Subject: Re: [RFC PATCH v3 09/12] net: add support for skbs with unreadable frags

From: Stanislav Fomichev <[email-masked]>
Date: Mon, 6 Nov 2023 10:47:56 -0800

On 11/05, Mina Almasry wrote:
> For device memory TCP, we expect the skb headers to be available in host
> memory for access, and we expect the skb frags to be in device memory
> and unaccessible to the host. We expect there to be no mixing and
> matching of device memory frags (unaccessible) with host memory frags
> (accessible) in the same skb.
> 
> Add a skb->devmem flag which indicates whether the frags in this skb
> are device memory frags or not.
> 
> __skb_fill_page_desc() now checks frags added to skbs for page_pool_iovs,
> and marks the skb as skb->devmem accordingly.
> 
> Add checks through the network stack to avoid accessing the frags of
> devmem skbs and avoid coalescing devmem skbs with non devmem skbs.
> 
> Signed-off-by: Willem de Bruijn <[email-masked]>
> Signed-off-by: Kaiyuan Zhang <[email-masked]>
> Signed-off-by: Mina Almasry <[email-masked]>
> 
> ---
>  include/linux/skbuff.h | 14 +++++++-
>  include/net/tcp.h      |  5 +--
>  net/core/datagram.c    |  6 ++++
>  net/core/gro.c         |  5 ++-
>  net/core/skbuff.c      | 77 ++++++++++++++++++++++++++++++++++++------
>  net/ipv4/tcp.c         |  6 ++++
>  net/ipv4/tcp_input.c   | 13 +++++--
>  net/ipv4/tcp_output.c  |  5 ++-
>  net/packet/af_packet.c |  4 +--
>  9 files changed, 115 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 1fae276c1353..8fb468ff8115 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -805,6 +805,8 @@ typedef unsigned char *sk_buff_data_t;
>   *	@csum_level: indicates the number of consecutive checksums found in
>   *		the packet minus one that have been verified as
>   *		CHECKSUM_UNNECESSARY (max 3)
> + *	@devmem: indicates that all the fragments in this skb are backed by
> + *		device memory.
>   *	@dst_pending_confirm: need to confirm neighbour
>   *	@decrypted: Decrypted SKB
>   *	@slow_gro: state present at GRO time, slower prepare step required
> @@ -991,7 +993,7 @@ struct sk_buff {
>  #if IS_ENABLED(CONFIG_IP_SCTP)
>  	__u8			csum_not_inet:1;
>  #endif
> -
> +	__u8			devmem:1;
>  #if defined(CONFIG_NET_SCHED) || defined(CONFIG_NET_XGRESS)
>  	__u16			tc_index;	/* traffic control index */
>  #endif
> @@ -1766,6 +1768,12 @@ static inline void skb_zcopy_downgrade_managed(struct sk_buff *skb)
>  		__skb_zcopy_downgrade_managed(skb);
>  }
>  
> +/* Return true if frags in this skb are not readable by the host. */
> +static inline bool skb_frags_not_readable(const struct sk_buff *skb)
> +{
> +	return skb->devmem;

bikeshedding: should we also rename 'devmem' sk_buff flag to 'not_readable'?
It better communicates the fact that the stack shouldn't dereference the
frags (because it has 'devmem' fragments or for some other potential
future reason).