Re: [PATCH rdma-next] IB/core: Add option to limit user mad receive list

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Apr 02, 2024 at 12:50:21PM +0300, Leon Romanovsky wrote:
> From: Michael Guralnik <michaelgur@nvidia.com>
> 
> ib_umad is keeping the received MAD packets in a list that is not
> limited in size. As the extraction of packets from this list is done
> from user-space application, there is no way to guarantee the extraction
> rate to be faster than the rate of incoming packets. This can cause to
> the list to grow uncontrollably.
> 
> As a solution, let's add new ysfs control knob for the users to limit
> the number of received MAD packets in the list.
> 
> Packets received when the list is full would be dropped. Sent packets
> that are queued on the receive list for whatever reason, like timed out
> sends, are not dropped even when the list is full.
> 
> Signed-off-by: Michael Guralnik <michaelgur@nvidia.com>
> Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> ---
>  .../ABI/stable/sysfs-class-infiniband         | 12 ++++
>  drivers/infiniband/core/user_mad.c            | 63 ++++++++++++++++++-
>  2 files changed, 72 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/ABI/stable/sysfs-class-infiniband b/Documentation/ABI/stable/sysfs-class-infiniband
> index 694f23a03a28..0ea9d590ab0e 100644
> --- a/Documentation/ABI/stable/sysfs-class-infiniband
> +++ b/Documentation/ABI/stable/sysfs-class-infiniband
> @@ -275,6 +275,18 @@ Description:
>  		=============== ===========================================
>  
>  
> +What:		/sys/class/infiniband_mad/umad<N>/max_recv_list_size
> +Date:		January, 2024
> +KernelVersion:	v6.9
> +Contact:	linux-rdma@vger.kernel.org
> +Description:
> +		(RW) Limit the size of the list of MAD packets waiting to be
> +		     read by the user-space agent.
> +		     The default value is 0, which means unlimited list size.
> +		     Packets received when the list is full will be silently
> +		     dropped.

I'm really not keen on this as a tunable, when we get to future
designs it may be hard to retain this specific behavior.

Why do we need a tunable? Can we just set it to something large and be
done with it?

Jason