This knob is not available when the size of ‘struct page’ (a structure defined in include/linux/mm_types.h) is not power of two (an unusual system config could result in this).
Enable (set to 1) or disable (set to 0) HugeTLB Vmemmap Optimization (HVO).
Once enabled, the vmemmap pages of subsequent allocation of HugeTLB pages from buddy allocator will be optimized (7 pages per 2MB HugeTLB page and 4095 pages per 1GB HugeTLB page), whereas already allocated HugeTLB pages will not be optimized. When those optimized HugeTLB pages are freed from the HugeTLB pool to the buddy allocator, the vmemmap pages representing that range needs to be remapped again and the vmemmap pages discarded earlier need to be rellocated again. If your use case is that HugeTLB pages are allocated ‘on the fly’ (e.g. never explicitly allocating HugeTLB pages with ‘nr_hugepages’ but only set ‘nr_overcommit_hugepages’, those overcommitted HugeTLB pages are allocated ‘on the fly’) instead of being pulled from the HugeTLB pool, you should weigh the benefits of memory savings against the more overhead (~2x slower than before) of allocation or freeing HugeTLB pages between the HugeTLB pool and the buddy allocator. Another behavior to note is that if the system is under heavy memory pressure, it could prevent the user from freeing HugeTLB pages from the HugeTLB pool to the buddy allocator since the allocation of vmemmap pages could be failed, you have to retry later if your system encounter this situation.
Once disabled, the vmemmap pages of subsequent allocation of HugeTLB pages from buddy allocator will not be optimized meaning the extra overhead at allocation time from buddy allocator disappears, whereas already optimized HugeTLB pages will not be affected. If you want to make sure there are no optimized HugeTLB pages, you can set “nr_hugepages” to 0 first and then disable this. Note that writing 0 to nr_hugepages will make any “in use” HugeTLB pages become surplus pages. So, those surplus pages are still optimized until they are no longer in use. You would need to wait for those surplus pages to be released before there are no optimized pages in the system.