On Thu, May 7, 2026 at 8:51 AM Ankit Jain <[email protected]> wrote: > > When an application locks SO_RCVBUF, it disables TCP window auto-tuning. > However, the kernel still applies dynamic truesize penalties to the > scaling_ratio. > > For small packets, this penalty drops the scaling_ratio to 1. This > reduces the advertised window and causes Silly Window Syndrome (SWS) > along with 504 Gateway Timeouts in applications like Tomcat. > > This patch bypasses the truesize penalty if SOCK_RCVBUF_LOCK is set. > To prevent memory exhaustion from large aggregate payloads, the penalty > is still applied for GRO packets (skb->len > len). > > Fixes: a2cbb1603943 ("tcp: Update window clamping condition") > Reported-by: Karen Badiryan <[email protected]> > Signed-off-by: Ankit Jain <[email protected]>
I still do not see why the current behavior has a 'bug'. I think it is quite sane/reasonable. Your selftest does not show anything wrong IMO. If you want to avoid SWS, perhaps the application needs to _not_ use SO_RCVBUF with a value smaller than the device MTU? Applications using SO_RCVBUF with tiny values can not expect kernel behavior to be stable, since rcvbuf management depends on metadata size, which can vary between kernels versions/options. If a kernel change is needed, I would rather enforce a sane sk_rcvbuf floor when the MSS is learnt at accept()/connect() time. ( TCP_SKB_MIN_TRUESIZE / SOCK_MIN_RCVBUF definitions )
