netmap: Fix queue stalls with generic interfaces

In emulated mode, the FreeBSD netmap port attempts to perform zero-copy
transmission.  This works as follows: the kernel ring is populated with
mbuf headers to which netmap buffers are attached.  When transmitting,
the mbuf refcount is initialized to 2, and when the counter value has
been decremented to 1 netmap infers that the driver has freed the mbuf
and thus transmission is complete.

This scheme does not generalize to the situation where netmap is
attaching to a software interface which may transmit packets among
multiple "queues", as is the case with bridge or lagg interfaces.  In
that case, we would be relying on backing hardware drivers to free
transmitted mbufs promptly, but this isn't guaranteed; a driver may
reasonably defer freeing a small number of transmitted buffers
indefinitely.  If such a buffer ends up at the tail of a netmap transmit
ring, further transmits can end up blocked indefinitely.

Fix the problem by removing the zero-copy scheme (which is also not
implemented in the Linux port of netmap).  Instead, the kernel ring is
populated with regular mbuf clusters into which netmap buffers are
copied by nm_os_generic_xmit_frame().  The refcounting scheme is
preserved, and this lets us avoid allocating a fresh cluster per
transmitted packet in the common case.  If the transmit ring is full, a
callout is used to free the "stuck" mbuf, avoiding the queue deadlock
described above.

Furthermore, when recycling mbuf clusters, be sure to fully reinitialize
the mbuf header instead of simply re-setting M_PKTHDR.  Some software
interfaces, like if_vlan, may set fields in the header which should be
reset before the mbuf is reused.

Reviewed by:	vmaffione
MFC after:	1 month
Sponsored by:	Zenarmor
Sponsored by:	OPNsense
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D38065

1 files changed, 37 insertions, 14 deletions

diff --git a/sys/dev/netmap/netmap_kern.h b/sys/dev/netmap/netmap_kern.h
index 7c68c79c61ef..04b9c199e1dc 100644
--- a/sys/dev/netmap/netmap_kern.h
+++ b/sys/dev/netmap/netmap_kern.h
@@ -501,6 +501,9 @@ struct netmap_kring {
 	struct mbuf	**tx_pool;
 	struct mbuf	*tx_event;	/* TX event used as a notification */
 	NM_LOCK_T	tx_event_lock;	/* protects the tx_event mbuf */
+#ifdef __FreeBSD__
+	struct callout	tx_event_callout;
+#endif
 	struct mbq	rx_queue;       /* intercepted rx mbufs. */
 
 	uint32_t	users;		/* existing bindings for this ring */
@@ -2381,39 +2384,59 @@ ptnet_sync_tail(struct nm_csb_ktoa *ktoa, struct netmap_kring *kring)
  *
  * We allocate mbufs with m_gethdr(), since the mbuf header is needed
  * by the driver. We also attach a customly-provided external storage,
- * which in this case is a netmap buffer. When calling m_extadd(), however
- * we pass a NULL address, since the real address (and length) will be
- * filled in by nm_os_generic_xmit_frame() right before calling
- * if_transmit().
+ * which in this case is a netmap buffer.
  *
  * The dtor function does nothing, however we need it since mb_free_ext()
  * has a KASSERT(), checking that the mbuf dtor function is not NULL.
  */
 
-static void void_mbuf_dtor(struct mbuf *m) { }
+static inline void
+nm_generic_mbuf_dtor(struct mbuf *m)
+{
+	uma_zfree(zone_clust, m->m_ext.ext_buf);
+}
 
 #define SET_MBUF_DESTRUCTOR(m, fn)	do {		\
 	(m)->m_ext.ext_free = (fn != NULL) ?		\
-	    (void *)fn : (void *)void_mbuf_dtor;	\
+	    (void *)fn : (void *)nm_generic_mbuf_dtor;	\
 } while (0)
 
 static inline struct mbuf *
-nm_os_get_mbuf(if_t ifp, int len)
+nm_os_get_mbuf(if_t ifp __unused, int len)
 {
 	struct mbuf *m;
+	void *buf;
 
-	(void)ifp;
-	(void)len;
+	KASSERT(len <= MCLBYTES, ("%s: len %d", __func__, len));
 
 	m = m_gethdr(M_NOWAIT, MT_DATA);
-	if (m == NULL) {
-		return m;
+	if (__predict_false(m == NULL))
+		return (NULL);
+	buf = uma_zalloc(zone_clust, M_NOWAIT);
+	if (__predict_false(buf == NULL)) {
+		m_free(m);
+		return (NULL);
 	}
+	m_extadd(m, buf, MCLBYTES, nm_generic_mbuf_dtor, NULL, NULL, 0,
+	    EXT_NET_DRV);
+	return (m);
+}
+
+static inline void
+nm_os_mbuf_reinit(struct mbuf *m)
+{
+	void *buf;
 
-	m_extadd(m, NULL /* buf */, 0 /* size */, void_mbuf_dtor,
-		 NULL, NULL, 0, EXT_NET_DRV);
+	KASSERT((m->m_flags & M_EXT) != 0,
+	    ("%s: mbuf %p has no external storage", __func__, m));
+	KASSERT(m->m_ext.ext_size == MCLBYTES,
+	    ("%s: mbuf %p has wrong external storage size %u", __func__, m,
+	    m->m_ext.ext_size));
 
-	return m;
+	buf = m->m_ext.ext_buf;
+	m_init(m, M_NOWAIT, MT_DATA, M_PKTHDR);
+	m_extadd(m, buf, MCLBYTES, nm_generic_mbuf_dtor, NULL, NULL, 0,
+	    EXT_NET_DRV);
 }
 
 #endif /* __FreeBSD__ */