Merge branch 'tun-tap-vhost-net-apply-qdisc-backpressure-on-full-ptr_ring-to-reduce-tx-drops'

Simon Schippers says:

====================
tun/tap & vhost-net: apply qdisc backpressure on full ptr_ring to reduce TX drops

This patch series deals with tun/tap & vhost-net which drop incoming
SKBs whenever their internal ptr_ring buffer is full. Instead, with this
patch series, the associated netdev queue is stopped, but only when the
new IFF_BACKPRESSURE flag is set and a qdisc is attached. Without the
flag, or if no qdisc is present, the existing behavior is preserved. The
XDP transmit path is not affected. This patch series touches tun/tap and
vhost-net, as they share common logic and must be updated together.
Modifying only one of them would break the other.

By applying proper backpressure, this change allows the connected qdisc to
operate correctly, as reported in [1], and significantly improves
performance in real-world scenarios, as demonstrated in our paper [2]. For
example, we observed a 36% TCP throughput improvement for an OpenVPN
connection between Germany and the USA.

The previous version of this work was applied and then reverted in 7.2,
because the backpressure was unconditional: it caused a significant
throughput drop in an IPv6 multicast testcase with multiple iperf3 TCP
threads sending on Brett Sheffield's librecast testbed [3]. This version
therefore makes the behavior opt-in via IFF_BACKPRESSURE from the very
first patch, so that a tun/tap device which does not set the flag behaves
exactly as before.

The series is ordered so that no patch changes how packets are handled
unless the flag is set:

- Patch 1 adds the flag only. It has no effect yet: TUNSETIFF silently
  masks it off, as it does for any flag outside TUN_FEATURES, until
  patch 5 adds it there.
- Patches 2 and 3 add the consumer side, which wakes a stopped netdev
  queue. __tun_wake_queue() returns early unless IFF_BACKPRESSURE is set,
  and no queue is stopped at this point anyway.
- Patch 4 is a pure ptr_ring refactor required by patch 5.
- Patch 5 adds the queue stopping, gated on IFF_BACKPRESSURE, together
  with the wake needed when the flag is cleared again, and only there is
  the flag added to TUN_FEATURES.

That way no intermediate commit changes the behavior of an existing
tun/tap user beyond the added checks, and bisecting inside the series can
not hit the regression that led to the revert.

[1] https://unix.stackexchange.com/questions/762935/traffic-shaping-ineffective-on-tun-device
[2] https://cni.etit.tu-dortmund.de/storages/cni-etit/r/Research/Publications/2025/Gebauer_2025_VTCFall/Gebauer_VTCFall2025_AuthorsVersion.pdf
[3] https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@karahi.librecast.net/
====================

Link: https://patch.msgid.link/20260803183641.96882-1-simon.schippers@tu-dortmund.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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