Re: Possible bug? DOM-U network stopped working after fatal error reported in DOM0

["Roger Pau Monné" <roger.pau@citrix.com>]

On Sun, Dec 26, 2021 at 02:06:55AM +0800, G.R. wrote:
> > > Thanks. I've raised this on freensd-net for advice [0]. IMO netfront
> > > shouldn't receive an mbuf that crosses a page boundary, but if that's
> > > indeed a legit mbuf I will figure out the best way to handle it.
> > >
> > > I have a clumsy patch (below) that might solve this, if you want to
> > > give it a try.
> >
> > Applied the patch and it worked like a charm!
> > Thank you so much for your quick help!
> > Wish you a wonderful holiday!
> 
> I may have said too quickly...
> With the patch I can attach the iscsi disk and neither the dom0 nor
> the NAS domU complains this time.
> But when I attempt to mount the attached disk it reports I/O errors randomly.
> By randomly I mean different disks behave differently...
> I don't see any error logs from kernels this time.
> (most of the iscsi disks are NTFS FS and mounted through the user mode
> fuse library)
> But since I have a local backup copy of the image, I can confirm that
> mounting that backup image does not result in any I/O error.
> Looks like something is still broken here...

Indeed. That patch was likely too simple, and didn't properly handle
the split of mbuf data buffers.

I have another version based on using sglist, which I think it's also
a worthwhile change for netfront. Can you please give it a try? I've
done a very simple test and seems fine, but you certainly have more
interesting cases.

You will have to apply it on top of a clean tree, without any of the
other patches applied.

Thanks, Roger.
---
diff --git a/sys/dev/xen/netfront/netfront.c b/sys/dev/xen/netfront/netfront.c
index 8dba5a8dc6d5..37ea7b1fa059 100644
--- a/sys/dev/xen/netfront/netfront.c
+++ b/sys/dev/xen/netfront/netfront.c
@@ -33,6 +33,8 @@ __FBSDID("$FreeBSD$");
 #include "opt_inet.h"
 #include "opt_inet6.h"
 
+#include <sys/types.h>
+
 #include <sys/param.h>
 #include <sys/sockio.h>
 #include <sys/limits.h>
@@ -40,6 +42,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/malloc.h>
 #include <sys/module.h>
 #include <sys/kernel.h>
+#include <sys/sglist.h>
 #include <sys/socket.h>
 #include <sys/sysctl.h>
 #include <sys/taskqueue.h>
@@ -199,6 +202,12 @@ struct netfront_txq {
 	struct taskqueue 	*tq;
 	struct task       	defrtask;
 
+	struct sglist 		*segments;
+	struct mbuf_refcount {
+		struct m_tag 	tag;
+		u_int 		count;
+	}			refcount_tag[NET_TX_RING_SIZE + 1];
+
 	bool			full;
 };
 
@@ -301,6 +310,38 @@ xn_get_rx_ref(struct netfront_rxq *rxq, RING_IDX ri)
 	return (ref);
 }
 
+#define MTAG_REFCOUNT 0
+
+static void mbuf_grab(uint32_t cookie, struct mbuf *m)
+{
+	struct mbuf_refcount *ref;
+
+	ref = (struct mbuf_refcount *)m_tag_locate(m, cookie, MTAG_REFCOUNT,
+	    NULL);
+	KASSERT(ref != NULL, ("Cannot find refcount"));
+	ref->count++;
+}
+
+static void mbuf_release(uint32_t cookie, struct mbuf *m)
+{
+	struct mbuf_refcount *ref;
+
+	ref = (struct mbuf_refcount *)m_tag_locate(m, cookie, MTAG_REFCOUNT,
+	    NULL);
+	KASSERT(ref != NULL, ("Cannot find refcount"));
+	KASSERT(ref->count > 0, ("Invalid reference count"));
+
+	if(--ref->count == 0)
+		m_freem(m);
+}
+
+static void tag_free(struct m_tag *t)
+{
+	struct mbuf_refcount *ref = (struct mbuf_refcount *)t;
+
+	KASSERT(ref->count == 0, ("Free mbuf tag with pending refcnt"));
+}
+
 #define IPRINTK(fmt, args...) \
     printf("[XEN] " fmt, ##args)
 #ifdef INVARIANTS
@@ -778,7 +819,7 @@ disconnect_txq(struct netfront_txq *txq)
 static void
 destroy_txq(struct netfront_txq *txq)
 {
-
+	sglist_free(txq->segments);
 	free(txq->ring.sring, M_DEVBUF);
 	buf_ring_free(txq->br, M_DEVBUF);
 	taskqueue_drain_all(txq->tq);
@@ -826,6 +867,11 @@ setup_txqs(device_t dev, struct netfront_info *info,
 		for (i = 0; i <= NET_TX_RING_SIZE; i++) {
 			txq->mbufs[i] = (void *) ((u_long) i+1);
 			txq->grant_ref[i] = GRANT_REF_INVALID;
+			m_tag_setup(&txq->refcount_tag[i].tag,
+			    (unsigned long)txq, MTAG_REFCOUNT,
+			    sizeof(txq->refcount_tag[i]) -
+			    sizeof(txq->refcount_tag[i].tag));
+			txq->refcount_tag[i].tag.m_tag_free = &tag_free;
 		}
 		txq->mbufs[NET_TX_RING_SIZE] = (void *)0;
 
@@ -874,10 +920,18 @@ setup_txqs(device_t dev, struct netfront_info *info,
 			device_printf(dev, "xen_intr_alloc_and_bind_local_port failed\n");
 			goto fail_bind_port;
 		}
+
+		txq->segments = sglist_alloc(MAX_TX_REQ_FRAGS, M_WAITOK);
+		if (txq->segments == NULL) {
+			device_printf(dev, "failed to allocate sglist\n");
+			goto fail_sglist;
+		}
 	}
 
 	return (0);
 
+fail_sglist:
+	xen_intr_unbind(&txq->xen_intr_handle);
 fail_bind_port:
 	taskqueue_drain_all(txq->tq);
 fail_start_thread:
@@ -1041,7 +1095,7 @@ xn_release_tx_bufs(struct netfront_txq *txq)
 		if (txq->mbufs_cnt < 0) {
 			panic("%s: tx_chain_cnt must be >= 0", __func__);
 		}
-		m_free(m);
+		mbuf_release((unsigned long)txq, m);
 	}
 }
 
@@ -1311,7 +1365,7 @@ xn_txeof(struct netfront_txq *txq)
 			txq->mbufs[id] = NULL;
 			add_id_to_freelist(txq->mbufs, id);
 			txq->mbufs_cnt--;
-			m_free(m);
+			mbuf_release((unsigned long)txq, m);
 			/* Only mark the txq active if we've freed up at least one slot to try */
 			ifp->if_drv_flags &= ~IFF_DRV_OACTIVE;
 		}
@@ -1507,22 +1561,6 @@ xn_get_responses(struct netfront_rxq *rxq,
 	return (err);
 }
 
-/**
- * \brief Count the number of fragments in an mbuf chain.
- *
- * Surprisingly, there isn't an M* macro for this.
- */
-static inline int
-xn_count_frags(struct mbuf *m)
-{
-	int nfrags;
-
-	for (nfrags = 0; m != NULL; m = m->m_next)
-		nfrags++;
-
-	return (nfrags);
-}
-
 /**
  * Given an mbuf chain, make sure we have enough room and then push
  * it onto the transmit ring.
@@ -1530,16 +1568,22 @@ xn_count_frags(struct mbuf *m)
 static int
 xn_assemble_tx_request(struct netfront_txq *txq, struct mbuf *m_head)
 {
-	struct mbuf *m;
 	struct netfront_info *np = txq->info;
 	struct ifnet *ifp = np->xn_ifp;
-	u_int nfrags;
-	int otherend_id;
+	u_int nfrags, i;
+	int otherend_id, rc;
+
+	sglist_reset(txq->segments);
+	rc = sglist_append_mbuf(txq->segments, m_head);
+	if (rc != 0) {
+		m_freem(m_head);
+		return (rc);
+	}
 
 	/**
 	 * Defragment the mbuf if necessary.
 	 */
-	nfrags = xn_count_frags(m_head);
+	nfrags = txq->segments->sg_nseg;
 
 	/*
 	 * Check to see whether this request is longer than netback
@@ -1551,6 +1595,8 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct mbuf *m_head)
 	 * the Linux network stack.
 	 */
 	if (nfrags > np->maxfrags) {
+		struct mbuf *m;
+
 		m = m_defrag(m_head, M_NOWAIT);
 		if (!m) {
 			/*
@@ -1561,11 +1607,15 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct mbuf *m_head)
 			return (EMSGSIZE);
 		}
 		m_head = m;
+		sglist_reset(txq->segments);
+		rc = sglist_append_mbuf(txq->segments, m_head);
+		if (rc != 0) {
+			m_freem(m_head);
+			return (rc);
+		}
+		nfrags = txq->segments->sg_nseg;
 	}
 
-	/* Determine how many fragments now exist */
-	nfrags = xn_count_frags(m_head);
-
 	/*
 	 * Check to see whether the defragmented packet has too many
 	 * segments for the Linux netback driver.
@@ -1604,14 +1654,15 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct mbuf *m_head)
 	 * the fragment pointers. Stop when we run out
 	 * of fragments or hit the end of the mbuf chain.
 	 */
-	m = m_head;
 	otherend_id = xenbus_get_otherend_id(np->xbdev);
-	for (m = m_head; m; m = m->m_next) {
+	for (i = 0; i < nfrags; i++) {
 		netif_tx_request_t *tx;
 		uintptr_t id;
 		grant_ref_t ref;
 		u_long mfn; /* XXX Wrong type? */
+		struct sglist_seg *seg;
 
+		seg = &txq->segments->sg_segs[i];
 		tx = RING_GET_REQUEST(&txq->ring, txq->ring.req_prod_pvt);
 		id = get_id_from_freelist(txq->mbufs);
 		if (id == 0)
@@ -1621,17 +1672,22 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct mbuf *m_head)
 		if (txq->mbufs_cnt > NET_TX_RING_SIZE)
 			panic("%s: tx_chain_cnt must be <= NET_TX_RING_SIZE\n",
 			    __func__);
-		txq->mbufs[id] = m;
+		if (i == 0)
+			m_tag_prepend(m_head, &txq->refcount_tag[id].tag);
+		mbuf_grab((unsigned long)txq, m_head);
+		txq->mbufs[id] = m_head;
 		tx->id = id;
 		ref = gnttab_claim_grant_reference(&txq->gref_head);
 		KASSERT((short)ref >= 0, ("Negative ref"));
-		mfn = virt_to_mfn(mtod(m, vm_offset_t));
+		mfn = atop(seg->ss_paddr);
 		gnttab_grant_foreign_access_ref(ref, otherend_id,
 		    mfn, GNTMAP_readonly);
 		tx->gref = txq->grant_ref[id] = ref;
-		tx->offset = mtod(m, vm_offset_t) & (PAGE_SIZE - 1);
+		tx->offset = seg->ss_paddr & PAGE_MASK;
+		KASSERT(tx->offset + seg->ss_len <= PAGE_SIZE,
+		    ("mbuf segment crosses a page boundary"));
 		tx->flags = 0;
-		if (m == m_head) {
+		if (i == 0) {
 			/*
 			 * The first fragment has the entire packet
 			 * size, subsequent fragments have just the
@@ -1640,7 +1696,7 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct mbuf *m_head)
 			 * subtracting the sizes of the other
 			 * fragments.
 			 */
-			tx->size = m->m_pkthdr.len;
+			tx->size = m_head->m_pkthdr.len;
 
 			/*
 			 * The first fragment contains the checksum flags
@@ -1654,12 +1710,12 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct mbuf *m_head)
 			 * so we have to test for CSUM_TSO
 			 * explicitly.
 			 */
-			if (m->m_pkthdr.csum_flags
+			if (m_head->m_pkthdr.csum_flags
 			    & (CSUM_DELAY_DATA | CSUM_TSO)) {
 				tx->flags |= (NETTXF_csum_blank
 				    | NETTXF_data_validated);
 			}
-			if (m->m_pkthdr.csum_flags & CSUM_TSO) {
+			if (m_head->m_pkthdr.csum_flags & CSUM_TSO) {
 				struct netif_extra_info *gso =
 					(struct netif_extra_info *)
 					RING_GET_REQUEST(&txq->ring,
@@ -1667,7 +1723,7 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct mbuf *m_head)
 
 				tx->flags |= NETTXF_extra_info;
 
-				gso->u.gso.size = m->m_pkthdr.tso_segsz;
+				gso->u.gso.size = m_head->m_pkthdr.tso_segsz;
 				gso->u.gso.type =
 					XEN_NETIF_GSO_TYPE_TCPV4;
 				gso->u.gso.pad = 0;
@@ -1677,9 +1733,9 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct mbuf *m_head)
 				gso->flags = 0;
 			}
 		} else {
-			tx->size = m->m_len;
+			tx->size = seg->ss_len;
 		}
-		if (m->m_next)
+		if (i != nfrags - 1)
 			tx->flags |= NETTXF_more_data;
 
 		txq->ring.req_prod_pvt++;