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/[email protected]/ Link to v2: https://lore.kernel.org/all/[email protected]/ 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
