netdev - RE: [RFC PATCH v3 09/12] net: add support for skbs with unreadable frags

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Date: Wed, 8 Nov 2023 14:43:23 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Mina Almasry' <almasrymina@...gle.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-arch@...r.kernel.org"
	<linux-arch@...r.kernel.org>, "linux-kselftest@...r.kernel.org"
	<linux-kselftest@...r.kernel.org>, "linux-media@...r.kernel.org"
	<linux-media@...r.kernel.org>, "dri-devel@...ts.freedesktop.org"
	<dri-devel@...ts.freedesktop.org>, "linaro-mm-sig@...ts.linaro.org"
	<linaro-mm-sig@...ts.linaro.org>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Jesper Dangaard Brouer <hawk@...nel.org>, "Ilias
 Apalodimas" <ilias.apalodimas@...aro.org>, Arnd Bergmann <arnd@...db.de>,
	David Ahern <dsahern@...nel.org>, Willem de Bruijn
	<willemdebruijn.kernel@...il.com>, Shuah Khan <shuah@...nel.org>, "Sumit
 Semwal" <sumit.semwal@...aro.org>, Christian König
	<christian.koenig@....com>, Shakeel Butt <shakeelb@...gle.com>, "Jeroen de
 Borst" <jeroendb@...gle.com>, Praveen Kaligineedi <pkaligineedi@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>, Kaiyuan Zhang <kaiyuanz@...gle.com>
Subject: RE: [RFC PATCH v3 09/12] net: add support for skbs with unreadable
 frags

From: Mina Almasry
> Sent: 06 November 2023 02:44
> 
> 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.
> 
...
> 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);
>  }

Doesn't that bloat struct sk_buff?
I'm not sure there are any spare bits available.
Although CONFIG_NET_SWITCHDEV and CONFIG_NET_SCHED seem to
already add padding.

	David

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