Hi
I've run into a problem with using mmap(..., MAP_ANONYMOUS | MAP_POPULATE | MAP_HUGETLB). If there are no huge pages available due to vm.nr_hugepages (or hugetlb.2MB.rsvd.limit_in_bytes cgroup setting) then the mmap call fails and I can gracefully fall back to 4KB pages. However, if neither of the above apply but hugetlb.2MB.limit_in_bytes prevents pages being mapped, then it appears that MAP_POPULATE is silently ignored (according to mincore), and rather than being able to gracefully fall back, attempting to use the memory results in SIGBUS.
Is that expected behaviour? I don't see anything in the mmap(2) man page about it being best-effort (in contrast to MAP_LOCKED, which explicitly says the call won't fail if it can't lock the memory).
This is on Linux 5.8 on Ubuntu 20.04. I can provide sample code if it's of interest, or test on a newer kernel if it'll help.
Thanks
Bruce
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Bruce Merry
Senior Science Processing Developer
SARAO
