fuse: lock page in mkwrite

Lock the page in fuse_page_mkwrite() to protect against a race with
fuse_writepage() where the page is redirtied before the actual writeback
begins.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/file.c

@@ -1618,14 +1618,17 @@ static void fuse_vma_close(struct vm_area_struct *vma)
 static int fuse_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 {
 	struct page *page = vmf->page;
-	/*
-	 * Don't use page->mapping as it may become NULL from a
-	 * concurrent truncate.
-	 */
-	struct inode *inode = vma->vm_file->f_mapping->host;
+	struct inode *inode = file_inode(vma->vm_file);
+
+	file_update_time(vma->vm_file);
+	lock_page(page);
+	if (page->mapping != inode->i_mapping) {
+		unlock_page(page);
+		return VM_FAULT_NOPAGE;
+	}
 
 	fuse_wait_on_page_writeback(inode, page->index);
-	return 0;
+	return VM_FAULT_LOCKED;
 }
 
 static const struct vm_operations_struct fuse_file_vm_ops = {