[PATCH net v3 0/2] tcp: protect locked SO_RCVBUF from Silly Window Syndrome

[Ankit Jain]

This patch series fixes Silly Window Syndrome (SWS) for sockets using 
locked SO_RCVBUF.

When applications like Tomcat lock SO_RCVBUF, receiving small packets 
causes the memory truesize penalty to drop the scaling_ratio to 1. 
This shrinks the internal window clamp and leads to 504 Gateway Timeouts.

Patch 1 bypasses this penalty for locked sockets, except for GRO packets.
Patch 2 adds a packetdrill test to validate this fix.

Link to v1:
https://lore.kernel.org/all/20260427152756.1205-1-ankit-aj.jain@xxxxxxxxxxxx/
Link to v2:
https://lore.kernel.org/all/20260504144945.13477-1-ankit-aj.jain@xxxxxxxxxxxx/

v2 -> v3:
 - Changed GRO detection from checking tp->advmss to skb->len > len 
   based on Eric Dumazet's suggestion. This correctly detects GRO 
   packets even if they contain tiny segments.
 - Updated packetdrill script. Removed the ad-hoc mss 48 configuration. 
   It now uses a standard 1460 MSS and sends varying packet sizes 
   (600, 700, 800 bytes) to naturally trigger the scaling_ratio 
   recalculation.

Testing:
 - Verified fix in a live Java/Tomcat environment (504 timeouts resolved).
 - Passed the newly added packetdrill test demonstrating the clamp bypass.
 - Passed upstream regression tests: tcp_rcv_neg_window.pkt,
   tcp_rcv_wnd_shrink_allowed.pkt, tcp_rcv_wnd_shrink_nomem.pkt,
   tcp_rcv_zero_wnd_fin.pkt, and tcp_rcv_big_endseq.pkt

Ankit Jain (2):
  tcp: protect locked SO_RCVBUF from Silly Window Syndrome
  selftests/net: add packetdrill test for locked SO_RCVBUF SWS

 net/ipv4/tcp_input.c                          |  7 ++++-
 .../net/packetdrill/tcp_locked_rcvbuf_sws.pkt | 29 +++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_locked_rcvbuf_sws.pkt

-- 
2.53.0