** Changed in: ubuntu-z-systems
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1848492
Title:
Change Config Option CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE for s390x
from yes to no
Status in Ubuntu on IBM z Systems:
Fix Committed
Status in linux package in Ubuntu:
In Progress
Status in linux source package in Bionic:
Fix Committed
Status in linux source package in Disco:
Fix Committed
Status in linux source package in Eoan:
Fix Committed
Bug description:
SRU Justification:
==================
[Impact]
* By default all standby memory that is configured to an LPAR is used
at boot (IPL) time.
* But the standby memory is meant to be added and used on demand later
(after IPL).
[Fix]
* 0001-Ubuntu-Config-Disabled-CONFIG_MEMORY_HOTPLUG_DEFAULT.patch
[Test Case]
* Configure an LPAR in it's Activation Profile with a certain amount
of 'Initial' memory and some 'Reserved' memory on top.
* Re-activate the LPAR to make the changes take effect and boot (IPL)
Ubuntu.
* Check with the help of free whether the reported memory is only the
initial memory (actual) or initial+reserved (desired).
[Regression Potential]
* There is a certain regression potential, since some systems may have
Initial and Reserved memory already defined today and have it already
all in use under Linux (today obviously w/o having it dynamically
added).
* If such a system got re-activated and rebooted, the memory size will
then be what's defined as Initial memory and no longer
Initial+Reserved.
* To be able to use the same size of the memory than before, the
Reserved memory need to be either dynamically be added or the Initial
memory re-configured to the sum of former Initial+Reserved.
* The latter case requires a re-activation of the LPAR, but since this
corner case only happens after a reboot, a downtime is already in
place for the re-activation.
* But overall the regression potential is low since it's for s390x
only and the 'Reserved' memory option in the Activation Profile is
rarely used.
[Other Info]
* In the past this config option was diabled for ppc64el, too.
__________
The Ubuntu kernel configuration contains
CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y for s390.
This causes all standby memory configured for an LPAR to be used at IPL time.
Note that this is definetaly not what a user wants on s390.
The normal process is: define a certain amount of "base" memory which the
system should use when IPL'ed, and define also a certain amount of "standby"
memory which can be (also only partially) be additionaly used when necessary.
Also note that it is *not* possible to add additional memory to a runnung
system, besides the amount of memory specified as "standby".
Therefore please change the kernel config option
CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE to "n" for s390.
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