[PATCH net-next v4 0/9] vsock: updates for SO_RCVLOWAT handling

Arseniy Krasnov
2022-08-19T05:22:34Z

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 | 33 +++++++-
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 | 108 +++++++++++++++++++++++++++
7 files changed, 162 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.

v3 -> v4:
1) vsock_set_rcvlowat() logic changed. Previous version required
assigned transport and always called its 'set_rcvlowat' callback
(if present). Now, assignment is not needed.
2) 0003,0004,0005,0006,0007,0008 - commit messages updated.
3) 0009 - small refactoring and style fixes.
4) RFC tag was removed.

--
2.25.1