Re: [RFC PATCH v2 0/9] vsock: updates for SO_RCVLOWAT handling

[Vishnu Dasa <vdasa@vmware.com>]

> On Jul 27, 2022, at 5:37 AM, Stefano Garzarella <sgarzare@redhat.com> wrote:
> 
> Hi Arseniy,
> 
> On Mon, Jul 25, 2022 at 07:54:05AM +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://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.open-std.org%2Fjtc1%2Fsc22%2Fopen%2Fn4217.pdf&amp;data=05%7C01%7Cvdasa%40vmware.com%7C5ad2c6759fd8439e938708da6fccbee4%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C1%7C637945222450930014%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=60hG3DiYufOCv1DuufSdujiLEKDNou1Ztyah3GPbOLI%3D&amp;reserved=0, 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.
> 
> Nice, it looks good!
> 
>> 
>> 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.
> 
> I CCed VMCI maintainers to the patch and also to this cover, maybe
> better to keep them in the loop for next versions.
> 
> (Jorgen's and Rajesh's emails bounced back, so I'm CCing here only
> Bryan, Vishnu, and pv-drivers@vmware.com)

Hi Stefano,
Jorgen and Rajesh are no longer with VMware.  There's a patch in
flight to remove Rajesh from the MAINTAINERS file (Jorgen is already
removed).

Thanks,
Vishnu