[BUG] Hot-adding memory in VMWare RHEL guest does not work.

Hot-adding memory in VMWare RHEL guest does not work.

Issue:

While trying to hot-add memory in VMware RHEL guest, the following errors are observed:

VMCIUtil: Updating context id from 0xffffffff to 0x537a2346 on event 0.
Hotplug Mem Device
System RAM resource 40000000 - 7fffffff cannot be added
ACPI:memory_hp:add_memory failed
ACPI:memory_hp:Error in acpi_memory_enable_device
acpi_memhotplug: probe of PNP0C80:00 failed with error -22
        

Environment:

• Red Hat Enterprise Linux, version 6.2 and up

Resolution:

Red Hat engineering is working on developing a resolution for this issue. A potential workaround to alleviate/mitigate the observed failures is to attempt to free and compact memory just before the hot-add operation:

echo 3 > /proc/sys/vm/drop_caches && echo 1 > /proc/sys/vm/compact_memory
        

Root Cause:

The prevailing theory is that these failures are due to memory fragmentation on the guest. If, while the system is hot-adding a new memory section, it finds itself unable to allocate order-9 contiguous memory blocks (2MB) due to memory fragmentation, it will fail to complete the memory hot-add procedure.

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