Subject: Re: [RFC PATCH v2 0/6] Improve SOCK_SEQPACKET receive logic

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 4 Jul 2021 05:54:29 -0400

On Sun, Jul 04, 2021 at 12:23:03PM +0300, Arseny Krasnov wrote:
> 
> On 04.07.2021 11:30, Michael S. Tsirkin wrote:
> > On Sun, Jul 04, 2021 at 11:08:13AM +0300, Arseny Krasnov wrote:
> >> 	This patchset modifies receive logic for SOCK_SEQPACKET.
> >> Difference between current implementation and this version is that
> >> now reader is woken up when there is at least one RW packet in rx
> >> queue of socket and data is copied to user's buffer, while merged
> >> approach wake up user only when whole message is received and kept
> >> in queue. New implementation has several advantages:
> >>  1) There is no limit for message length. Merged approach requires
> >>     that length must be smaller than 'peer_buf_alloc', otherwise
> >>     transmission will stuck.
> >>  2) There is no need to keep whole message in queue, thus no
> >>     'kmalloc()' memory will be wasted until EOR is received.
> >>
> >>     Also new approach has some feature: as fragments of message
> >> are copied until EOR is received, it is possible that part of
> >> message will be already in user's buffer, while rest of message
> >> still not received. And if user will be interrupted by signal or
> >> timeout with part of message in buffer, it will exit receive loop,
> >> leaving rest of message in queue. To solve this problem special
> >> callback was added to transport: it is called when user was forced
> >> to leave exit loop and tells transport to drop any packet until
> >> EOR met.
> > Sorry about commenting late in the game.  I'm a bit lost
> >
> >
> > SOCK_SEQPACKET
> > Provides sequenced, reliable, bidirectional, connection-mode transmission paths for records. A record can be sent using one or more output operations and received using one or more input operations, but a single operation never transfers part of more than one record. Record boundaries are visible to the receiver via the MSG_EOR flag.
> >
> > it's supposed to be reliable - how is it legal to drop packets?
> 
> Sorry, seems i need to rephrase description. "Packet" here means fragment of record(message) at transport
> 
> layer. As this is SEQPACKET mode, receiver could get only whole message or error, so if only several fragments
> 
> of message was copied (if signal received for example) we can't return it to user - it breaks SEQPACKET sense. I think,
> 
> in this case we can drop rest of record's fragments legally.
> 
> 
> Thank You

Would not that violate the reliable property? IIUC it's only ok to
return an error if socket gets closed. Just like e.g. TCP ...

> >
> >
> >> When EOR is found, this mode is disabled and normal packet
> >> processing started. Note, that when 'drop until EOR' mode is on,
> >> incoming packets still inserted in queue, reader will be woken up,
> >> tries to copy data, but nothing will be copied until EOR found.
> >> It was possible to drain such unneeded packets it rx work without
> >> kicking user, but implemented way is simplest. Anyway, i think
> >> such cases are rare.
> >
> >>     New test also added - it tries to copy to invalid user's
> >> buffer.
> >>
> >> Arseny Krasnov (16):
> >>  af_vsock/virtio/vsock: change seqpacket receive logic
> >>  af_vsock/virtio/vsock: remove 'seqpacket_has_data' callback
> >>  virtio/vsock: remove 'msg_count' based logic
> >>  af_vsock/virtio/vsock: add 'seqpacket_drop()' callback
> >>  virtio/vsock: remove record size limit for SEQPACKET
> >>  vsock_test: SEQPACKET read to broken buffer
> >>
> >>  drivers/vhost/vsock.c                   |   2 +-
> >>  include/linux/virtio_vsock.h            |   7 +-
> >>  include/net/af_vsock.h                  |   4 +-
> >>  net/vmw_vsock/af_vsock.c                |  44 ++++----
> >>  net/vmw_vsock/virtio_transport.c        |   2 +-
> >>  net/vmw_vsock/virtio_transport_common.c | 103 ++++++++-----------
> >>  net/vmw_vsock/vsock_loopback.c          |   2 +-
> >>  tools/testing/vsock/vsock_test.c        | 120 ++++++++++++++++++++++
> >>  8 files changed, 193 insertions(+), 91 deletions(-)
> >>
> >>  v1 -> v2:
> >>  Patches reordered and reorganized.
> >>
> >> Signed-off-by: Arseny Krasnov <[email-masked]>
> >> ---
> >>  cv.txt | 0
> >>  1 file changed, 0 insertions(+), 0 deletions(-)
> >>  create mode 100644 cv.txt
> >>
> >> diff --git a/cv.txt b/cv.txt
> >> new file mode 100644
> >> index 000000000000..e69de29bb2d1
> >> -- 
> >> 2.25.1
> >