Re: [RFC PATCH v1 0/2] send credit update during setting SO_RCVLOWAT

[Stefano Garzarella]

On Wed, Nov 08, 2023 at 10:20:02AM +0300, Arseniy Krasnov wrote:
> Hello,
>
>                               DESCRIPTION
>
> This patchset fixes old problem with hungup of both rx/tx sides and adds
> test for it. This happens due to non-default SO_RCVLOWAT value and
> deferred credit update in virtio/vsock. Link to previous old patchset:
> https://lore.kernel.org/netdev/39b2e9fd-601b-189d-39a9-914e5574524c@xxxxxxxxxxxxxx/
>
> Here is what happens step by step:
>
>                                  TEST
>
>                            INITIAL CONDITIONS
>
> 1) Vsock buffer size is 128KB.
> 2) Maximum packet size is also 64KB as defined in header (yes it is
>   hardcoded, just to remind about that value).
> 3) SO_RCVLOWAT is default, e.g. 1 byte.
>
>
>                                 STEPS
>
>            SENDER                              RECEIVER
> 1) sends 128KB + 1 byte in a
>   single buffer. 128KB will
>   be sent, but for 1 byte
>   sender will wait for free
>   space at peer. Sender goes
>   to sleep.
>
>
> 2)                                     reads 64KB, credit update not sent
> 3)                                     sets SO_RCVLOWAT to 64KB + 1
> 4)                                     poll() -> wait forever, there is
>                                       only 64KB available to read.
>
> So in step 4) receiver also goes to sleep, waiting for enough data or
> connection shutdown message from the sender. Idea to fix it is that rx
> kicks tx side to continue transmission (and may be close connection)
> when rx changes number of bytes to be woken up (e.g. SO_RCVLOWAT) and
> this value is bigger than number of available bytes to read.
>
> I've added small test for this, but not sure as it uses hardcoded value

Thanks for adding the test!

> for maximum packet length, this value is defined in kernel header and
> used to control deferred credit update. And as this is not available to
> userspace, I can't control test parameters correctly (if one day this
> define will be changed - test may become useless).

I see, I'll leave a comment in the patch!

Thanks,
Stefano

> Head for this patchset is:
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=ff269e2cd5adce4ae14f883fc9c8803bc43ee1e9
>
> Arseniy Krasnov (2):
>  virtio/vsock: send credit update during setting SO_RCVLOWAT
>  vsock/test: SO_RCVLOWAT + deferred credit update test
>
> drivers/vhost/vsock.c                   |   2 +
> include/linux/virtio_vsock.h            |   1 +
> net/vmw_vsock/virtio_transport.c        |   2 +
> net/vmw_vsock/virtio_transport_common.c |  31 ++++++
> net/vmw_vsock/vsock_loopback.c          |   2 +
> tools/testing/vsock/vsock_test.c        | 131 ++++++++++++++++++++++++
> 6 files changed, 169 insertions(+)
>
> --
> 2.25.1