On Mon, 2012-04-09 at 15:34 -0400, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Date: Mon, 09 Apr 2012 21:11:12 +0200
>
> > I think Marc posted stack traces showing problem on transmit side.
> ...
> > I dont really understand how it can happen, with MTU=1500
>
> Depending upon the configuration and the driver, wireless can need
> more headroom. For encryption an extra 8 bytes are necessary, and the
> driver may request a variable amount of extra headroom via
> ->hw.extra_tx_headroom
>
> What wireless device are we dealing with again?
Problem seems related to tailroom needed by mac80211
(IEEE80211_ENCRYPT_TAILROOM = 18 bytes)
So we must reallocate skb->head, thats impressive nobody cares.
[ 3007.249687] ieee80211_skb_resize(skb=ffff8802329846e8) cloned=1 head_need=0 tail_need=18 skb->len=1494 ksize=4096 tailroom=0 headroom=2282
[ 3007.249693] ieee80211_skb_resize(skb=ffff8802329846e8) cloned=0 head_need=0 tail_need=0 skb->len=1526 ksize=8192 tailroom=64 headroom=2250
Ouch... skb_tailroom() seems wrong ... it seems pskb_expand_head() is really suboptimal.
It appears tcp_sendmsg() tries to fill skb completely, with no available tailroom :
if (skb_tailroom(skb) > 0) {
/* We have some space in skb head. Superb! */
if (copy > skb_tailroom(skb))
copy = skb_tailroom(skb);
err = skb_add_data_nocache(sk, skb, from, copy);
if (err)
goto do_fault;
} else {
Shouldnt we take into account dev->needed_tailroom ?
I'll submit a pskb_expand_head() fix asap.
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