Re: [PATCH] mlx5: reduce stack usage in mlx5_setup_tc

[Tariq Toukan <ttoukan.linux@xxxxxxxxx>]

On 17/01/2023 19:28, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> Clang warns about excessive stack usage on 32-bit targets:
>
> drivers/net/ethernet/mellanox/mlx5/core/en_main.c:3597:12: error: stack frame size (1184) exceeds limit (1024) in 'mlx5e_setup_tc' [-Werror,-Wframe-larger-than]
> static int mlx5e_setup_tc(struct net_device *dev, enum tc_setup_type type,
>
> It turns out that both the mlx5e_setup_tc_mqprio_dcb() function and
> the mlx5e_safe_switch_params() function it calls have a copy of
> 'struct mlx5e_params' on the stack, and this structure is fairly
> large.
>
> Use dynamic allocation for both.
>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>   .../net/ethernet/mellanox/mlx5/core/en_main.c | 36 ++++++++++++-------
>   1 file changed, 23 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> index 6bb0fdaa5efa..e5198c26e383 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> @@ -2993,37 +2993,42 @@ static int mlx5e_switch_priv_channels(struct mlx5e_priv *priv,
>   	return err;
>   }
>   
> -int mlx5e_safe_switch_params(struct mlx5e_priv *priv,
> +noinline_for_stack int mlx5e_safe_switch_params(struct mlx5e_priv *priv,
>   			     struct mlx5e_params *params,
>   			     mlx5e_fp_preactivate preactivate,
>   			     void *context, bool reset)
>   {
> -	struct mlx5e_channels new_chs = {};
> +	struct mlx5e_channels *new_chs;
>   	int err;
>   
>   	reset &= test_bit(MLX5E_STATE_OPENED, &priv->state);
>   	if (!reset)
>   		return mlx5e_switch_priv_params(priv, params, preactivate, context);
>   
> -	new_chs.params = *params;
> +	new_chs = kzalloc(sizeof(*new_chs), GFP_KERNEL);
> +	if (!new_chs)
> +		return -ENOMEM;
> +	new_chs->params = *params;
>   
> -	mlx5e_selq_prepare_params(&priv->selq, &new_chs.params);
> +	mlx5e_selq_prepare_params(&priv->selq, &new_chs->params);
>   
> -	err = mlx5e_open_channels(priv, &new_chs);
> +	err = mlx5e_open_channels(priv, new_chs);
>   	if (err)
>   		goto err_cancel_selq;
>   
> -	err = mlx5e_switch_priv_channels(priv, &new_chs, preactivate, context);
> +	err = mlx5e_switch_priv_channels(priv, new_chs, preactivate, context);
>   	if (err)
>   		goto err_close;
>   
> +	kfree(new_chs);
>   	return 0;
>   
>   err_close:
> -	mlx5e_close_channels(&new_chs);
> +	mlx5e_close_channels(new_chs);
>   
>   err_cancel_selq:
>   	mlx5e_selq_cancel(&priv->selq);
> +	kfree(new_chs);
>   	return err;
>   }
>   
> @@ -3419,10 +3424,10 @@ static void mlx5e_params_mqprio_reset(struct mlx5e_params *params)
>   	mlx5e_params_mqprio_dcb_set(params, 1);
>   }
>   
> -static int mlx5e_setup_tc_mqprio_dcb(struct mlx5e_priv *priv,
> +static noinline_for_stack int mlx5e_setup_tc_mqprio_dcb(struct mlx5e_priv *priv,
>   				     struct tc_mqprio_qopt *mqprio)
>   {
> -	struct mlx5e_params new_params;
> +	struct mlx5e_params *new_params;
>   	u8 tc = mqprio->num_tc;
>   	int err;
>   
> @@ -3431,10 +3436,13 @@ static int mlx5e_setup_tc_mqprio_dcb(struct mlx5e_priv *priv,
>   	if (tc && tc != MLX5E_MAX_NUM_TC)
>   		return -EINVAL;
>   
> -	new_params = priv->channels.params;
> -	mlx5e_params_mqprio_dcb_set(&new_params, tc ? tc : 1);
> +	new_params = kmemdup(&priv->channels.params,
> +			     sizeof(priv->channels.params), GFP_KERNEL);
> +	if (!new_params)
> +		return -ENOMEM;
> +	mlx5e_params_mqprio_dcb_set(new_params, tc ? tc : 1);
>   
> -	err = mlx5e_safe_switch_params(priv, &new_params,
> +	err = mlx5e_safe_switch_params(priv, new_params,
>   				       mlx5e_num_channels_changed_ctx, NULL, true);
>   

Is this change really required, even after new_chs are dynamically 
allocated?
As this code pattern of static local new_params repeats in all callers 
of mlx5e_safe_switch_params, let's not change this one alone if not 
necessary.


Same for the noinline_for_stack. Are they really needed even after using 
dynamic allocation for new_chs?

>   	if (!err && priv->mqprio_rl) {
> @@ -3445,6 +3453,8 @@ static int mlx5e_setup_tc_mqprio_dcb(struct mlx5e_priv *priv,
>   
>   	priv->max_opened_tc = max_t(u8, priv->max_opened_tc,
>   				    mlx5e_get_dcb_num_tc(&priv->channels.params));
> +
> +	kfree(new_params);
>   	return err;
>   }
>   
> @@ -3533,7 +3543,7 @@ static struct mlx5e_mqprio_rl *mlx5e_mqprio_rl_create(struct mlx5_core_dev *mdev
>   	return rl;
>   }
>   
> -static int mlx5e_setup_tc_mqprio_channel(struct mlx5e_priv *priv,
> +static noinline_for_stack int mlx5e_setup_tc_mqprio_channel(struct mlx5e_priv *priv,
>   					 struct tc_mqprio_qopt_offload *mqprio)
>   {
>   	mlx5e_fp_preactivate preactivate;