bcachefs CLI: question on use of mlockall()

[Thomas Bertschinger <tahbertschinger@gmail.com>]

The mlockall(MCL_FUTURE) call at the beginning of the bcachefs CLI
tool seems problematic in some cases. When running commands as an
unprivileged user with the default mlock limit (8 MiB), it can cause
ENOMEM failures, for example:

$ ulimit -l
8192
$ fallocate -l $((1024 ** 3)) test-image
$ bcachefs format test-image
...
error opening test-img: ENOMEM_fs_other_alloc


While this can be worked around by running the bcachefs utility as
root, or by raising the default limit, I think commands like this
should work for unprivileged users under a common default configuration.

I wonder if the mlockall() can be targeted to the scenarios where it's
really needed, instead of applying to every bcachefs command? Some
ideas on how to target it: 
- only call it if running as root
- check that getrlimit(RLIMIT_MEMLOCK, ...) is at a "high enough" value
first
- or, only call it for subcommands that actually need it

Any thoughts / comments? Would a better, or complementary approach, be
to reduce the amount of memory mapped for operations like 'format'?

Thanks!

- Thomas Bertschinger