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

[Arseny Krasnov]

On 04.07.2021 12:54, Michael S. Tsirkin wrote:
> 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 ...
>
Sorry for late answer, yes You're right, seems this is unwanted drop...

Lets wait for Stefano Garzarella feedback


Thank You

>
>>>
>>>> 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 <arseny.krasnov@xxxxxxxxxxxxx>
>>>> ---
>>>>  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
>