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20.4. Known Issues
SELinux
MLS policy is not supported with kernel version 4.14
SELinux
Multi-Level Security (MLS) Policy denies unknown classes and permissions, and kernel version 4.14 in the kernel-alt packages recognizes the map permission, which is not defined in any policy. Consequently, every command on a system with active MLS policy and SELinux
in enforcing mode terminates with the Segmentation fault
error. A lot of SELinux
denial warnings occurs on systems with active MLS policy and SELinux
in permissive mode. The combination of SELinux
MLS policy with kernel version 4.14 is not supported.
kdump
saves the vmcore
only if mpt3sas
is blacklisted
When
kdump
kernel loads the mpt3sas
driver, the kdump
kernel crashes and fails to save the vmcore
on certain POWER9 systems. To work around this problem, blacklist mpt3sas
from the kdump
kernel environment by appending the module_blacklist=mpt3sas
string to the KDUMP_COMMANDLINE_APPEND
variable in the /etc/sysconfig/kdump
file:
KDUMP_COMMANDLINE_APPEND="irqpoll maxcpus=1 ... module_blacklist=mpt3sas"
Then restart the
kdump
service to pick up the changes to the configuration file by running the systemctl restart
command as the root
user:
~]# systemctl restart kdump.service
As a result,
kdump
is now able to save the vmcore
on the POWER9 systems. (BZ#1496273)
Recovering from OOM situation fails due to incorrect function of OOM-killer
Recovering from an out-of-memory (OOM) situation does not work correctly on systems with large amounts of memory. Kernel's OOM-killer kills the process using the most memory and frees the memory to be used again. However, sometimes the OOM-killer does not wait long enough before killing a second process. Eventually, the OOM-killer kills all the processes on the system and logs this error:
Kernel panic - not syncing: Out of memory and no killable processes...
If this happens, the operating system must be rebooted. There is no available workaround. (BZ#1405748)
HTM is disabled for guests running on IBM POWER systems
The Hardware Transactional Memory (HTM) feature currently prevents migrating guest virtual machines from IBM POWER8 to IBM POWER9 hosts, and has therefore been disabled by default. As a consequence, guest virtual machines running on IBM POWER8 and IBM POWER9 hosts cannot use HTM, unless the feature is manually enabled.
To do so, change the default
pseries-rhel7.5
machine type of these guests to pseries-rhel7.4
. Note that guests configured this way cannot be migrated from an IBM POWER8 host to an IBM POWER9 host. (BZ#1525599)
Migrating guests with huge pages from IBM POWER8 to IBM POWER9 fails
IBM POWER8 hosts can only use 16MB and 16GB huge pages, but these huge-page sizes are not supported on IBM POWER9. As a consequence, migrating a guest from an IBM POWER8 host to an IBM POWER9 host fails if the guest is configured with static huge pages.
To work around this problem, disable huge pages on the guest and reboot it prior to migration. (BZ#1538959)
modprobe
succeeds to load kernel modules with incorrect parameters
When attempting to load a kernel module with an incorrect parameter using the
modprobe
command, the incorrect parameter is ignored, and the module loads as expected on Red Hat Enterprise Linux 7 for ARM and for IBM Power LE (POWER9).
Note that this is a different behavior compared to Red Hat Enterprise Linux for traditional architectures, such as AMD64 and Intel 64, IBM Z and IBM Power Systems. On these systems,
modprobe
exits with an error, and the module with an incorrect parameter does not load in the described situation.
On all architectures, an error message is recorded in the
dmesg
output. (BZ#1449439)