When the driver fails to allocate a new Rx buffer, it passes an empty Rx
descriptor (contains zero address and size) to the device and marks it
as invalid by setting the skb pointer in the descriptor's metadata to
NULL.

After processing enough Rx descriptors, the driver will try to process
the invalid descriptor, but will return immediately seeing that the skb
pointer is NULL. Since the driver no longer passes new Rx descriptors to
the device, the Rx queue will eventually become full and the device will
start to drop packets.

Fix this by recycling the received packet if allocation of the new
packet failed. This means that allocation is no longer performed at the
end of the Rx routine, but at the start, before tearing down the DMA
mapping of the received packet.

Remove the comment about the descriptor being zeroed as it is no longer
correct. This is OK because we either use the descriptor as-is (when
recycling) or overwrite its address and size fields with that of the
newly allocated Rx buffer.

The issue was discovered when a process ("perf") consumed too much
memory and put the system under memory pressure. It can be reproduced by
injecting slab allocation failures [1]. After the fix, the Rx queue no
longer comes to a halt.

[1]
 # echo 10 > /sys/kernel/debug/failslab/times
 # echo 1000 > /sys/kernel/debug/failslab/interval
 # echo 100 > /sys/kernel/debug/failslab/probability

 FAULT_INJECTION: forcing a failure.
 name failslab, interval 1000, probability 100, space 0, times 8
 [...]
 Call Trace:
  <IRQ>
  dump_stack_lvl+0x34/0x44
  should_fail.cold+0x32/0x37
  should_failslab+0x5/0x10
  kmem_cache_alloc_node+0x23/0x190
  __alloc_skb+0x1f9/0x280
  __netdev_alloc_skb+0x3a/0x150
  mlxsw_pci_rdq_skb_alloc+0x24/0x90
  mlxsw_pci_cq_tasklet+0x3dc/0x1200
  tasklet_action_common.constprop.0+0x9f/0x100
  __do_softirq+0xb5/0x252
  irq_exit_rcu+0x7a/0xa0
  common_interrupt+0x83/0xa0
  </IRQ>
  asm_common_interrupt+0x1e/0x40
 RIP: 0010:cpuidle_enter_state+0xc8/0x340
 [...]
 mlxsw_spectrum2 0000:06:00.0: Failed to alloc skb for RDQ

Fixes: eda6500a ("mlxsw: Add PCI bus implementation")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20211024064014.1060919-1-idosch@idosch.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>