Re: [ovs-dev] [PATCH v8 09/13] dp-packet: Handle multi-seg mbufs in resize__().

[Eelco Chaudron]

On 22 Jun 2018, at 21:04, Lam, Tiago wrote:

On 18/06/2018 14:06, Eelco Chaudron wrote:


On 11 Jun 2018, at 18:21, Tiago Lam wrote:

When enabled with DPDK OvS relies on mbufs allocated by mempools to
receive and output data on DPDK ports. Until now, each OvS dp_packet
has
had only one mbuf associated, which is allocated with the maximum
possible size, taking the MTU into account. This approach, however,
doesn't allow us to increase the allocated size in an mbuf, if 
needed,
since an mbuf is allocated and initialised upon mempool creation.
Thus,
in the current implementatin this is dealt with by calling
OVS_NOT_REACHED() and terminating OvS.

To avoid this, and allow the (already) allocated space to be better
used, dp_packet_resize__() now tries to use the available room, both
the
tailroom and the headroom, to make enough space for the new data.
Since
this happens for packets of source DPBUF_DPDK, the single-segment 
mbuf
case mentioned above is also covered by this new aproach in
resize__().

Signed-off-by: Tiago Lam <tiago....@intel.com>
---
 lib/dp-packet.c | 48 
++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 46 insertions(+), 2 deletions(-)

diff --git a/lib/dp-packet.c b/lib/dp-packet.c
index 399fadb..d0fab94 100644
--- a/lib/dp-packet.c
+++ b/lib/dp-packet.c
@@ -237,9 +237,51 @@ dp_packet_resize__(struct dp_packet *b, size_t
new_headroom, size_t new_tailroom
     new_allocated = new_headroom + dp_packet_size(b) + 
new_tailroom;

     switch (b->source) {
+    /* When resizing mbufs, both a single mbuf and multi-segment
mbufs (where
+     * data is not contigously held in memory), both the headroom 
and
the
+     * tailroom available will be used to make more space for where
data needs
+     * to be inserted. I.e if there's not enough headroom, data may
be shifted
+     * right if there's enough tailroom.
+     * However, this is not bulletproof and in some cases the space
available
+     * won't be enough - in those cases, an error should be 
returned
and the
+     * packet dropped. */
     case DPBUF_DPDK:
-        OVS_NOT_REACHED();
+    {
+        size_t miss_len;
+
+        if (new_headroom == dp_packet_headroom(b)) {
+            /* This is a tailroom adjustment. Since there's no
tailroom space
+             * left, try and shift data towards the head to free up
tail space,
+             * if there's enough headroom */
+
+            miss_len = new_tailroom - dp_packet_tailroom(b);
+
+            if (miss_len <= new_headroom) {
+                dp_packet_shift(b, -miss_len);
+            } else {
+                /* XXX: Handle error case and report error to 
caller
*/
+                OVS_NOT_REACHED();
Should we add another fragment here, asking as dp_packet_set_size() 
can
free buffers?

I think I've covered this in my reply to the cover letter and to patch
06/13, we can continue the discussion there.

+            }
+        } else {

Can it also be possible that we need to adjust both tail and head 
room?

My understanding is that dp_packet_resize__() should be called
internally only, by either dp_packet_prealloc_tailroom() or
dp_packet_prealloc_headroom(), thus it should only change one of those
at a time. The case that handles `DPBUF_MALLOC` makes the same 
assumption.

I re-read the code and you and DPBUF_MALLOC are handling the case where 
both change.
So ignore my comment :)

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