netdev - Re: [net-next 8/8] stmmac: limit max_mtu in case of 4KiB and use __netdev_alloc

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Date:	Tue, 18 Oct 2011 12:18:30 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Giuseppe CAVALLARO <peppe.cavallaro@...com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [net-next 8/8] stmmac: limit max_mtu in case of 4KiB and use
 __netdev_alloc_skb.

Le mardi 18 octobre 2011 à 12:01 +0200, Giuseppe CAVALLARO a écrit :
> Problem using big mtu around 4096 bytes is you end allocating (4096
> +NET_SKB_PAD + NET_IP_ALIGN + sizeof(struct skb_shared_info) bytes ->
> 8192 bytes : order-1 pages
> 
> It's better to limit the mtu to SKB_MAX_HEAD(NET_SKB_PAD),
> to have no more than one page per skb.
> 
> Also the patch changes the netdev_alloc_skb_ip_align() done in
> init_dma_desc_rings() and uses a variant allowing GFP_KERNEL allocations
> allowing the driver to load even in case of memory pressure.
> 
> Reported-by: Eric Dumazet <eric.dumazet@...il.com>
> Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@...com>
> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c |   13 +++++++++----
>  1 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index 1848a16..f5ca3be 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -474,11 +474,13 @@ static void init_dma_desc_rings(struct net_device *dev)
>  	for (i = 0; i < rxsize; i++) {
>  		struct dma_desc *p = priv->dma_rx + i;
>  
> -		skb = netdev_alloc_skb_ip_align(dev, bfsize);
> +		skb = __netdev_alloc_skb(dev, bfsize + NET_IP_ALIGN,
> +					 GFP_KERNEL);
>  		if (unlikely(skb == NULL)) {
>  			pr_err("%s: Rx init fails; skb is NULL\n", __func__);
>  			break;
>  		}
> +		skb_reserve(skb, NET_IP_ALIGN);
>  		priv->rx_skbuff[i] = skb;
>  		priv->rx_skbuff_dma[i] = dma_map_single(priv->device, skb->data,
>  						bfsize, DMA_FROM_DEVICE);
> @@ -1176,12 +1178,15 @@ static inline void stmmac_rx_refill(struct stmmac_priv *priv)
>  
>  			skb = __skb_dequeue(&priv->rx_recycle);
>  			if (skb == NULL)
> -				skb = netdev_alloc_skb_ip_align(priv->dev,
> -								bfsize);
> +				skb = __netdev_alloc_skb(priv->dev, bfsize +
> +							 NET_IP_ALIGN,
> +							 GFP_KERNEL);
>  

No, you cant do that in softirq context. We cant sleep here and must use
GFP_ATOMIC

Only the init_dma_desc_rings() part is OK, we run in process context and
are allowed to sleep in memory allocations (GFP_KERNEL)


>  			if (unlikely(skb == NULL))
>  				break;
>  
> +			skb_reserve(skb, NET_IP_ALIGN);
> +
>  			priv->rx_skbuff[entry] = skb;
>  			priv->rx_skbuff_dma[entry] =
>  			    dma_map_single(priv->device, skb->data, bfsize,
> @@ -1401,7 +1406,7 @@ static int stmmac_change_mtu(struct net_device *dev, int new_mtu)
>  	if (priv->plat->enh_desc)
>  		max_mtu = JUMBO_LEN;
>  	else
> -		max_mtu = BUF_SIZE_4KiB;
> +		max_mtu = SKB_MAX_HEAD(NET_SKB_PAD);
>  

minor nit (since NET_IP_ALIGN is 0 anyway on modern x86)
max_mtu = SKB_MAX_HEAD(NET_SKB_PAD + NET_IP_ALIGN);


>  	if ((new_mtu < 46) || (new_mtu > max_mtu)) {
>  		pr_err("%s: invalid MTU, max MTU is: %d\n", dev->name, max_mtu);


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