[ovs-dev] [PATCH v5 08/14] dp-packet: Handle multi-seg mbufs in resize__().

[Darrell Ball]

Thanks for the patch.

A few queries inline.

On Wed, Jul 11, 2018 at 11:23 AM, Tiago Lam <tiago.lam at intel.com> 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.lam at intel.com>
> Acked-by: Eelco Chaudron <echaudro at redhat.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 d6e19eb..87af459 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();
>

Previously, it was a coding error to call this function for a DPDK mbuf
case, which is pretty
clear. But with this patch, presumably that is not longer the case and the
calling the API is
now ok for DPDK mbufs.



> +    {
> +        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();
>


This will not just drop the packet, it will fail the daemon, because a
packet cannot be resized.
If the system is completely depleted of memory, that may be ok, but in the
case, no such
assumption is implied.

Also, why is XXX still left in the patch?

Also, the preexisting API handles two cases:
1/ Tailroom only adjustment
2/ headroom and/or tailroom adjustment

meaning it handles all cases.

The new DPDK addition (part of the same API) defines 2 cases

1/ tailroom only adjustment
2/ headroom only adjustment

So, it looks like a different API, that also does not handle all cases.



> +            }
> +        } else {
> +            /* Otherwise, this is a headroom adjustment. Try to shift data
> +             * towards the tail to free up head space, if there's enough
> +             * tailroom */
> +
> +            miss_len = new_headroom - dp_packet_headroom(b);
>
> +
> +            if (miss_len <= new_tailroom) {
> +                dp_packet_shift(b, miss_len);
> +            } else {
> +                /* XXX: Handle error case and report error to caller */
> +                OVS_NOT_REACHED();
>


same comments as above.



> +            }
> +        }
> +
> +        new_base = dp_packet_base(b);
> +
> +        break;
> +    }
>      case DPBUF_MALLOC:
>          if (new_headroom == dp_packet_headroom(b)) {
>              new_base = xrealloc(dp_packet_base(b), new_allocated);
> @@ -263,7 +305,9 @@ dp_packet_resize__(struct dp_packet *b, size_t
> new_headroom, size_t new_tailroom
>          OVS_NOT_REACHED();
>      }
>
> -    dp_packet_set_allocated(b, new_allocated);
> +    if (b->source != DPBUF_DPDK) {
> +        dp_packet_set_allocated(b, new_allocated);
> +    }
>      dp_packet_set_base(b, new_base);
>
>      new_data = (char *) new_base + new_headroom;
> --
> 2.7.4
>
> _______________________________________________
> dev mailing list
> dev at openvswitch.org
> https://mail.openvswitch.org/mailman/listinfo/ovs-dev
>