Re: [PATCH net-next v3 1/3] selftests: fix OOM problem in msg_zerocopy selftest

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

zijianzhang@ wrote:
> From: Zijian Zhang <zijianzhang@bytedance.com>
> 
> In selftests/net/msg_zerocopy.c, it has a while loop keeps calling sendmsg
> on a socket with MSG_ZEROCOPY flag, and it will recv the notifications
> until the socket is not writable. Typically, it will start the receiving
> process after around 30+ sendmsgs. However, because of the
> commit dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale")
> the sender is always writable and does not get any chance to run recv
> notifications. The selftest always exits with OUT_OF_MEMORY because the
> memory used by opt_skb exceeds the core.sysctl_optmem_max.
> 
> According to our experiments, this problem can be mitigated by open the
> DEBUG_LOCKDEP configuration for the kernel. But it will makes the
> notifications disordered even in good commits before
> commit dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale").
> 
> We introduce "cfg_notification_limit" to force sender to receive
> notifications after some number of sendmsgs. And, notifications may not
> come in order, because of the reason we present above. We have order
> checking code managed by cfg_verbose.
> 
> Signed-off-by: Zijian Zhang <zijianzhang@bytedance.com>
> Signed-off-by: Xiaochun Lu <xiaochun.lu@bytedance.com>
> ---
>  tools/testing/selftests/net/msg_zerocopy.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/net/msg_zerocopy.c b/tools/testing/selftests/net/msg_zerocopy.c
> index bdc03a2097e8..ba6c257f689c 100644
> --- a/tools/testing/selftests/net/msg_zerocopy.c
> +++ b/tools/testing/selftests/net/msg_zerocopy.c
> @@ -85,6 +85,7 @@ static bool cfg_rx;
>  static int  cfg_runtime_ms	= 4200;
>  static int  cfg_verbose;
>  static int  cfg_waittime_ms	= 500;
> +static int  cfg_notification_limit = 32;
>  static bool cfg_zerocopy;
>  
>  static socklen_t cfg_alen;
> @@ -95,6 +96,8 @@ static char payload[IP_MAXPACKET];
>  static long packets, bytes, completions, expected_completions;
>  static int  zerocopied = -1;
>  static uint32_t next_completion;
> +/* The number of sendmsgs which have not received notified yet */
> +static uint32_t sendmsg_counter;

minor: comment typo. Consider a more self documenting variable and
drop the comment

static int sends_since_notify;