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Date:   Thu, 14 Dec 2023 10:56:04 +0100
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Arseniy Krasnov <avkrasnov@salutedevices.com>
Cc:     Stefan Hajnoczi <stefanha@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Bobby Eshleman <bobby.eshleman@bytedance.com>,
        kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel@sberdevices.ru, oxffffaa@gmail.com
Subject: Re: [PATCH net-next v9 0/4] send credit update during setting
 SO_RCVLOWAT
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References: <20231214091947.395892-1-avkrasnov@salutedevices.com>
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On Thu, Dec 14, 2023 at 12:19:43PM +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@sberdevices.ru/
>
>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
>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).
>
>Head for this patchset is:
>https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=9bab51bd662be4c3ebb18a28879981d69f3ef15a
>
>Link to v1:
>https://lore.kernel.org/netdev/20231108072004.1045669-1-avkrasnov@salutedevices.com/
>Link to v2:
>https://lore.kernel.org/netdev/20231119204922.2251912-1-avkrasnov@salutedevices.com/
>Link to v3:
>https://lore.kernel.org/netdev/20231122180510.2297075-1-avkrasnov@salutedevices.com/
>Link to v4:
>https://lore.kernel.org/netdev/20231129212519.2938875-1-avkrasnov@salutedevices.com/
>Link to v5:
>https://lore.kernel.org/netdev/20231130130840.253733-1-avkrasnov@salutedevices.com/
>Link to v6:
>https://lore.kernel.org/netdev/20231205064806.2851305-1-avkrasnov@salutedevices.com/
>Link to v7:
>https://lore.kernel.org/netdev/20231206211849.2707151-1-avkrasnov@salutedevices.com/
>Link to v8:
>https://lore.kernel.org/netdev/20231211211658.2904268-1-avkrasnov@salutedevices.com/
>
>Changelog:
>v1 -> v2:
> * Patchset rebased and tested on new HEAD of net-next (see hash above).
> * New patch is added as 0001 - it removes return from SO_RCVLOWAT set
>   callback in 'af_vsock.c' when transport callback is set - with that
>   we can set 'sk_rcvlowat' only once in 'af_vsock.c' and in future do
>   not copy-paste it to every transport. It was discussed in v1.
> * See per-patch changelog after ---.
>v2 -> v3:
> * See changelog after --- in 0003 only (0001 and 0002 still same).
>v3 -> v4:
> * Patchset rebased and tested on new HEAD of net-next (see hash above).
> * See per-patch changelog after ---.
>v4 -> v5:
> * Change patchset tag 'RFC' -> 'net-next'.
> * See per-patch changelog after ---.
>v5 -> v6:
> * New patch 0003 which sends credit update during reading bytes from
>   socket.
> * See per-patch changelog after ---.
>v6 -> v7:
> * Patchset rebased and tested on new HEAD of net-next (see hash above).
> * See per-patch changelog after ---.
>v7 -> v8:
> * See per-patch changelog after ---.
>v8 -> v9:
> * Patchset rebased and tested on new HEAD of net-next (see hash above).
> * Add 'Fixes' tag for the current 0002.
> * Reorder patches by moving two fixes first.
>
>Arseniy Krasnov (4):
>  virtio/vsock: fix logic which reduces credit update messages
>  virtio/vsock: send credit update during setting SO_RCVLOWAT
>  vsock: update SO_RCVLOWAT setting callback
>  vsock/test: two tests to check credit update logic

This order will break the bisectability, since now patch 2 will not
build if patch 3 is not applied.

So you need to implement in patch 2 `set_rcvlowat` and in patch 3
updated it to `notify_set_rcvlowat`, otherwise we always need to
backport patch 3 in stable branches, that should be applied before
patch 2.

You have 2 options:
a. move patch 3 before patch 2 without changing the code
b. change patch 2 to use `set_rcvlowat` and updated that code in patch 3

I don't have a strong opinion, but I slightly prefer option a. BTW that
forces us to backport more patches on stable branches, so I'm fine with
option b as well.

That said:
Nacked-by: Stefano Garzarella <sgarzare@redhat.com>