Use of TX offload flags MBUF_FAST_FREE and MULTI_SEGS

[Morten Brørup]

>From: Stephen Hemminger [mailto:stephen at networkplumber.org] 
>Sent: Tuesday, 7 October 2025 09.59
>
>To help with definition of fast free flag, I would like the documentation to be better.
>
>Something like:
>Fast free flag allows driver to avoid expensive atomic operators on ref count and assume all mbufs in a burst are from the same pool.

IMO, for a driver to really benefit, the mbufs must also be direct, so the driver can simply free all the mbufs after they have been transmitted, and avoid this complex part of rte_pktmbuf_prefree_seg() [2]:
	if (!RTE_MBUF_DIRECT(m)) {
		rte_pktmbuf_detach(m);
		if (RTE_MBUF_HAS_EXTBUF(m) &&
		    RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
		    __rte_pktmbuf_pinned_extbuf_decref(m))
			return NULL;
	}

[2]: https://elixir.bootlin.com/dpdk/v25.07/source/lib/mbuf/rte_mbuf.h#L1430

>
>Should also add debug asserts in drivers implenting fast free.
>
>On Mon, Oct 6, 2025, 16:40 Morten Brørup <mb at smartsharesystems.com> wrote:
>>> From: Bruce Richardson [mailto:bruce.richardson at intel.com]
>>> Sent: Friday, 3 October 2025 11.18
>>> Subject: Minutes of techboard meeting, 2025-10-01
>>
>>> * Use of FAST_FREE and multi-buffer/scattered mbuf flags
>>>   - The flags for enabling fast-free and supporting multi-mbuf packets
>>> are
>>>     now documented incompatible
>>>   - Previously they were not defined as incompatible, but that seems to
>>>     have been assumed for some usages.
>>>   - Techboard discussed how best to resolve this incompatibility with
>>>     regards to:
>>>     - ensuring correctness
>>>     - avoiding major churn to DPDK code
>>>     - avoiding churn to end-user code
>>>   - Options discussed:
>>>     1 change definition back to not have the settings incompatible:
>>> this
>>>       necessitates checking drivers for correctness
>>>     2 keep as explicitly incompatible and report error if both
>>> specified:
>>>       this could break end-user apps, and requires changes to example
>>> apps
>>>     3 drop the fast-free flag if multi-segment mbufs are also
>>> specified:
>>>       "hides" the issue, but probably minimises changes. Would need to
>>>       decide whether the dropping of flag done in drivers vs ethdev
>>> level.
>>>       Pros and cons to both options. Needs clear documenting.
>>>   - No firm decision reached, will discuss more over email.
>>
>>IMO, the patch [1] making MBUF_FAST_FREE and MULTI_SEGS explicitly incompatible should be reverted, at least for RC1.
>>That will take the project back to the state it was in before we started this discussion.
>>And all the examples broken by the patch (because they use both TX offloads) will not need fixing.
>>
>>[1]: https://patchwork.dpdk.org/project/dpdk/patch/20250803194218.683318-3-mb@smartsharesystems.com/
>>