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

[David Ahern]

On 11/6/23 11:47 AM, Stanislav Fomichev wrote:
> 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 <willemb@xxxxxxxxxx>
>> Signed-off-by: Kaiyuan Zhang <kaiyuanz@xxxxxxxxxx>
>> Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
>>
>> ---
>>  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).

+1.

Also, the flag on the skb is an optimization - a high level signal that
one or more frags is in unreadable memory. There is no requirement that
all of the frags are in the same memory type.