Dan, thanks for looking at the patch and for your question!
I tried to explain this a bit in comment #1, not too much detailed though: "[...]closing session should be somehow added to gc [...] and then (the gc) performs the clean-up. To include a session in gc, the function session_add_to_gc_queue() should be called. [...] (this) patch proactively puts a session in gc on session_remove_fifo(). This function is always called in the event of Release a session - when the ssh ends, for example. [...]" Being a bit more specific: when SSH ends, for example, a Release event is sent through dbus and systemd-logind captures it, in the function manager_message_handler(). >From there, the function session_remove_fifo() is called. That point is our >"bootstrap" to add the closing session on gc, our first addition there. But unfortunately due to an unpredictable timing of cgroup become empty for that session, the gc might fail to remove the session right in that moment. So, we need to re-add the closing session to gc from...within the gc! In my experiments, 2 or 3 re-additions are enough to get the session removed. It wouldn't be necessary to do these 2 steps above IF the cgroup empty notification was working fine. In that case, after the cgroup for the closing session become empty, a notification handler would be triggered in the systemd-logind and that handler would add the session to gc, and then gc would clean it up. But, since in Trusty we have upstart as our init system (and systemd is highly patched to co-exist in this fashion) and cgroup empty notifications were broken in systemd (until a complete refactor of cgroup handling in systemd), our approach here is the less expensive one. In summary, the above 2 steps [adding the session to gc in session_remove_fifo() and re-adding it from within gc] are the most important parts of this patch - without them we leak entire sessions and eventually the logind process gets OOM'ed. Thanks, Guilherme -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to systemd in Ubuntu. https://bugs.launchpad.net/bugs/1750013 Title: systemd-logind: memory leaks on session's connections (trusty-only) Status in systemd package in Ubuntu: Fix Released Status in systemd source package in Trusty: In Progress Status in systemd source package in Xenial: Fix Released Status in systemd source package in Artful: Fix Released Status in systemd source package in Bionic: Fix Released Bug description: Below the SRU request form. Please refer to the Original Description to a more comprehensive explanation of the problem observed. [Impact] * systemd-logind tool is leaking memory at each session connected. The issues happens in systemd from Trusty (14.04) only. * Three issues observed: - systemd-logind is leaking entire sessions, i.e, the sessions are not feeed after they're closed. In order to fix that, we proactively add the sessions to systemd garbage collector (gc) when they are closed. Also, part of the fix is to make cgmanager package a dependency. Refer to comment #1 to a more thorough explanation of the issue and the fix. - a small memory leak was observed in the session creation logic of systemd-logind. The fix for that is the addition of an appropriate free() call. Refer to comment #2 to more details on the issue and fix. - another small memory leak was observed in the cgmanager glue code of systemd-logind - this code is only present in this specific Ubuntu release of the package, due to necessary compatibility layer with upstart init system. The fix is to properly call free() in 2 functions. Refer to comment #3 to a deep exposition of the issue and the fix. [Test Case] * The basic test-case is to run the following loop from a remote machine: while true; do ssh <hostname-target> "whoami"; done * It's possible to watch the increase in memory consumption from "systemd-logind" process in the target machine. One can use the "ps uax" command to verify the RSS of the process, or count its anonymous pages from /proc/<logind_pid>/smaps. [Regression Potential] * Since the fixes are small and not intrusive, the potential for regressions are low. More regression considerations on comments #1, #2 and #3 for each fix. * A potential small regressson is performance-wise, since now we add sessions to garbage collector proactively. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1750013/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : [email protected] Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
