Re: [PATCH net-next v9 0/4] send credit update during setting SO_RCVLOWAT

[Stefano Garzarella]

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@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
> 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@xxxxxxxxxxxxxxxxx/
> Link to v2:
> https://lore.kernel.org/netdev/20231119204922.2251912-1-avkrasnov@xxxxxxxxxxxxxxxxx/
> Link to v3:
> https://lore.kernel.org/netdev/20231122180510.2297075-1-avkrasnov@xxxxxxxxxxxxxxxxx/
> Link to v4:
> https://lore.kernel.org/netdev/20231129212519.2938875-1-avkrasnov@xxxxxxxxxxxxxxxxx/
> Link to v5:
> https://lore.kernel.org/netdev/20231130130840.253733-1-avkrasnov@xxxxxxxxxxxxxxxxx/
> Link to v6:
> https://lore.kernel.org/netdev/20231205064806.2851305-1-avkrasnov@xxxxxxxxxxxxxxxxx/
> Link to v7:
> https://lore.kernel.org/netdev/20231206211849.2707151-1-avkrasnov@xxxxxxxxxxxxxxxxx/
> Link to v8:
> https://lore.kernel.org/netdev/20231211211658.2904268-1-avkrasnov@xxxxxxxxxxxxxxxxx/
>
> 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@xxxxxxxxxx>