On 27.07.2022 15:37, Stefano Garzarella 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://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. > > Nice, it looks good! Thank You! > >> >> 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@xxxxxxxxxx) Ok, i'll CC them in the next version > >> >> 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. > > I left a couple of comments in some patches, but it seems to me to be in a good state :-) > > I would just suggest a bit of a re-organization of the series (the patches are fine, just the order): > - introduce vsock_set_rcvlowat() > - disabling it for hv_sock > - use 'target' in virtio transports > - use 'target' in vmci transports > - use sock_rcvlowat in vsock_poll() > I think is better to pass sock_rcvlowat() as 'target' when the > transports are already able to use it > - add vsock_data_ready() > - use vsock_data_ready() in virtio transports > - use vsock_data_ready() in vmci transports > - tests > > What do you think? No problem! I think i can wait for reply from VMWare guys before preparing v3 > > Thanks, > Stefano >
