Subject: Re: [PATCH net 2/2] macvtap: limit head length of skb allocated

From: Greg Rose <[email-masked]>
Date: Tue, 12 Nov 2013 09:45:01 -0800

On Tue, 12 Nov 2013 18:02:57 +0800
Jason Wang <[email-masked]> wrote:

> We currently use hdr_len as a hint of head length which is advertised
> by guest. But when guest advertise a very big value, it can lead to
> an 64K+ allocating of kmalloc() which has a very high possibility of
> failure when host memory is fragmented or under heavy stress. The
> huge hdr_len also reduce the effect of zerocopy or even disable if a
> gso skb is linearized in guest.
> 
> To solves those issues, this patch introduces an upper limit
> (PAGE_SIZE) of the head, which guarantees an order 0 allocation each
> time.
> 
> Cc: Stefan Hajnoczi <[email-masked]>
> Cc: Michael S. Tsirkin <[email-masked]>
> Signed-off-by: Jason Wang <[email-masked]>
> ---
> The patch was needed for stable.
> ---
>  drivers/net/macvtap.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index 9dccb1e..7ee6f9d 100644
> --- a/drivers/net/macvtap.c
> +++ b/drivers/net/macvtap.c
> @@ -523,6 +523,11 @@ static inline struct sk_buff
> *macvtap_alloc_skb(struct sock *sk, size_t prepad, int noblock, int
> *err) {
>  	struct sk_buff *skb;
> +	int good_linear = SKB_MAX_HEAD(prepad);
> +
> +	/* Don't use huge linear part */
> +	if (linear > good_linear)
> +		linear = good_linear;
>  
>  	/* Under a page?  Don't bother with paged skb. */
>  	if (prepad + len < PAGE_SIZE || !linear)

I see no problem with this or the tuntap patch except that in both
cases kernel coding style would prefer that you align the local
variable declarations in a reverse pyramid, longest at the beginning,
shortest at the end.

- Greg