RE: mbuf fast-free requirements analysis

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com]
> > Sent: Monday, 15 December 2025 15.41
> >
> > >
> > > Executive Summary:
> > >
> > > My analysis shows that the mbuf library is not a barrier for fast-
> > freeing
> > > segmented packet mbufs, and thus fast-free of jumbo frames is
> > possible.
> > >
> > >
> > > Detailed Analysis:
> > >
> > > The purpose of the mbuf fast-free Tx optimization is to reduce
> > > rte_pktmbuf_free_seg() to something much simpler in the ethdev
> > drivers, by
> > > eliminating the code path related to indirect mbufs.
> > > Optimally, we want to simplify the ethdev driver's function that
> > frees the
> > > transmitted mbufs, so it can free them directly to their mempool
> > without
> > > accessing the mbufs themselves.
> > >
> > > If the driver cannot access the mbuf itself, it cannot determine
> > which
> > > mempool it belongs to.
> > > We don't want the driver to access every mbuf being freed; but if all
> > > mbufs of a Tx queue belong to the same mempool, the driver can
> > determine
> > > which mempool by looking into just one of the mbufs.
> > >
> > > REQUIREMENT 1: The mbufs of a Tx queue must come from the same
> > mempool.
> > >
> > >
> > > When an mbuf is freed to its mempool, some of the fields in the mbuf
> > must
> > > be initialized.
> > > So, for fast-free, this must be done by the driver's function that
> > > prepares the Tx descriptor.
> > > This is a requirement to the driver, not a requirement to the
> > application.
> > >
> > > Now, let's dig into the code for freeing an mbuf.
> > > Note: For readability purposes, I'll cut out some code and comments
> > > unrelated to this topic.
> > >
> > > static __rte_always_inline void
> > > rte_pktmbuf_free_seg(struct rte_mbuf *m)
> > > {
> > > 	m = rte_pktmbuf_prefree_seg(m);
> > > 	if (likely(m != NULL))
> > > 		rte_mbuf_raw_free(m);
> > > }
> > >
> > >
> > > rte_mbuf_raw_free(m) is simple, so nothing to gain there:
> > >
> > > /**
> > >  * Put mbuf back into its original mempool.
> > >  *
> > >  * The caller must ensure that the mbuf is direct and properly
> > >  * reinitialized (refcnt=1, next=NULL, nb_segs=1), as done by
> > >  * rte_pktmbuf_prefree_seg().
> > >  */
> > > static __rte_always_inline void
> > > rte_mbuf_raw_free(struct rte_mbuf *m)
> > > {
> > > 	rte_mbuf_history_mark(m, RTE_MBUF_HISTORY_OP_LIB_FREE);
> > > 	rte_mempool_put(m->pool, m);
> > > }
> > >
> > > Note that the description says that the mbuf must be direct.
> > > This is not entirely accurate; the mbuf is allowed to use a pinned
> > > external buffer, if the mbuf holds the only reference to it.
> > > (Most of the mbuf library functions have this documentation
> > inaccuracy,
> > > which should be fixed some day.)
> > >
> > > So, the fast-free optimization really comes down to
> > > rte_pktmbuf_prefree_seg(m), which must not return NULL.
> > >
> > > Let's dig into that.
> > >
> > > /**
> > >  * Decrease reference counter and unlink a mbuf segment
> > >  *
> > >  * This function does the same than a free, except that it does not
> > >  * return the segment to its pool.
> > >  * It decreases the reference counter, and if it reaches 0, it is
> > >  * detached from its parent for an indirect mbuf.
> > >  *
> > >  * @return
> > >  *   - (m) if it is the last reference. It can be recycled or freed.
> > >  *   - (NULL) if the mbuf still has remaining references on it.
> > >  */
> > > static __rte_always_inline struct rte_mbuf *
> > > rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> > > {
> > > 	bool refcnt_not_one;
> > >
> > > 	refcnt_not_one = unlikely(rte_mbuf_refcnt_read(m) != 1);
> > > 	if (refcnt_not_one && __rte_mbuf_refcnt_update(m, -1) != 0)
> > > 		return NULL;
> > >
> > > 	if (unlikely(!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;
> > > 	}
> > >
> > > 	if (refcnt_not_one)
> > > 		rte_mbuf_refcnt_set(m, 1);
> > > 	if (m->nb_segs != 1)
> > > 		m->nb_segs = 1;
> > > 	if (m->next != NULL)
> > > 		m->next = NULL;
> > >
> > > 	return m;
> > > }
> > >
> > > This function can only succeed (i.e. return non-NULL) when 'refcnt'
> > is 1
> > > (or reaches 0).
> > >
> > > REQUIREMENT 2: The driver must hold the only reference to the mbuf,
> > > i.e. 'm->refcnt' must be 1.
> > >
> > >
> > > When the function succeeds, it initializes the mbuf fields as
> > required by
> > > rte_mbuf_raw_free() before returning.
> > >
> > > Now, since the driver has exclusive access to the mbuf, it is free to
> > > initialize the 'm->next' and 'm->nb_segs' at any time.
> > > It could do that when preparing the Tx descriptor.
> > >
> > > This is very interesting, because it means that fast-free does not
> > > prohibit segmented packets!
> > > (But the driver must have sufficient Tx descriptors for all segments
> > in
> > > the mbuf.)
> > >
> > >
> > > Now, lets dig into rte_pktmbuf_prefree_seg()'s block handling non-
> > direct
> > > mbufs, i.e. cloned mbufs and mbufs with external buffer:
> > >
> > > 	if (unlikely(!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;
> > > 	}
> > >
> > > Starting with rte_pktmbuf_detach():
> > >
> > > static inline void rte_pktmbuf_detach(struct rte_mbuf *m)
> > > {
> > > 	struct rte_mempool *mp = m->pool;
> > > 	uint32_t mbuf_size, buf_len;
> > > 	uint16_t priv_size;
> > >
> > > 	if (RTE_MBUF_HAS_EXTBUF(m)) {
> > > 		/*
> > > 		 * The mbuf has the external attached buffer,
> > > 		 * we should check the type of the memory pool where
> > > 		 * the mbuf was allocated from to detect the pinned
> > > 		 * external buffer.
> > > 		 */
> > > 		uint32_t flags = rte_pktmbuf_priv_flags(mp);
> > >
> > > 		if (flags & RTE_PKTMBUF_POOL_F_PINNED_EXT_BUF) {
> > > 			/*
> > > 			 * The pinned external buffer should not be
> > > 			 * detached from its backing mbuf, just exit.
> > > 			 */
> > > 			return;
> > > 		}
> > > 		__rte_pktmbuf_free_extbuf(m);
> > > 	} else {
> > > 		__rte_pktmbuf_free_direct(m);
> > > 	}
> > > 	priv_size = rte_pktmbuf_priv_size(mp);
> > > 	mbuf_size = (uint32_t)(sizeof(struct rte_mbuf) + priv_size);
> > > 	buf_len = rte_pktmbuf_data_room_size(mp);
> > >
> > > 	m->priv_size = priv_size;
> > > 	m->buf_addr = (char *)m + mbuf_size;
> > > 	rte_mbuf_iova_set(m, rte_mempool_virt2iova(m) + mbuf_size);
> > > 	m->buf_len = (uint16_t)buf_len;
> > > 	rte_pktmbuf_reset_headroom(m);
> > > 	m->data_len = 0;
> > > 	m->ol_flags = 0;
> > > }
> > >
> > > The only quick and simple code path through this function is when the
> > mbuf
> > > uses a pinned external buffer:
> > > 	if (RTE_MBUF_HAS_EXTBUF(m)) {
> > > 		uint32_t flags = rte_pktmbuf_priv_flags(mp);
> > > 		if (flags & RTE_PKTMBUF_POOL_F_PINNED_EXT_BUF)
> > > 			return;
> > >
> > > REQUIREMENT 3: The mbuf must not be cloned or use a non-pinned
> > external
> > > buffer.
> > >
> > >
> > > Continuing with the next part of rte_pktmbuf_prefree_seg()'s block:
> > >  		if (RTE_MBUF_HAS_EXTBUF(m) &&
> > > 				RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
> > > 				__rte_pktmbuf_pinned_extbuf_decref(m))
> > > 			return NULL;
> > >
> > > Continuing with the next part of the block in
> > rte_pktmbuf_prefree_seg():
> > >
> > > /**
> > >  * @internal Handle the packet mbufs with attached pinned external
> > buffer
> > >  * on the mbuf freeing:
> > >  *
> > >  *  - return zero if reference counter in shinfo is one. It means
> > there is
> > >  *  no more reference to this pinned buffer and mbuf can be returned
> > to
> > >  *  the pool
> > >  *
> > >  *  - otherwise (if reference counter is not one), decrement
> > reference
> > >  *  counter and return non-zero value to prevent freeing the backing
> > mbuf.
> > >  *
> > >  * Returns non zero if mbuf should not be freed.
> > >  */
> > > static inline int __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf
> > *m)
> > > {
> > > 	struct rte_mbuf_ext_shared_info *shinfo;
> > >
> > > 	/* Clear flags, mbuf is being freed. */
> > > 	m->ol_flags = RTE_MBUF_F_EXTERNAL;
> > > 	shinfo = m->shinfo;
> > >
> > > 	/* Optimize for performance - do not dec/reinit */
> > > 	if (likely(rte_mbuf_ext_refcnt_read(shinfo) == 1))
> > > 		return 0;
> > >
> > > 	/*
> > > 	 * Direct usage of add primitive to avoid
> > > 	 * duplication of comparing with one.
> > > 	 */
> > > 	if (likely(rte_atomic_fetch_add_explicit(&shinfo->refcnt, -1,
> > > 				     rte_memory_order_acq_rel) - 1))
> > > 		return 1;
> > >
> > > 	/* Reinitialize counter before mbuf freeing. */
> > > 	rte_mbuf_ext_refcnt_set(shinfo, 1);
> > > 	return 0;
> > > }
> > >
> > > Essentially, if the mbuf does use a pinned external buffer,
> > > rte_pktmbuf_prefree_seg() only succeeds if that pinned external
> > buffer is
> > > only referred to by the mbuf.
> > >
> > > REQUIREMENT 4: If the mbuf uses a pinned external buffer, the mbuf
> > must
> > > hold the only reference to that pinned external buffer, i.e. in that
> > case,
> > > 'm->shinfo->refcnt' must be 1.
> > >
> > >
> > > Please review.
> > >
> > > If I'm not mistaken, the mbuf library is not a barrier for fast-
> > freeing
> > > segmented packet mbufs, and thus fast-free of jumbo frames is
> > possible.
> > >
> > > We need a driver developer to confirm that my suggested approach -
> > > resetting the mbuf fields, incl. 'm->nb_segs' and 'm->next', when
> > > preparing the Tx descriptor - is viable.
> >
> > Great analysis, makes a lot of sense to me.
> > Shall we add then a special API to make PMD maintainers life a bit
> > easier:
> > Something like rte_mbuf_fast_free_prep(mp, mb), that will optionally
> > check
> > that requirements outlined  above are satisfied for given mbuf and
> > also reset mbuf fields to expected values?
> 
> Good idea, Konstantin.
> 
> Detailed suggestion below.
> Note that __rte_mbuf_raw_sanity_check_mp() is used to checks the requirements
> after 'nb_segs' and 'next' have been initialized.
> 
> /**
>  * Reinitialize an mbuf for freeing back into the mempool.
>  *
>  * The caller must ensure that the mbuf comes from the specified mempool,
>  * is direct and only referred to by the caller (refcnt=1).
>  *
>  * This function is used by drivers in their transmit function for mbuf fast release
>  * when the transmit descriptor is initialized,
>  * so the driver can call rte_mbuf_raw_free()
>  * when the packet segment has been transmitted.
>  *
>  * @see RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE
>  *
>  * @param mp
>  *   The mempool to which the mbuf belong.
>  * @param m
>  *   The mbuf being reinitialized.
>  */
> static __rte_always_inline void
> rte_mbuf_raw_prefree_seg(const struct rte_mempool *mp, struct rte_mbuf *m)
> {
> 	if (m->nb_segs != 1)
> 		m->nb_segs = 1;
> 	if (m->next != NULL)
> 		m->next = NULL;
> 
> 	__rte_mbuf_raw_sanity_check_mp(m, mp);
> 	rte_mbuf_history_mark(mbuf,
> RTE_MBUF_HISTORY_OP_LIB_PREFREE_RAW);
> }

Thanks Morten, though should we really panic if condition is not met?
Might be just do check first and return an error.
 
> 
> /**
>  * Reinitialize a bulk of mbufs for freeing back into the mempool.
>  *
>  * The caller must ensure that the mbufs come from the specified mempool,
>  * are direct and only referred to by the caller (refcnt=1).
>  *
>  * This function is used by drivers in their transmit function for mbuf fast release
>  * when the transmit descriptors are initialized,
>  * so the driver can call rte_mbuf_raw_free_bulk()
>  * when the packet segments have been transmitted.
>  *
>  * @see RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE
>  *
>  * @param mp
>  *   The mempool to which the mbufs belong.
>  * @param mbufs
>  *   Array of pointers to mbufs being reinitialized.
>  *   The array must not contain NULL pointers.
>  * @param count
>  *   Array size.
>  */
> static __rte_always_inline void
> rte_mbuf_raw_prefree_seg_bulk(const struct rte_mempool *mp, struct rte_mbuf
> **mbufs, unsigned int count)
> {
> 	for (unsigned int idx = 0; idx < count; idx++) {
> 		struct rte_mbuf *m = mbufs[idx];
> 
> 		if (m->nb_segs != 1)
> 			m->nb_segs = 1;
> 		if (m->next != NULL)
> 			m->next = NULL;
> 
> 		__rte_mbuf_raw_sanity_check_mp(m, mp);
> 	}
> 	rte_mbuf_history_mark_bulk(mbufs, count,
> RTE_MBUF_HISTORY_OP_LIB_PREFREE_RAW);
> }
> 
> > Konstantin
> >
> >