RE: Intel PMD fast free layer violation

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Monday, 16 March 2026 09.45
> 
> On Sat, Mar 14, 2026 at 08:41:27AM +0100, Morten Brørup wrote:
> > Bruce,
> >
> > I haven't looked at the Next-net-intel tree, so it might already have
> been fixed...
> >
> > ci_tx_free_bufs_vec() in drivers/net/intel/common/tx.h has:
> >
> > 	/* is fast-free enabled? */
> > 	struct rte_mempool *mp =
> > 			likely(txq->fast_free_mp != (void *)UINTPTR_MAX) ?
> > 			txq->fast_free_mp :
> > 			(txq->fast_free_mp = txep[0].mbuf->pool);
> >
> > 	if (mp != NULL && (n & 31) == 0) {
> > 		void **cache_objs;
> > 		struct rte_mempool_cache *cache =
> rte_mempool_default_cache(mp, rte_lcore_id());
> >
> > 		if (cache == NULL)
> > 			goto normal;
> >
> > 		cache_objs = &cache->objs[cache->len];
> >
> > 		if (n > RTE_MEMPOOL_CACHE_MAX_SIZE) {
> > 			rte_mempool_ops_enqueue_bulk(mp, (void *)txep, n);
> > 			goto done;
> > 		}
> >
> > 		/* The cache follows the following algorithm
> > 		 *   1. Add the objects to the cache
> > 		 *   2. Anything greater than the cache min value (if it
> > 		 *   crosses the cache flush threshold) is flushed to the
> ring.
> > 		 */
> > 		/* Add elements back into the cache */
> > 		uint32_t copied = 0;
> > 		/* n is multiple of 32 */
> > 		while (copied < n) {
> > 			memcpy(&cache_objs[copied], &txep[copied], 32 *
> sizeof(void *));
> > 			copied += 32;
> > 		}
> > 		cache->len += n;
> >
> > 		if (cache->len >= cache->flushthresh) {
> > 			rte_mempool_ops_enqueue_bulk(mp, &cache->objs[cache-
> >size],
> > 					cache->len - cache->size);
> > 			cache->len = cache->size;
> > 		}
> > 		goto done;
> > 	}
> >
> > It should be replaced by:
> >
> > 	/* is fast-free enabled? */
> > 	struct rte_mempool *mp =
> > 			likely(txq->fast_free_mp != (void *)UINTPTR_MAX) ?
> > 			txq->fast_free_mp :
> > 			(txq->fast_free_mp = txep[0].mbuf->pool);
> >
> > 	if (mp != NULL) {
> > 		rte_mbuf_raw_free_bulk(mp, txep, n);
> > 		goto done;
> > 	}
> >
> > This removes a layer violation and adds the missing mbuf sanity
> checks and mbuf history marks due to that layer violation.
> > And it implements fast-free for bulks not a multiple of 32.
> >
> Right, agreed that this would be better. Apart from the missing checks
> and
> history, the current code is functionally correct right now, though?

Although it follows an older caching algorithm, it is functionally correct, and doesn't break any invariants of the current caching algorithm.
In short: No real bugs here.

> Unless
> it's likely to break, I'd rather take this patch in 26.07 rather than
> risk
> changing this code post-rc2 of this release. Any issue with that?

I have been working on an improved mempool caching algorithm with other invariants, which I plan to propose for 26.07.
That will depend on fixing this driver.
Fixing the driver now would avoid that dependency, which is only "nice to have".

I don't have any "must have"-grade issues with postponing this patch to 26.07.
So I'll leave it up to you.

> 
> /Bruce