Until recently, the CPUs only having 4 1GB TLB entries. I'm sure we
still have customers using that generation of CPUs. 2MB pages perform
better than 1GB pages on the previous generation of hardware, and I
haven't seen numbers for the next generation yet.
I read that somewhere else before, yet we have heavy 1 GiB page users,
especially in the context of VMs and DPDK.
I wonder if those users actually benchmarked. Or whether the memory
savings worked out so well for them that the loss of TLB performance
So, it only works for hugetlbfs in case uffd is not in place (-> no
per-process data in the page table) and we have an actual shared mappings.
When unsharing, we zap the PUD entry, which will result in allocating a
per-process page table on next fault.
I think uffd was a huge mistake. It should have been a filesystem
instead of a hack on the side of anonymous memory.
I will rephrase my previous statement "hugetlbfs just doesn't raise these
problems because we are special casing it all over the place already". For
example, not allowing to swap such pages. Disallowing MADV_DONTNEED. Special
Sure, that's why I want to drag this feature out of "oh this is a
hugetlb special case" and into "this is something Linux supports".