mm: provide helper for finishing mkwrite faults

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.

Link: http://lkml.kernel.org/r/1479460644-25076-16-git-send-email-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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
 
 /*

mm/memory.c

@@ -2269,6 +2269,38 @@ oom:
 	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
@@ -2285,16 +2317,7 @@ 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;
@@ -2304,7 +2327,6 @@ 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);
 
@@ -2318,26 +2340,17 @@ 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)))) {
 			unlock_page(vmf->page);
-			pte_unmap_unlock(vmf->pte, vmf->ptl);
 			put_page(vmf->page);
-			return 0;
+			return tmp;
 		}
-		page_mkwrite = 1;
-	}
-
-	wp_page_reuse(vmf);
-	if (!page_mkwrite)
+	} else {
+		wp_page_reuse(vmf);
 		lock_page(vmf->page);
+	}
 	fault_dirty_shared_page(vma, vmf->page);
 	put_page(vmf->page);