netdev - Re: [PATCH net-next 1/2] net: move the hsize check to the else block in skb

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Message-ID: <CADvbK_c4wUUp0We=Cv9jczzACAurZnu0_pRen7Oa=4k8bKcCMw@mail.gmail.com>
Date:   Tue, 12 Jan 2021 15:20:44 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     network dev <netdev@...r.kernel.org>,
        "linux-sctp @ vger . kernel . org" <linux-sctp@...r.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net-next 1/2] net: move the hsize check to the else block
 in skb_segment

On Tue, Jan 12, 2021 at 12:26 AM Alexander Duyck
<alexander.duyck@...il.com> wrote:
>
> On Mon, Jan 11, 2021 at 4:45 AM Xin Long <lucien.xin@...il.com> wrote:
> >
> > After commit 89319d3801d1 ("net: Add frag_list support to skb_segment"),
> > it goes to process frag_list when !hsize in skb_segment(). However, when
> > using skb frag_list, sg normally should not be set. In this case, hsize
> > will be set with len right before !hsize check, then it won't go to
> > frag_list processing code.
> >
> > So the right thing to do is move the hsize check to the else block, so
> > that it won't affect the !hsize check for frag_list processing.
> >
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > ---
> >  net/core/skbuff.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index 7626a33..ea79359 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -3855,8 +3855,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
> >                 hsize = skb_headlen(head_skb) - offset;
> >                 if (hsize < 0)
> >                         hsize = 0;
> > -               if (hsize > len || !sg)
> > -                       hsize = len;
> >
> >                 if (!hsize && i >= nfrags && skb_headlen(list_skb) &&
> >                     (skb_headlen(list_skb) == len || sg)) {
>
> So looking at the function it seems like the only spot where the
> standard path actually reads the hsize value is right here, and it is
> overwritten before we exit the non-error portion of the if statement.
> I wonder if we couldn't save ourselves a few cycles and avoid an
> unnecessary assignment by replacing the "!hsize" with a check for
> "hsize <= 0" and just move the entire set of checks above down into
> the lower block.
>
> > @@ -3901,6 +3899,9 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
> >                         skb_release_head_state(nskb);
> >                         __skb_push(nskb, doffset);
> >                 } else {
> > +                       if (hsize > len || !sg)
> > +                               hsize = len;
> > +
>
> Then you could essentially just add the "if (hsize < 0)" piece here as
> an "else if" check and avoid the check if it isn't needed.
Look correct, will post v2. Thanks!

>
> >                         nskb = __alloc_skb(hsize + doffset + headroom,
> >                                            GFP_ATOMIC, skb_alloc_rx_flag(head_skb),
> >                                            NUMA_NO_NODE);
> > --
> > 2.1.0
> >