linux-kernel - Re: [PATCH 3/3] UML - Correctly handle skb allocation failures

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Message-Id: <20070926151051.802008d5.akpm@linux-foundation.org>
Date:	Wed, 26 Sep 2007 15:10:51 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeff Dike <jdike@...toit.com>
Cc:	linux-kernel@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net, caker@...ode.com
Subject: Re: [PATCH 3/3] UML - Correctly handle skb allocation failures

On Wed, 26 Sep 2007 17:46:13 -0400
Jeff Dike <jdike@...toit.com> wrote:

> Handle memory allocation failures when reading packets.
> 
> We have to read something from the host, even if we can't allocate any
> memory.  If we don't, the host side of the device may fill up and stop
> delivering interrupts because no new packets can be queued.
> 
> A single sk_buff is allocated whenever an MTU is seen which is larger
> than any seen earlier.  This is used to read packets if there is a
> memory allocation failure.
> 
> The large MTU check is done from eth_configure, which is called when a
> interface is added to the system, and from uml_net_change_mtu, which
> is called when an existing interface has its MTU changed.
> 
> Signed-off-by: Jeff Dike <jdike@...ux.intel.com>
> ---
>  arch/um/drivers/net_kern.c |   60 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 60 insertions(+)
> 
> Index: linux-2.6.20/arch/um/drivers/net_kern.c
> ===================================================================
> --- linux-2.6.20.orig/arch/um/drivers/net_kern.c	2007-09-26 16:48:21.000000000 -0400
> +++ linux-2.6.20/arch/um/drivers/net_kern.c	2007-09-26 16:56:16.000000000 -0400
> @@ -34,6 +34,48 @@ static inline void set_ether_mac(struct 
>  static DEFINE_SPINLOCK(opened_lock);
>  static LIST_HEAD(opened);
>  
> +/*
> + * The throwaway skb is used when we can't allocate an skb.  The
> + * packet is read into throwaway in order to get the data off the
> + * connection to the host.
> + * It is reallocated whenever an MTU is seen which is larger than
> + * anything seen before.  update_throwaway_skb is called from
> + * eth_configure for new interfaces and from uml_net_change_mtu for
> + * MTU changes on existing interfaces.
> + */
> +static DEFINE_SPINLOCK(throwaway_lock);
> +static struct sk_buff *throwaway;
> +static int throwaway_max;
> +
> +static int update_throwaway_skb(int max)
> +{
> +	struct sk_buff *new;
> +	int err = 0;
> +
> +	spin_lock(&throwaway_lock);
> +
> +	if (max <= throwaway_max)
> +		goto out;
> +
> +	err = -ENOMEM;
> +	new = dev_alloc_skb(max);
> +	if (new == NULL)
> +		goto out;
> +
> +	skb_put(new, max);
> +
> +	kfree_skb(throwaway);
> +	throwaway = new;
> +	throwaway_max = max;
> +	err = 0;
> +out:
> +	spin_unlock(&throwaway_lock);
> +
> +	return err;
> +}
> +
> +int npackets = 0;

Unneeded initialisation?

Maybe not a good name for a global symbol ;)

I worry that the memory at "throwaway" can get thrown away...

>  static int uml_net_rx(struct net_device *dev)
>  {
>  	struct uml_net_private *lp = dev->priv;
> @@ -42,7 +84,14 @@ static int uml_net_rx(struct net_device 
>  
>  	/* If we can't allocate memory, try again next round. */
>  	skb = dev_alloc_skb(lp->max_packet);
> +	if ((++npackets % 100) == 0){
> +		kfree_skb(skb);
> +		skb = NULL;
> +	}
> +
>  	if (skb == NULL) {
> +		throwaway->dev = dev;
> +		(*lp->read)(lp->fd, throwaway, lp);

...  while other code is still using it.  Are you sure we don't need
throwaway_lock here?


>  		lp->stats.rx_dropped++;
>  		return 0;
>  	}
> @@ -240,6 +289,13 @@ static int uml_net_set_mac(struct net_de
>  
>  static int uml_net_change_mtu(struct net_device *dev, int new_mtu)
>  {
> +	struct uml_net_private *lp = dev->priv;
> +	int err;
> +
> +	err = update_throwaway_skb(lp->max_packet);
> +	if (err)
> +		return err;
> +
>  	dev->mtu = new_mtu;
>  
>  	return 0;
> @@ -447,6 +503,10 @@ static void eth_configure(int n, void *i
>  	dev->watchdog_timeo = (HZ >> 1);
>  	dev->irq = UM_ETH_IRQ;
>  
> +	err = update_throwaway_skb(lp->max_packet);
> +	if (err)
> +		goto out_undo_user_init;
> +
>  	rtnl_lock();
>  	err = register_netdevice(dev);
>  	rtnl_unlock();
-
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