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

[Eric Dumazet]

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.

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>