On Tue, Jan 12, 2021 at 12:26 AM Alexander Duyck
<alexander.duyck@xxxxxxxxx> wrote:
>
> On Mon, Jan 11, 2021 at 4:45 AM Xin Long <lucien.xin@xxxxxxxxx> 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@xxxxxxxxx>
> > ---
> > 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
> >
