Re: [PATCH net-next 2/2] net: optimize skb_postpull_rcsum()

[Vladimir Oltean <olteanv@gmail.com>]

On Thu, Dec 02, 2021 at 06:51:47AM -0800, Eric Dumazet wrote:
> Hi Vladimir
> > I am seeing some errors after this patch, and I am not able to
> > understand why. Specifically, __skb_gro_checksum_complete() hits this
> > condition:
> 
> There were two patches, one for GRO, one for skb_postpull_rcsum()
> 
> I am a bit confused by your report. Which one is causing problems ?

I'm sorry, indeed it seems that I missed to provide that info.
Anyway, it is the skb_postpull_rcsum() call from the DSA switch driver,
that I pointed to, which seems to be problematic.

[  754.211845] mscc_felix 0000:00:00.5 swp0: hw csum failure
[  754.217670] skb len=64 headroom=118 headlen=64 tailroom=1546
[  754.217670] mac=(84,14) net=(98,20) trans=118
[  754.217670] shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
[  754.217670] csum(0x0 ip_summed=2 complete_sw=0 valid=0 level=0)
[  754.217670] hash(0x0 sw=0 l4=0) proto=0x0800 pkttype=0 iif=9
[  754.246905] dev name=swp0 feat=0x0x0002000000195829
[  754.253200] skb headroom: 00000000: ef be ad de ef be ad de ef be ad de ef be ad de
[  754.261751] skb headroom: 00000010: ef be ad de ef be ad de ef be ad de ef be ad de
[  754.269444] skb headroom: 00000020: ef be ad de ef be ad de ef be ad de ef be ad de
[  754.277135] skb headroom: 00000030: ef be ad de ef be ad de ef be ad de ef be ad de
[  754.284826] skb headroom: 00000040: 88 80 00 0a 00 3e 6b e3 36 a1 01 80 00 00 00 0f
[  754.292516] skb headroom: 00000050: 00 10 00 00 d2 ee 27 92 2d 6c 6a b6 a6 22 19 47
[  754.300207] skb headroom: 00000060: 08 00 45 00 00 54 2d 7c 00 00 40 01 03 d9 c0 a8
[  754.307897] skb headroom: 00000070: 64 02 c0 a8 64 01
[  754.312971] skb linear:   00000000: 00 00 60 4d 03 af 00 01 77 eb a8 61 00 00 00 00
[  754.320662] skb linear:   00000010: b6 e2 06 00 00 00 00 00 10 11 12 13 14 15 16 17
[  754.328352] skb linear:   00000020: 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27
[  754.336042] skb linear:   00000030: 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37
[  754.343732] skb tailroom: 00000000: 00 00 00 00 00 00 00 00 00 00 ef be ad de ef be
[  754.351423] skb tailroom: 00000010: ad de ef be ad de ef be ad de ef be ad de ef be
(irrelevant tailroom trimmed)
[  755.088130] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.16.0-rc3-next-20211202-07010-ga9b9500ffaac-dirty #1531
[  755.098169] Hardware name: LS1028A RDB Board (DT)
[  755.102885] Call trace:
[  755.105333]  dump_backtrace+0x0/0x1ac
[  755.109016]  show_stack+0x18/0x70
[  755.112341]  dump_stack_lvl+0x68/0x84
[  755.116022]  dump_stack+0x18/0x34
[  755.119345]  netdev_rx_csum_fault+0x60/0x64
[  755.123549]  __skb_checksum_complete+0x104/0x10c
[  755.128180]  icmp_rcv+0x9c/0x3f0
[  755.131421]  ip_protocol_deliver_rcu+0x40/0x220
[  755.135965]  ip_local_deliver_finish+0x68/0x84
[  755.140421]  ip_local_deliver+0x7c/0x120
[  755.144353]  ip_sublist_rcv_finish+0x48/0x70
[  755.148643]  ip_sublist_rcv+0x168/0x1f0
[  755.152489]  ip_list_rcv+0xf8/0x1a0
[  755.155985]  __netif_receive_skb_list_core+0x184/0x214
[  755.161142]  netif_receive_skb_list_internal+0x180/0x29c
[  755.166471]  napi_complete_done+0x68/0x1bc
[  755.170581]  gro_cell_poll+0x80/0xa0
[  755.174176]  __napi_poll+0x38/0x184
[  755.177674]  net_rx_action+0xe8/0x280
[  755.181347]  __do_softirq+0x124/0x2a0
[  755.185019]  __irq_exit_rcu+0xe4/0x100
[  755.188782]  irq_exit_rcu+0x10/0x1c
[  755.192278]  el1_interrupt+0x38/0x84
[  755.195864]  el1h_64_irq_handler+0x18/0x24
[  755.199972]  el1h_64_irq+0x78/0x7c
[  755.203380]  cpuidle_enter_state+0x12c/0x2f0
[  755.207671]  cpuidle_enter+0x38/0x50
[  755.211256]  do_idle+0x214/0x29c
[  755.214495]  cpu_startup_entry+0x24/0x80
[  755.218428]  rest_init+0xe4/0xf4
[  755.221664]  arch_call_rest_init+0x10/0x1c
[  755.225778]  start_kernel+0x628/0x668
[  755.229450]  __primary_switched+0xc0/0xc8

> > There seems to be a disparity when the skb->csum is calculated by
> > skb_postpull_rcsum as zero. Before, it was calculated as 0xffff.
> 
> skb->csum is 32bit, so there are about 2^16 different values for a
> given Internet checksum

I meant 0xffffffff, sorry. It is visible in the skb_dump output that it
was 0xffffffff before and now it is 0.

> > Do you have some suggestions as to what may be wrong? Thanks.
> 
> What kind of traffic is triggering the fault ? TCP, UDP, something else ?

The simplest to reproduce would be for ICMP. I'm pretty sure I had a
stack trace with TCP as well, but I don't seem to be able to reproduce
that right now.

> Do you have a stack trace to provide, because it is not clear from
> where the issue is detected.