> On Jul 27, 2022, at 5:37 AM, Stefano Garzarella <sgarzare@xxxxxxxxxx> 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&data=05%7C01%7Cvdasa%40vmware.com%7C5ad2c6759fd8439e938708da6fccbee4%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C1%7C637945222450930014%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=60hG3DiYufOCv1DuufSdujiLEKDNou1Ztyah3GPbOLI%3D&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@xxxxxxxxxx) 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
