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Message-ID: <Z87Qv2Jf3p3MeXRC@lore-desk>
Date: Mon, 10 Mar 2025 12:45:03 +0100
From: Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To: arthur@...hurfabre.com
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org, jakub@...udflare.com,
hawk@...nel.org, yan@...udflare.com, jbrandeburg@...udflare.com,
thoiland@...hat.com, lbiancon@...hat.com,
Arthur Fabre <afabre@...udflare.com>
Subject: Re: [PATCH RFC bpf-next 16/20] trait: Support sk_buffs
> From: Arthur Fabre <afabre@...udflare.com>
>
> Hide the space used by traits from skb_headroom(): that space isn't
> actually usable.
>
> Preserve the trait store in pskb_expand_head() by copying it ahead of
> the new headroom. The struct xdp_frame at the start of the headroom
> isn't needed anymore, so we can overwrite it with traits, and introduce
> a new flag to indicate traits are stored at the start of the headroom.
>
> Cloned skbs share the same packet data and headroom as the original skb,
> so changes to traits in one would be reflected in the other.
> Is that ok?
> Are there cases where we would want a clone to have different traits?
> For now, prevent clones from using traits.
>
> Signed-off-by: Arthur Fabre <afabre@...udflare.com>
> ---
> include/linux/skbuff.h | 25 +++++++++++++++++++++++--
> net/core/skbuff.c | 25 +++++++++++++++++++++++--
> 2 files changed, 46 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index d7dfee152ebd26ce87a230222e94076aca793adc..886537508789202339c925b5613574de67b7e43c 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -39,6 +39,7 @@
> #include <net/net_debug.h>
> #include <net/dropreason-core.h>
> #include <net/netmem.h>
> +#include <net/trait.h>
>
> /**
> * DOC: skb checksums
> @@ -729,6 +730,8 @@ enum skb_traits_type {
> SKB_TRAITS_NONE,
> /* Trait store in headroom, offset by sizeof(struct xdp_frame) */
> SKB_TRAITS_AFTER_XDP,
> + /* Trait store at start of headroom */
> + SKB_TRAITS_AT_HEAD,
> };
>
> /**
> @@ -1029,7 +1032,7 @@ struct sk_buff {
> __u8 csum_not_inet:1;
> #endif
> __u8 unreadable:1;
> - __u8 traits_type:1; /* See enum skb_traits_type */
> + __u8 traits_type:2; /* See enum skb_traits_type */
> #if defined(CONFIG_NET_SCHED) || defined(CONFIG_NET_XGRESS)
> __u16 tc_index; /* traffic control index */
> #endif
> @@ -2836,6 +2839,18 @@ static inline void *pskb_pull(struct sk_buff *skb, unsigned int len)
>
> void skb_condense(struct sk_buff *skb);
>
> +static inline void *skb_traits(const struct sk_buff *skb)
> +{
> + switch (skb->traits_type) {
> + case SKB_TRAITS_AFTER_XDP:
> + return skb->head + _XDP_FRAME_SIZE;
> + case SKB_TRAITS_AT_HEAD:
> + return skb->head;
> + default:
> + return NULL;
> + }
> +}
> +
> /**
> * skb_headroom - bytes at buffer head
> * @skb: buffer to check
> @@ -2844,7 +2859,13 @@ void skb_condense(struct sk_buff *skb);
> */
> static inline unsigned int skb_headroom(const struct sk_buff *skb)
> {
> - return skb->data - skb->head;
> + int trait_size = 0;
> + void *traits = skb_traits(skb);
> +
> + if (traits)
> + trait_size = traits_size(traits);
> +
> + return skb->data - skb->head - trait_size;
I am not fully aware of all possible use-cases, but do we really need to
store hw medata traits (e.g. hw rx checksum or hw rx hash) in the skb
headroom when we convert the xdp_frame/xdp_buff in the skb? All of these
fields already have dedicated fields in the skb struct. Moreover, we need
to set them in order to have a real performance improvements when we execute
XDP_PASS. Something like:
https://lore.kernel.org/bpf/01ce17910fdd7b693c23132663fa884d5ec7f440.1726935917.git.lorenzo@kernel.org/
Regards,
Lorenzo
> }
>
> /**
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 7b03b64fdcb276f68ce881d1d8da8e4c6b897efc..83f58517738e8ff12990c28b09336ed44f4be32a 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -1515,6 +1515,19 @@ static struct sk_buff *__skb_clone(struct sk_buff *n, struct sk_buff *skb)
> atomic_inc(&(skb_shinfo(skb)->dataref));
> skb->cloned = 1;
>
> + /* traits would end up shared with the clone,
> + * and edits would be reflected there.
> + *
> + * Is that ok? What if the original skb and the clone take different paths?
> + * Does that even happen?
> + *
> + * If that's not ok, we could copy the traits and store them in an extension header
> + * for clones.
> + *
> + * For now, pretend the clone doesn't have any traits.
> + */
> + skb->traits_type = SKB_TRAITS_NONE;
> +
> return n;
> #undef C
> }
> @@ -2170,7 +2183,7 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> unsigned int osize = skb_end_offset(skb);
> unsigned int size = osize + nhead + ntail;
> long off;
> - u8 *data;
> + u8 *data, *head;
> int i;
>
> BUG_ON(nhead < 0);
> @@ -2187,10 +2200,18 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> goto nodata;
> size = SKB_WITH_OVERHEAD(size);
>
> + head = skb->head;
> + if (skb->traits_type != SKB_TRAITS_NONE) {
> + head = skb_traits(skb) + traits_size(skb_traits(skb));
> + /* struct xdp_frame isn't needed in the headroom, drop it */
> + memcpy(data, skb_traits(skb), traits_size(skb_traits(skb)));
> + skb->traits_type = SKB_TRAITS_AT_HEAD;
> + }
> +
> /* Copy only real data... and, alas, header. This should be
> * optimized for the cases when header is void.
> */
> - memcpy(data + nhead, skb->head, skb_tail_pointer(skb) - skb->head);
> + memcpy(data + nhead, head, skb_tail_pointer(skb) - head);
>
> memcpy((struct skb_shared_info *)(data + size),
> skb_shinfo(skb),
>
> --
> 2.43.0
>
>
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