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

Stefan Hajnoczi
2022-08-23T20:01:59Z

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On Fri, Aug 19, 2022 at 05:21:58AM +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

Hi Arseniy,
Stefano will be online again on Monday. I suggest we wait for him to
review this series. If it's urgent, please let me know and I'll take a
look.

Thanks,
Stefan

> 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

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