Kernel SRU request for changing kernel config submitted:
https://lists.ubuntu.com/archives/kernel-team/2019-October/thread.html#104813
changing status to 'In Progress'
** Description changed:
+ 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.
** Changed in: linux (Ubuntu)
Status: New => In Progress
** Changed in: ubuntu-z-systems
Status: Triaged => In Progress
** Changed in: ubuntu-z-systems
Assignee: (unassigned) => Frank Heimes (frank-heimes)
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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
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