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8e0e8bef48
A recent cleanup changed the behaviour of tcp_set_window_clamp(). This
looks unintentional, and affects MPTCP selftests, e.g. some tests
re-establishing a connection after a disconnect are now unstable.
Before the cleanup, this operation was done:
new_rcv_ssthresh = min(tp->rcv_wnd, new_window_clamp);
tp->rcv_ssthresh = max(new_rcv_ssthresh, tp->rcv_ssthresh);
The cleanup used the 'clamp' macro which takes 3 arguments -- value,
lowest, and highest -- and returns a value between the lowest and the
highest allowable values. This then assumes ...
lowest (rcv_ssthresh) <= highest (rcv_wnd)
... which doesn't seem to be always the case here according to the MPTCP
selftests, even when running them without MPTCP, but only TCP.
For example, when we have ...
rcv_wnd < rcv_ssthresh < new_rcv_ssthresh
... before the cleanup, the rcv_ssthresh was not changed, while after
the cleanup, it is lowered down to rcv_wnd (highest).
During a simple test with TCP, here are the values I observed:
new_window_clamp (val) rcv_ssthresh (lo) rcv_wnd (hi)
117760 (out) 65495 < 65536
128512 (out) 109595 > 80256 => lo > hi
1184975 (out) 328987 < 329088
113664 (out) 65483 < 65536
117760 (out) 110968 < 110976
129024 (out) 116527 > 109696 => lo > hi
Here, we can see that it is not that rare to have rcv_ssthresh (lo)
higher than rcv_wnd (hi), so having a different behaviour when the
clamp() macro is used, even without MPTCP.
Note: new_window_clamp is always out of range (rcv_ssthresh < rcv_wnd)
here, which seems to be generally the case in my tests with small
connections.
I then suggests reverting this part, not to change the behaviour.
Fixes:
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|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.