Re: [PATCH v3] nvmet-tcp: report a bounded MDTS instead of "no limit"

[Maurizio Lombardi]

On Sat Sep 19, 2026 at 5:00 AM CEST, Ing. Alfonso Kuen Arroyo wrote:
> On Tue Sep 01, 2026, Maurizio Lombardi wrote:
>> #define NVMET_TCP_MAXH2CDATA 0x400000 /* 16M arbitrary limit */
>>
>> So the comment should be fixed, it says NVMET_TCP_MAXH2CDATA is 16M.
>
> Agreed - the value is 4 MiB and the comment is stale; it predates this
> patch. I'll send a separate one-line patch for the comment rather than
> fold an unrelated change into this one.
>
>> Also, it's not entirely clear to me how the target enforces this. Suppose
>> an host sends a 2 MiB command, violating the MDTS setting, what happens?
>
> It doesn't enforce it, and this patch doesn't add enforcement. MDTS is a
> limit the host is required to honour (the base spec says the host shall
> not submit a command exceeding it), and nvmet has no per-command check
> against it, neither in the core nor in the transports.

And indeed this is what's missing, because the specification explicitely
says that "if a command is submitted that exceeds [MDTS], then the
command is aborted with a status code of Invalid Field in Command."
(Identify – Identify Controller Data Structure, I/O Command Set
Independent)

Maybe it can be added separatedly with a dedicated patch.

>
> What the target does have is the hard limit in nvmet_tcp_map_data():
> since 4a3f00262a04 a data length above NVMET_TCP_MAXH2CDATA fails with
> NVME_SC_SGL_INVALID_DATA | DNR before any buffer is allocated. So today
> there are two numbers: the one the target will actually accept (4 MiB)
> and the one it advertises (none). The patch makes them the same number,
> so a compliant host never builds the command that the second check would
> reject. With MDTS = 4 MiB, a 2 MiB command is within the limit and is
> served normally;

Yes, but your patch sets NVMET_RDMA_MAX_MDTS to 8, which means 1MiB, not
4MiB, so a 2MiB command should be rejected, even if it's under the MAXH2CDATA
limit of 4MiB; or am I missing something?

Maurizio