Subject: Re: [PATCH net 1/2] tuntap: limit head length of skb allocated

From: Jason Wang <[email-masked]>
Date: Tue, 12 Nov 2013 09:21:12 -0500 (EST)

----- 原始邮件 -----
> 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/tun.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 7cb105c..5537b65 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -954,6 +954,11 @@ static struct sk_buff *tun_alloc_skb(struct tun_file
> *tfile,
>  	struct sock *sk = tfile->socket.sk;
>  	struct sk_buff *skb;
>  	int err;
> +	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)
> --
> 1.8.3.2
> 

Looks like reduce the linear here will cause callers' iov_pages() under estimation.
Will send V2.
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