | 67 ++++++++++++++++++++++++++++++++----------------------
>  2 files changed, 41 insertions(+), 27 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index fb128beecdac..685ff1c57f2b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -615,6 +615,7 @@ static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
>  int alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
>  		struct page *page);
>  int finish_fault(struct vm_fault *vmf);
> +int finish_mkwrite_fault(struct vm_fault *vmf);
>  #endif
>  
>  /*
> diff --git a/mm/memory.c b/mm/memory.c
> index 06aba4203104..1517ff91c743 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2270,6 +2270,38 @@ static int wp_page_copy(struct vm_fault *vmf)
>  	return VM_FAULT_OOM;
>  }
>  
> +/**
> + * finish_mkwrite_fault - finish page fault for a shared mapping, making PTE
> + *			  writeable once the page is prepared
> + *
> + * @vmf: structure describing the fault
> + *
> + * This function handles all that is needed to finish a write page fault in a
> + * shared mapping due to PTE being read-only once the mapped page is prepared.
> + * It handles locking of PTE and modifying it. The function returns
> + * VM_FAULT_WRITE on success, 0 when PTE got changed before we acquired PTE
> + * lock.
> + *
> + * The function expects the page to be locked or other protection against
> + * concurrent faults / writeback (such as DAX radix tree locks).
> + */
> +int finish_mkwrite_fault(struct vm_fault *vmf)
> +{
> +	WARN_ON_ONCE(!(vmf->vma->vm_flags & VM_SHARED));
> +	vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, vmf->address,
> +				       &vmf->ptl);
> +	/*
> +	 * We might have raced with another page fault while we released the
> +	 * pte_offset_map_lock.
> +	 */
> +	if (!pte_same(*vmf->pte, vmf->orig_pte)) {
> +		pte_unmap_unlock(vmf->pte, vmf->ptl);
> +		return 0;
> +	}
> +	wp_page_reuse(vmf);
> +	return VM_FAULT_WRITE;
> +}
> +
>  /*
>   * Handle write page faults for VM_MIXEDMAP or VM_PFNMAP for a VM_SHARED
>   * mapping
> @@ -2286,16 +2318,7 @@ static int wp_pfn_shared(struct vm_fault *vmf)
>  		ret = vma->vm_ops->pfn_mkwrite(vma, vmf);
>  		if (ret & VM_FAULT_ERROR)
>  			return ret;
> -		vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> -				vmf->address, &vmf->ptl);
> -		/*
> -		 * We might have raced with another page fault while we
> -		 * released the pte_offset_map_lock.
> -		 */
> -		if (!pte_same(*vmf->pte, vmf->orig_pte)) {
> -			pte_unmap_unlock(vmf->pte, vmf->ptl);
> -			return 0;
> -		}
> +		return finish_mkwrite_fault(vmf);
>  	}
>  	wp_page_reuse(vmf);
>  	return VM_FAULT_WRITE;
> @@ -2305,7 +2328,6 @@ static int wp_page_shared(struct vm_fault *vmf)
>  	__releases(vmf->ptl)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
> -	int page_mkwrite = 0;
>  
>  	get_page(vmf->page);
>  
> @@ -2319,26 +2341,17 @@ static int wp_page_shared(struct vm_fault *vmf)
>  			put_page(vmf->page);
>  			return tmp;
>  		}
> -		/*
> -		 * Since we dropped the lock we need to revalidate
> -		 * the PTE as someone else may have changed it.  If
> -		 * they did, we just return, as we can count on the
> -		 * MMU to tell us if they didn't also make it writable.
> -		 */
> -		vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> -						vmf->address, &vmf->ptl);
> -		if (!pte_same(*vmf->pte, vmf->orig_pte)) {
> +		tmp = finish_mkwrite_fault(vmf);
> +		if (unlikely(!tmp || (tmp &
> +				      (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {

Looks like the second part of condition is never true here, right? Not
that it would matter, having the next patch in the queue.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov

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