linux-kernel - Re: [PATCH] nvme: Use non zero KATO for persistent discovery connections

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Message-ID: <CAKmqyKNtx-eq-dSgcJqoXjajX--QyBvqwfgKyfTfxiTiF-cFBA@mail.gmail.com>
Date: Wed, 3 Sep 2025 10:00:36 +1000
From: Alistair Francis <alistair23@...il.com>
To: Martin George <martinus.gpy@...il.com>
Cc: kbusch@...nel.org, axboe@...nel.dk, hch@....de, sagi@...mberg.me, 
	linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Alistair Francis <alistair.francis@....com>
Subject: Re: [PATCH] nvme: Use non zero KATO for persistent discovery connections

On Tue, Sep 2, 2025 at 8:35 PM Martin George <martinus.gpy@...il.com> wrote:
>
> On Tue, 2025-09-02 at 13:52 +1000, alistair23@...il.com wrote:
> > From: Alistair Francis <alistair.francis@....com>
> >
> > The NVMe Base Specification 2.1 states that:
> >
> > """
> > A host requests an explicit persistent connection ... by specifying a
> > non-zero Keep Alive Timer value in the Connect command.
> > """
> >
> > As such if we are starting a persistent connection to a discovery
> > controller and the KATO is currently 0 we need to update KATO to a
> > non
> > zero value to avoid continuous timeouts on the target.
> >
> >
>
> When would this ever happen? Won't nvme-cli & nvme/host/fabrics.c in

It occurs if you perform a `nvme connect` to the discovery nqn of a
Linux target.

> the kernel ensure a PDC (persistent discovery controller) would always
> have the KATO either default set to NVMF_DEF_DISC_TMO (i.e. 30s) or any
> positive int value & not zero?

The kernel doesn't set a default for discovery controllers (hence this patch).

nvme-cli will only set NVMF_DEF_DISC_TMO if the `--persistent`
connection option is supplied to `nvme discover`. But it doesn't set a
KATO for `nvme connect`, even though it's a persistent connection.

Note, that I think that is a bug in nvme-cli and it should be setting
a non zero KATO. I plan on patching that. At the same time if the
kernel knows it's a persistent discovery connection it should also be
setting a default KATO. The kernel is already using a default non-zero
KATO for non discovery nqns (see nvmf_parse_options()). This just
extends the default to apply to persistent discovery controllers.

>
> Do you have a test log for the above scenario where the KATO ends up
> being zero for a PDC?

I do, it's just a lot of keep-alive timeout prints on the target

Alistair

>
> -Martin