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

[Mina Almasry]

On Mon, Nov 6, 2023 at 3:37 PM David Ahern <dsahern@xxxxxxxxxx> wrote:
>
> 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.
>
>

Currently only tcp_recvmsg_devmem(), I think. tcp_recvmsg_locked()
delegates to tcp_recvmsg_devmem() if skb->devmem, and
tcp_recvmsg_devmem() net_err's if it finds a non-iov frag in the skb.
This is done for some simplicity, because iov's are given to the user
via cmsg, but pages are copied into the linear buffer. I think it
would be confusing for the user if we simultaneously copied some data
to the linear buffer and gave them a devmem cmsgs in the same
recvmsg() call.

So, my simplicity is:

1. in a single skb, all frags must be devmem or non-devmem, no mixing.
2. In a single recvmsg() call, we only process devmem or non-devmem
skbs, no mixing.

-- 
Thanks,
Mina