Thanks for the patch created by Shilong, and I have some more ideas to implement the default quota based on Shilong's patch.
1. the default quota setting is more like a normal setting (it's not related to specific user/group/project account, and it is more like a global setting),
then it's better to be implemented by some global setting, for example, by some special account or some specific quota configuration file.
2. To support the default quota based on the existing quota mechanism, we can create a special quota record for the non-enforced quota account,
the special record will have zero hard/soft limit and a flag at the high bits of the grace time (so it can be downgraded to previous Lustre version),
and enforce it with the global default quota setting dynamically.
Thanks!
Landed for 2.12