Need a test that verifies that mapping faults are 2MiB or 1GiB, and not 4KiB

[jagalactic]

The famfs kernel RFC v1 has fault counters that can be enabled via /sys/fs/famfs/... These were implemented because we had a bug in the kernel module at one point that resulted in 4K mapping faults rather than 2M, and it killed performance for many concurrent computational processes hammering on the same data frame(s) in memory.

Famfs files are constrained to multiples of 2MiB, and mmap addresses are constrained to 2MiB alignment - both for this reason. But we need a good way to catch a regression.

However, the counters proved controversial (see thread [1]) so they will probably be dropped from the next version of the patch set. Dan Williams pointed out a user space test that ndctl uses to verify something similar [2], but I'm hoping we can do better. The first place to look is the rest of the thread at [1] to see of a prescription becomes clear.

[1] https://lore.kernel.org/linux-fsdevel/3jwluwrqj6rwsxdsksfvdeo5uccgmnkh7rgefaeyxf2gu75344@ybhwncywkftx/T/#m69d2b66e54f9657c38e6e0a0da94ab4b3eca7586
[2] https://github.com/pmem/ndctl/blob/main/test/dax.sh#L31

[jagalactic]

Note our smoke tests access the fault counters, so they will need to adapt to the new method (whatever that ends up being)