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

[Arseniy Krasnov]

On 05.08.2022 19:05, Stefano Garzarella wrote:
> 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
Hello,
no problem

Thank You
> 
> 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
>