Re: [RFC PATCH v3 0/9] vsock: updates for SO_RCVLOWAT handling

[Stefano Garzarella <sgarzare@redhat.com>]

Hi Arseniy,
sorry but I didn't have time to review this series. I will definitely do 
it next Monday!

Have a nice weekend,
Stefano

On Wed, Aug 03, 2022 at 01:48:06PM +0000, Arseniy Krasnov wrote:
>Hello,
>
>This patchset includes some updates for SO_RCVLOWAT:
>
>1) af_vsock:
>   During my experiments with zerocopy receive, i found, that in some
>   cases, poll() implementation violates POSIX: when socket has non-
>   default SO_RCVLOWAT(e.g. not 1), poll() will always set POLLIN and
>   POLLRDNORM bits in 'revents' even number of bytes available to read
>   on socket is smaller than SO_RCVLOWAT value. In this case,user sees
>   POLLIN flag and then tries to read data(for example using  'read()'
>   call), but read call will be blocked, because  SO_RCVLOWAT logic is
>   supported in dequeue loop in af_vsock.c. But the same time,  POSIX
>   requires that:
>
>   "POLLIN     Data other than high-priority data may be read without
>               blocking.
>    POLLRDNORM Normal data may be read without blocking."
>
>   See https://www.open-std.org/jtc1/sc22/open/n4217.pdf, page 293.
>
>   So, we have, that poll() syscall returns POLLIN, but read call will
>   be blocked.
>
>   Also in man page socket(7) i found that:
>
>   "Since Linux 2.6.28, select(2), poll(2), and epoll(7) indicate a
>   socket as readable only if at least SO_RCVLOWAT bytes are available."
>
>   I checked TCP callback for poll()(net/ipv4/tcp.c, tcp_poll()), it
>   uses SO_RCVLOWAT value to set POLLIN bit, also i've tested TCP with
>   this case for TCP socket, it works as POSIX required.
>
>   I've added some fixes to af_vsock.c and virtio_transport_common.c,
>   test is also implemented.
>
>2) virtio/vsock:
>   It adds some optimization to wake ups, when new data arrived. Now,
>   SO_RCVLOWAT is considered before wake up sleepers who wait new data.
>   There is no sense, to kick waiter, when number of available bytes
>   in socket's queue < SO_RCVLOWAT, because if we wake up reader in
>   this case, it will wait for SO_RCVLOWAT data anyway during dequeue,
>   or in poll() case, POLLIN/POLLRDNORM bits won't be set, so such
>   exit from poll() will be "spurious". This logic is also used in TCP
>   sockets.
>
>3) vmci/vsock:
>   Same as 2), but i'm not sure about this changes. Will be very good,
>   to get comments from someone who knows this code.
>
>4) Hyper-V:
>   As Dexuan Cui mentioned, for Hyper-V transport it is difficult to
>   support SO_RCVLOWAT, so he suggested to disable this feature for
>   Hyper-V.
>
>Thank You
>
>Arseniy Krasnov(9):
> vsock: SO_RCVLOWAT transport set callback
> hv_sock: disable SO_RCVLOWAT support
> virtio/vsock: use 'target' in notify_poll_in callback
> vmci/vsock: use 'target' in notify_poll_in callback
> vsock: pass sock_rcvlowat to notify_poll_in as target
> vsock: add API call for data ready
> virtio/vsock: check SO_RCVLOWAT before wake up reader
> vmci/vsock: check SO_RCVLOWAT before wake up reader
> vsock_test: POLLIN + SO_RCVLOWAT test
>
> include/net/af_vsock.h                       |   2 +
> net/vmw_vsock/af_vsock.c                     |  38 +++++++++-
> net/vmw_vsock/hyperv_transport.c             |   7 ++
> net/vmw_vsock/virtio_transport_common.c      |   7 +-
> net/vmw_vsock/vmci_transport_notify.c        |  10 +--
> net/vmw_vsock/vmci_transport_notify_qstate.c |  12 +--
> tools/testing/vsock/vsock_test.c             | 107 +++++++++++++++++++++++++++
> 7 files changed, 166 insertions(+), 17 deletions(-)
>
> Changelog:
>
> v1 -> v2:
> 1) Patches for VMCI transport(same as for virtio-vsock).
> 2) Patches for Hyper-V transport(disabling SO_RCVLOWAT setting).
> 3) Waiting logic in test was updated(sleep() -> poll()).
>
> v2 -> v3:
> 1) Patches were reordered.
> 2) Commit message updated in 0005.
> 3) Check 'transport' pointer in 0001 for NULL.
> 4) Check 'value' in 0001 for > buffer_size.
>
>-- 
>2.25.1