[PATCH net-next v4 0/4] tun/tap: use kfree_skb_reason() to trace dropped skb

[Dongli Zhang <dongli.zhang@oracle.com>]

The commit c504e5c2f964 ("net: skb: introduce kfree_skb_reason()") has
introduced the kfree_skb_reason() to help track the reason.

The tun and tap are commonly used as virtio-net/vhost-net backend. This is to
use kfree_skb_reason() to trace the dropped skb for those two drivers. 

Changed since v1:
- I have renamed many of the reasons since v1. I make them as generic as
  possible so that they can be re-used by core networking and drivers.

Changed since v2:
- declare drop_reason as type "enum skb_drop_reason"
- handle the drop in skb_list_walk_safe() case for tap driver, and
  kfree_skb_list_reason() is introduced

Changed since v3 (only for PATCH 4/4):
- rename to TAP_FILTER and TAP_TXFILTER
- honor reverse xmas tree style declaration for 'drop_reason' in
  tun_net_xmit()


The following reasons are introduced.

- SKB_DROP_REASON_SKB_CSUM

This is used whenever there is checksum error with sk_buff.

- SKB_DROP_REASON_SKB_COPY_DATA

The kernel may (zero) copy the data to or from sk_buff, e.g.,
zerocopy_sg_from_iter(), skb_copy_datagram_from_iter() and
skb_orphan_frags_rx(). This reason is for the copy related error.

- SKB_DROP_REASON_SKB_GSO_SEG

Any error reported when GSO processing the sk_buff. It is frequent to process
sk_buff gso data and we introduce a new reason to handle that.
	
- SKB_DROP_REASON_SKB_PULL
- SKB_DROP_REASON_SKB_TRIM

It is frequent to pull to sk_buff data or trim the sk_buff data.

- SKB_DROP_REASON_DEV_HDR

Any driver may report error if there is any error in the metadata on the DMA
ring buffer.

- SKB_DROP_REASON_DEV_READY

The device is not ready/online or initialized to receive data.

- SKB_DROP_REASON_FULL_RING

Suggested by Eric Dumazet.

- SKB_DROP_REASON_TAP_FILTER

Suggested by Menglong Dong. For sk_buff dropped by (ebpf) filter directly
attached to tun/tap, e.g., via TUNSETFILTEREBPF.

- SKB_DROP_REASON_TAP_TXFILTER

Suggested by Menglong Dong. For sk_buff dropped by tx filter implemented at
tun/tap, e.g., check_filter()


This is the output for TUN device.

# cat /sys/kernel/debug/tracing/trace_pipe
          <idle>-0       [014] b.s3.   893.074829: kfree_skb: skbaddr=0000000013eea285 protocol=4 location=00000000036fe12c reason: FULL_RING
      vhost-4456-4469    [024] b..1.   893.230235: kfree_skb: skbaddr=0000000011eab049 protocol=2054 location=00000000036fe12c reason: FULL_RING
          arping-6811    [002] b..1.   893.235606: kfree_skb: skbaddr=000000000121f124 protocol=2054 location=00000000036fe12c reason: FULL_RING
          arping-6811    [002] b..1.   894.235682: kfree_skb: skbaddr=000000000121f124 protocol=2054 location=00000000036fe12c reason: FULL_RING
      vhost-4456-4469    [024] b..1.   894.291240: kfree_skb: skbaddr=00000000d093f0cd protocol=2054 location=00000000036fe12c reason: FULL_RING


This is the output for TAP device.

# cat /sys/kernel/debug/tracing/trace_pipe
          <idle>-0       [004] ..s1.  1584.564287: kfree_skb: skbaddr=00000000e0987862 protocol=0 location=00000000177d2c83 reason: NOT_SPECIFIED
          arping-9338    [018] ..s1.  1584.642082: kfree_skb: skbaddr=00000000856cd27d protocol=2054 location=00000000615812ac reason: FULL_RING
          arping-9338    [018] ..s1.  1585.642190: kfree_skb: skbaddr=00000000856cd27d protocol=2054 location=00000000615812ac reason: FULL_RING
          arping-9338    [018] ..s1.  1586.642271: kfree_skb: skbaddr=00000000856cd27d protocol=2054 location=00000000615812ac reason: FULL_RING
          arping-9338    [018] ..s1.  1587.642368: kfree_skb: skbaddr=00000000856cd27d protocol=2054 location=00000000615812ac reason: FULL_RING


 drivers/net/tap.c          | 35 +++++++++++++++++++++++++----------
 drivers/net/tun.c          | 38 ++++++++++++++++++++++++++++++--------
 include/linux/skbuff.h     | 21 +++++++++++++++++++++
 include/trace/events/skb.h | 10 ++++++++++
 net/core/skbuff.c          | 11 +++++++++--
 5 files changed, 95 insertions(+), 20 deletions(-)


Thank you very much!

Dongli Zhang