You can refer the below document.
Over time memory gets fragmented so it is possible to have a lot of free memory, but not enough continuous memory for the kernel zone to allocate large pages. In 11.4 we have memory reservation pools, which is a way of reserving memory for each kernel zone. Unfortunately the documentation is not complete for this feature yet, however there is a great blog from Joerg Moellenkamp on how to use this feature
Hope that helps
Thank you both Vishwanath and Chris, that is very helpful...