【 run_mlm.py 】attention_mask will be set to [1,1,...1] with DataCollatorForLanguageModeling 
tokenized_datasets has been padded when padding="max_length"
when get a dataloader, we will use DataCollatorForLanguageModeling with tokenizer.pad at first
tokenizer.pad will set attention_mask to all 1 because input_ids have already been padded
so i want to know whether the attention mask meet expectations.
Sorry, I don't really understand the question here. Could you clarify a bit? In case this is a question about the behavior of `DataCollatorForLanguageModeling`, it would be awesome if you could use the forum: https://discuss.huggingface.co/ .
Otherwise it would be great if you provide a code snippet showcasing the unexpected behavior. Thanks!<|||||>in run_mlm.py

first use:
```python
def tokenize_function(examples):
    # Remove empty lines
    examples["text"] = [line for line in examples["text"] if len(line) > 0 and not line.isspace()]
    return tokenizer(
                examples["text"],
                padding=padding,
                truncation=True,
                max_length=data_args.max_seq_length,
                # We use this option because DataCollatorForLanguageModeling (see below) is more efficient when it
                # receives the `special_tokens_mask`.
                return_special_tokens_mask=True,
     )
```

if i set padding="max_length", the inputs will be padded. that means the inputs will already be padded to [1, 4, 5, 6, 2, 0, 0..] and attention_mask will be set to [1, 1, 1, 1, 1, 0, 0...]. so when use DataCollatorForLanguageModeling, the inputs will be padded again (tokenizer.pad). the inputs will not change but the attention_mask will be [1, 1, 1, 1, 1, 1, 1..].
if i set padding="false", tokenizer.pad in DataCollatorForLanguageModeling will not pad the inputs to max_length. <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Hey @xieyuchen13 - I don't think the `attention_mask` will change from [1,1,1,...0,0,0] to [1,1,1,....1,1,1] => could you show me an example that proves otherwise? <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.