[ovs-dev] [PATCH v8 05/13] dp-packet: Fix data_len handling multi-seg mbufs.

[Tiago Lam]

When a dp_packet is from a DPDK source, and it contains multi-segment
mbufs, the data_len is not equal to the packet size, pkt_len. Instead,
the data_len of each mbuf in the chain should be considered while
distributing the new (provided) size.

To account for the above dp_packet_set_size() has been changed so that,
in the multi-segment mbufs case, only the data_len on the last mbuf of
the chain and the total size of the packet, pkt_len, are changed. The
data_len on the intermediate mbufs preceeding the last mbuf is not
changed by dp_packet_set_size(). Furthermore, in some cases
dp_packet_set_size() may be used to set a smaller size than the current
packet size, thus effectively trimming the end of the packet. In the
multi-segment mbufs case this may lead to lingering mbufs that may need
freeing.

__dp_packet_set_data() now also updates an mbufs' data_len after setting
the data offset. This is so that both fields are always in sync for each
mbuf in a chain.

Co-authored-by: Michael Qiu <qiudayu at chinac.com>
Co-authored-by: Mark Kavanagh <mark.b.kavanagh at intel.com>
Co-authored-by: Przemyslaw Lal <przemyslawx.lal at intel.com>
Co-authored-by: Marcin Ksiadz <mksiadz at gmail.com>
Co-authored-by: Yuanhan Liu <yliu at fridaylinux.org>

Signed-off-by: Michael Qiu <qiudayu at chinac.com>
Signed-off-by: Mark Kavanagh <mark.b.kavanagh at intel.com>
Signed-off-by: Przemyslaw Lal <przemyslawx.lal at intel.com>
Signed-off-by: Marcin Ksiadz <mksiadz at gmail.com>
Signed-off-by: Yuanhan Liu <yliu at fridaylinux.org>
Signed-off-by: Tiago Lam <tiago.lam at intel.com>
---
 lib/dp-packet.h | 56 +++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 45 insertions(+), 11 deletions(-)

diff --git a/lib/dp-packet.h b/lib/dp-packet.h
index 4c104b6..c301ed5 100644
--- a/lib/dp-packet.h
+++ b/lib/dp-packet.h
@@ -429,17 +429,39 @@ dp_packet_size(const struct dp_packet *b)
 static inline void
 dp_packet_set_size(struct dp_packet *b, uint32_t v)
 {
-    /* netdev-dpdk does not currently support segmentation; consequently, for
-     * all intents and purposes, 'data_len' (16 bit) and 'pkt_len' (32 bit) may
-     * be used interchangably.
-     *
-     * On the datapath, it is expected that the size of packets
-     * (and thus 'v') will always be <= UINT16_MAX; this means that there is no
-     * loss of accuracy in assigning 'v' to 'data_len'.
-     */
-    b->mbuf.data_len = (uint16_t)v;  /* Current seg length. */
-    b->mbuf.pkt_len = v;             /* Total length of all segments linked to
-                                      * this segment. */
+    if (b->source == DPBUF_DPDK) {
+        struct rte_mbuf *seg = &b->mbuf;
+        struct rte_mbuf *fmbuf = seg;
+        uint16_t pkt_len = v;
+        uint16_t seg_len;
+        uint16_t nb_segs = 0;
+
+        /* Trim 'v' length bytes from the end of the chained buffers, freeing
+           any buffers that may be left floating */
+        while (seg) {
+            seg_len = MIN(pkt_len, seg->data_len);
+            seg->data_len = seg_len;
+
+            pkt_len -= seg_len;
+            if (pkt_len <= 0) {
+                /* Free the rest of chained mbufs */
+                free_dpdk_buf((struct dp_packet *) seg->next);
+                seg->next = NULL;
+            } else if (!seg->next) {
+                seg->data_len = pkt_len;
+            }
+
+            nb_segs += 1;
+            seg = seg->next;
+        }
+
+        fmbuf->nb_segs = nb_segs;
+    } else {
+        b->mbuf.data_len = v;
+    }
+
+    /* Total length of all segments linked to this segment. */
+    b->mbuf.pkt_len = v;
 }
 
 static inline uint16_t
@@ -451,7 +473,19 @@ __packet_data(const struct dp_packet *b)
 static inline void
 __packet_set_data(struct dp_packet *b, uint16_t v)
 {
+    uint16_t prev_ofs = b->mbuf.data_off;
     b->mbuf.data_off = v;
+    int16_t ofs_diff = prev_ofs - b->mbuf.data_off;
+
+    /* When dealing with DPDK mbufs, keep data_off and data_len in sync. Thus,
+     * update data_len if the length changes with the move of data_off.
+     * However, if data_len is 0, there's no data to move and data_Len should
+     * remain 0. */
+
+    if (b->mbuf.data_len != 0) {
+        b->mbuf.data_len = MIN(b->mbuf.data_len + ofs_diff,
+                               b->mbuf.buf_len - b->mbuf.data_off);
+    }
 }
 
 static inline uint16_t
-- 
2.7.4