Re: [PATCH net] gro: fix napi_gro_frags() Fast GRO breakage due to IP alignment check

[Alexander Lobakin]

From: Eric Dumazet <edumazet@xxxxxxxxxx>
Date: Mon, 19 Apr 2021 13:05:16 +0200

> On Sun, Apr 18, 2021 at 1:43 PM Alexander Lobakin <alobakin@xxxxx> wrote:
> >
> > Commit 38ec4944b593 ("gro: ensure frag0 meets IP header alignment")
> > did the right thing, but missed the fact that napi_gro_frags() logics
> > calls for skb_gro_reset_offset() *before* pulling Ethernet header
> > to the skb linear space.
> > That said, the introduced check for frag0 address being aligned to 4
> > always fails for it as Ethernet header is obviously 14 bytes long,
> > and in case with NET_IP_ALIGN its start is not aligned to 4.
> >
> > Fix this by adding @nhoff argument to skb_gro_reset_offset() which
> > tells if an IP header is placed right at the start of frag0 or not.
> > This restores Fast GRO for napi_gro_frags() that became very slow
> > after the mentioned commit, and preserves the introduced check to
> > avoid silent unaligned accesses.
> >
> > Fixes: 38ec4944b593 ("gro: ensure frag0 meets IP header alignment")
> > Signed-off-by: Alexander Lobakin <alobakin@xxxxx>
> > ---
> >  net/core/dev.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 1f79b9aa9a3f..965d5f9b6fee 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -5914,7 +5914,7 @@ static struct list_head *gro_list_prepare(struct napi_struct *napi,
> >         return head;
> >  }
> >
> > -static void skb_gro_reset_offset(struct sk_buff *skb)
> > +static void skb_gro_reset_offset(struct sk_buff *skb, u32 nhoff)
> >  {
> >         const struct skb_shared_info *pinfo = skb_shinfo(skb);
> >         const skb_frag_t *frag0 = &pinfo->frags[0];
> > @@ -5925,7 +5925,7 @@ static void skb_gro_reset_offset(struct sk_buff *skb)
> >
> >         if (!skb_headlen(skb) && pinfo->nr_frags &&
> >             !PageHighMem(skb_frag_page(frag0)) &&
> > -           (!NET_IP_ALIGN || !(skb_frag_off(frag0) & 3))) {
> > +           (!NET_IP_ALIGN || !((skb_frag_off(frag0) + nhoff) & 3))) {
> >                 NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
> >                 NAPI_GRO_CB(skb)->frag0_len = min_t(unsigned int,
> >                                                     skb_frag_size(frag0),
> > @@ -6143,7 +6143,7 @@ gro_result_t napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
> >         skb_mark_napi_id(skb, napi);
> >         trace_napi_gro_receive_entry(skb);
> >
> > -       skb_gro_reset_offset(skb);
> > +       skb_gro_reset_offset(skb, 0);
> >
> >         ret = napi_skb_finish(napi, skb, dev_gro_receive(napi, skb));
> >         trace_napi_gro_receive_exit(ret);
> > @@ -6232,7 +6232,7 @@ static struct sk_buff *napi_frags_skb(struct napi_struct *napi)
> >         napi->skb = NULL;
> >
> >         skb_reset_mac_header(skb);
> > -       skb_gro_reset_offset(skb);
> > +       skb_gro_reset_offset(skb, hlen);
> >
> >         if (unlikely(skb_gro_header_hard(skb, hlen))) {
> >                 eth = skb_gro_header_slow(skb, hlen, 0);
> > --
> > 2.31.1
> >
> >
>
> Good catch, thanks.
>
> This has the unfortunate effect of increasing code length on x86,
> maybe we should have an exception to
> normal rules so that skb_gro_reset_offset() is inlined.

Agree. To mitigate this codegrowth we either go with NET_IP_ALIGN
ifdeffery or just inline skb_gro_reset_offset(). The function is
tiny itself, I don't see a reason to not do this.
Will drop v2 in a moment.

> Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>

Thanks,
Al