mm: Check NULL pointer Dereference in mm/filemap.c

index 20e5642..e81e264 100644
@@ -1300,6 +1300,9 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
 			retval = filemap_write_and_wait_range(mapping, pos,
 					pos + iov_length(iov, nr_segs) - 1);
 			if (!retval) {
+				if (unlikely(!mapping->a_ops ||
+					!mapping->a_ops->direct_IO))
+					goto out;
 				retval = mapping->a_ops->direct_IO(READ, iocb,
 							iov, pos, nr_segs);
 			}

@@ -1581,6 +1584,8 @@ retry_find:
 	return ret | VM_FAULT_LOCKED;
 
 no_cached_page:
+	if (unlikely(!mapping->a_ops || !mapping->a_ops->readpage))
+		return VM_FAULT_SIGBUS;
 	/*
 	 * We're only likely to ever get here if MADV_RANDOM is in
 	 * effect.

@@ -2103,6 +2108,8 @@ int pagecache_write_begin(struct file *file, struct address_space *mapping,
 {
 	const struct address_space_operations *aops = mapping->a_ops;
 
+	if (unlikely(!aops || !aops->write_begin))
+		return -EINVAL;
 	return aops->write_begin(file, mapping, pos, len, flags,
 							pagep, fsdata);
 }

@@ -2114,6 +2121,9 @@ int pagecache_write_end(struct file *file, struct address_space *mapping,
 {
 	const struct address_space_operations *aops = mapping->a_ops;
 
+	if (unlikely(!aops || !aops->write_end))
+		return -EINVAL;
+
 	mark_page_accessed(page);
 	return aops->write_end(file, mapping, pos, len, copied, page, fsdata);
 }

@@ -2161,6 +2171,9 @@ generic_file_direct_write(struct kiocb *iocb, const struct iovec *iov,
 		}
 	}
 
+	if (unlikely(!mapping->a_ops || !mapping->a_ops->direct_IO))
+		goto out;
+
 	written = mapping->a_ops->direct_IO(WRITE, iocb, iov, pos, *nr_segs);
 
 	/*