Is MAP_POPULATE supposed to fail silently?

[Bruce Merry <bmerry@ska.ac.za>]

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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
-- 
Bruce Merry
Senior Science Processing Developer
SARAO

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