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

[David Ahern]

On 11/6/23 3:18 PM, Mina Almasry wrote:
>>>>>> @@ -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.
>>
>> David: maybe there should be such a requirement (that they all are
>> unreadable)? Might be easier to support initially; we can relax later
>> on.
>>
> 
> Currently devmem == not_readable, and the restriction is that all the
> frags in the same skb must be either all readable or all unreadable
> (all devmem or all non-devmem).

What requires that restriction? In all of the uses of skb->devmem and
skb_frags_not_readable() what matters is if any frag is not readable,
then frag list walk or collapse is avoided.