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@xxxxxxxxxxxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> 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@xxxxxxxxxxxxxxx>
--
Kirill A. Shutemov
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