netdev - Re: [PATCH NET v4 3/7] ipv6: use skb_expand_head in ip6

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Date:   Sun, 22 Aug 2021 10:13:32 -0700
From:   Christoph Paasch <christoph.paasch@...il.com>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel@...nvz.org,
        Julian Wiedmann <jwi@...ux.ibm.com>
Subject: Re: [PATCH NET v4 3/7] ipv6: use skb_expand_head in ip6_xmit

On Sun, Aug 22, 2021 at 10:04 AM Christoph Paasch
<christoph.paasch@...il.com> wrote:
>
> Hello Vasily,
>
> On Fri, Aug 20, 2021 at 11:21 PM Vasily Averin <vvs@...tuozzo.com> wrote:
> >
> > On 8/21/21 1:44 AM, Christoph Paasch wrote:
> > > (resend without html - thanks gmail web-interface...)
> > > On Fri, Aug 20, 2021 at 3:41 PM Christoph Paasch
> > >> AFAICS, this is because pskb_expand_head (called from
> > >> skb_expand_head) is not adjusting skb->truesize when skb->sk is set
> > >> (which I guess is the case in this particular scenario). I'm not
> > >> sure what the proper fix would be though...
> >
> > Could you please elaborate?
> > it seems to me skb_realloc_headroom used before my patch called pskb_expand_head() too
> > and did not adjusted skb->truesize too. Am I missed something perhaps?
> >
> > The only difference in my patch is that skb_clone can be not called,
> > though I do not understand how this can affect skb->truesize.
>
> I *believe* that the difference is that after skb_clone() skb->sk is
> NULL and thus truesize will be adjusted.
>
> I will try to confirm that with some more debugging.

Yes indeed.

Before your patch:
[   19.154039] ip6_xmit before realloc truesize 4864 sk? 000000002ccd6868
[   19.155230] ip6_xmit after realloc truesize 5376 sk? 0000000000000000

skb->sk is not set and thus truesize will be adjusted.


With your change:
[   15.092933] ip6_xmit before realloc truesize 4864 sk? 00000000072930fd
[   15.094131] ip6_xmit after realloc truesize 4864 sk? 00000000072930fd

skb->sk is set and thus truesize is not adjusted.


Christoph

>
>
> Christoph
>
> >
> > Thank you,
> >         Vasily Averin