netdev - RE: [PATCH v4 bpf-next 06/22] xsk: introduce wrappers and helpers for supporting multi-buffer in Tx path

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Message-ID: <87r0q5c5e3.fsf@toke.dk>
Date: Wed, 21 Jun 2023 15:27:16 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: "Sarkar, Tirthendu" <tirthendu.sarkar@...el.com>, "Fijalkowski, Maciej"
 <maciej.fijalkowski@...el.com>, "bpf@...r.kernel.org"
 <bpf@...r.kernel.org>, "ast@...nel.org" <ast@...nel.org>,
 "daniel@...earbox.net" <daniel@...earbox.net>, "andrii@...nel.org"
 <andrii@...nel.org>
Cc: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "Karlsson, Magnus"
 <magnus.karlsson@...el.com>, "bjorn@...nel.org" <bjorn@...nel.org>,
 "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
 "simon.horman@...igine.com" <simon.horman@...igine.com>
Subject: RE: [PATCH v4 bpf-next 06/22] xsk: introduce wrappers and helpers
 for supporting multi-buffer in Tx path

"Sarkar, Tirthendu" <tirthendu.sarkar@...el.com> writes:

>> -----Original Message-----
>> From: Toke Høiland-Jørgensen <toke@...hat.com>
>> Sent: Tuesday, June 20, 2023 10:56 PM
>>>
>> Subject: Re: [PATCH v4 bpf-next 06/22] xsk: introduce wrappers and helpers
>> for supporting multi-buffer in Tx path
>> 
>> Maciej Fijalkowski <maciej.fijalkowski@...el.com> writes:
>> 
>> > From: Tirthendu Sarkar <tirthendu.sarkar@...el.com>
>> >
>> > In Tx path, xsk core reserves space for each desc to be transmitted in
>> > the completion queue and it's address contained in it is stored in the
>> > skb destructor arg. After successful transmission the skb destructor
>> > submits the addr marking completion.
>> >
>> > To handle multiple descriptors per packet, now along with reserving
>> > space for each descriptor, the corresponding address is also stored in
>> > completion queue. The number of pending descriptors are stored in skb
>> > destructor arg and is used by the skb destructor to update completions.
>> >
>> > Introduce 'skb' in xdp_sock to store a partially built packet when
>> > __xsk_generic_xmit() must return before it sees the EOP descriptor for
>> > the current packet so that packet building can resume in next call of
>> > __xsk_generic_xmit().
>> >
>> > Helper functions are introduced to set and get the pending descriptors
>> > in the skb destructor arg. Also, wrappers are introduced for storing
>> > descriptor addresses, submitting and cancelling (for unsuccessful
>> > transmissions) the number of completions.
>> >
>> > Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@...el.com>
>> > ---
>> >  include/net/xdp_sock.h |  6 ++++
>> >  net/xdp/xsk.c          | 74 ++++++++++++++++++++++++++++++------------
>> >  net/xdp/xsk_queue.h    | 19 ++++-------
>> >  3 files changed, 67 insertions(+), 32 deletions(-)
>> >
>> > diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
>> > index 36b0411a0d1b..1617af380162 100644
>> > --- a/include/net/xdp_sock.h
>> > +++ b/include/net/xdp_sock.h
>> > @@ -68,6 +68,12 @@ struct xdp_sock {
>> >  	u64 rx_dropped;
>> >  	u64 rx_queue_full;
>> >
>> > +	/* When __xsk_generic_xmit() must return before it sees the EOP
>> descriptor for the current
>> > +	 * packet, the partially built skb is saved here so that packet building
>> can resume in next
>> > +	 * call of __xsk_generic_xmit().
>> > +	 */
>> > +	struct sk_buff *skb;
>> 
>> What ensures this doesn't leak? IIUC, when the loop in
>> __xsk_generic_xmit() gets to the end of a batch, userspace will get an
>> EAGAIN error and be expected to retry the call later, right? But if
>> userspace never retries, could the socket be torn down with this pointer
>> still populated? I looked for something that would prevent this in
>> subsequent patches, but couldn't find it; am I missing something?
>> 
>> -Toke
>> 
>
> Thanks for catching this. We will add cleanup during socket termination in v5.

Awesome! :)

-Toke