linux-kernel - Re: [PATCH net-next v24 08/13] net: add support for skbs with unreadable frags

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Message-ID: <CANn89i+iXXUjOJCOF1nodjVH4eX8P-B26Kg+Uu-iM2_Hc=5D5g@mail.gmail.com>
Date: Wed, 4 Sep 2024 17:29:44 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
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Subject: Re: [PATCH net-next v24 08/13] net: add support for skbs with
 unreadable frags

On Wed, Sep 4, 2024 at 5:18 PM Mina Almasry <almasrymina@...gle.com> wrote:
>
> On Tue, Sep 3, 2024 at 2:40 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Sat, 31 Aug 2024 00:43:08 +0000 Mina Almasry wrote:
> > >  static inline bool tcp_skb_can_collapse_to(const struct sk_buff *skb)
> > >  {
> > > -     return likely(!TCP_SKB_CB(skb)->eor);
> > > +     return likely(!TCP_SKB_CB(skb)->eor && skb_frags_readable(skb));
> >
> > Do you remember why this is here?
>
> Yeah, to be honest, when I first implemented some of these checks I
> erred on the side of caution, and added checks around anything that
> looked concerning, some of these unnecessary checks got removed, but
> looks like this one didn't.
>
> > Both for Rx and Tx what should matter
> > is whether the "readability" matches, right? We can merge two unreadable
> > messages.
>
> Yes, you're right, only 'readability matches' should be the criteria
> here. `tcp_skb_can_collapse` already checks readability is matching
> correctly, so no issue there. The `tcp_skb_can_collapse_to` check
> you're commenting on here looks unnecessary. I will remove it and run
> that through some testing.
>
> As an aside, it looks to me like that tcp_skb_can_collapse_to
> callsites don't seem to be doing any collapsing. Unless I misread the
> code. It looks like tcp_skb_can_collapse_to is used as an eor check. I
> can rename the function to tcp_skb_is_eor() or something if that makes
> sense (in a separate patch). I think the name of the function confused
> me slightly and made me think I need to do a readability check.
>

Please do not use tcp_skb_is_eor()

tcp_skb_can_collapse_to() could be renamed to tcp_skb_can_aggregate_to()
or tcp_skb_can_append_to(), but EOR would not help at all.