CRIU does work on mainline kernels and has for several years. However, it is still being actively developed (userfaultfd is the example the article uses -- a ma

CRIU does work on mainline kernels and has for several years. However, it is still being actively developed (userfaultfd is the example the article uses -- a mainline feature which is still seeing lots of development). It doesn't usually require additional patches to work, though they do have a fairly big job (every new kernel feature needs to be added to CRIU so it can both checkpoint and restore it -- and not all features are trivial to checkpoint). It's entirely possible that programs using very new kernel features may struggle to get CRIU to work.