https://bugs.kde.org/show_bug.cgi?id=462267
Bug ID: 462267
Summary: KWin tabbox should not make the X server leak
"/memfd:xshmfence" filedescriptors until the X server
becomes unusable due to exhausted filedescriptors.
Classification: Plasma
Product: kwin
Version: 5.26.3
Platform: OpenSUSE
OS: Linux
Status: REPORTED
Severity: major
Priority: NOR
Component: tabbox
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
SUMMARY
Each showing of the task switcher window makes the X server leak resources,
overloading it over time.
STEPS TO REPRODUCE
1. Run KWin on X11 on Linux.
2. Observe the number of open filedescriptors of the X server which refer to
"/memfd:xshmfence", e.g. using "ls -la /proc/"$(pidof X)"/fd | grep
"/memfd:xshmfence" | wc -l". Note this number. For example, the number is 157.
3. Hold the left "alt" key. Press and release the "tab" key. Wait until the
task switcher window (it does not matter which style of task switcher window
you have configured, the problem seems to appear for any task switcher window
style). Then release the left "alt" key.
4. Observe the number of open filedescriptors of the X server which refer to
"/memfd:xshmfence" again, e.g. using "ls -la /proc/"$(pidof X)"/fd | grep
"/memfd:xshmfence" | wc -l". Note this number, too.
5. Repeat step 3.
6. Repeat step 4.
OBSERVED RESULT
It can be observed that the number of "xshmfence"-filedescriptors increased
each time the task switcher window was shown.
The observed increase in the number of filedescriptors is sometimes by +1,
sometimes by +2 per task switcher window show.
If sufficiently many shows of task switcher windows happen, then the limited
number of filedescriptors of the X server process (usually 1024) is exhausted.
After that, the KDE desktop becomes completely unusable and a restart of the X
server becomes necessary, loosing all the current open work. Depending on user
behavior, this may take some hours or days to happen. It may never happen for
users who do not use the task switcher. It does not happen if the user is
always fast enough when using the Alt+Tab shortcut such that the task switcher
window is never shown.
EXPECTED RESULT
It is expected that the number of "xshmfence"-filedescriptors does not increase
each time the task switcher window is shown.
If sufficiently many task switcher windows are shown, the limited number of
filedescriptors of the X server process (usually 1024) should not be exhausted.
The KDE desktop should not become completely unusable. A restart of the X
server should not become necessary.
SOFTWARE/OS VERSIONS
Linux Kernel: 6.0.9
KDE Plasma Version: 5.26.3
KDE Frameworks Version: 5.100.1
Qt Version: 5.15.7
ADDITIONAL INFORMATION
The bug https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1018176 is probably
the same.
Bug 443990 may be related.
This bug exists despite the fix to bug 406303.
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