netdev - Re: Socket buffer sizes with autotuning

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Message-ID: <d1c2719f0805072033u38dfffaes71aed3efbc8035c0@mail.gmail.com>
Date:	Wed, 7 May 2008 20:33:06 -0700
From:	"Jerry Chu" <hkchu@...gle.com>
To:	"David Miller" <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: Socket buffer sizes with autotuning

There seems to be quite a bit of complexity plus one additional pointer
field per skb_shared_info to make skb better track when a pkt leaves
the host. Now I wonder if it's really a better solution than my original,
simply checking dataref==1 approach which, although not bullet proof,
may be "good enough" for all practical purposes?

Jerry

On Wed, May 7, 2008 at 6:43 PM, David Miller <davem@...emloft.net> wrote:
> From: "Jerry Chu" <hkchu@...gle.com>
>  Date: Wed, 7 May 2008 18:37:01 -0700
>
>
>  > Ok, will give it a try. First i'll fix your patch to
>  > atomic_add()/atomic_sub() by
>  > skb_shinfo(skb)->gso_segs rather than always 1, in order for GSO/TSO to work.
>
>  That might not work.  gso_segs can change over time as retransmit
>  packets get split up due to SACKs etc.  it needs to be audited,
>  at the very least.
>
>
>  > One problem came up to my mind - it seems possible for __kfree_skb() to
>  > access skb_shinfo(skb)->in_flight whose tp has been freed up since only the
>  > original skb's on TCP's rexmit list have the owner set and socket
>  > held. One solution
>  > is for TCP to zap skb_shinfo(skb)->in_flight field when it's ready to
>  > free up skb.
>  > I can hack sock_wfree() to do this, but I don't know how to do it right.
>
>  There will be references to the socket, so this should be ok.
>
>  If it isn't we can adjust the count and zap the pointer in
>  skb_orphan().
>
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