[issue1726] tmpfs "malloc limit exceeded" panic

[Venkatesh Srinivas (via DragonFly issue tracker)]

Venkatesh Srinivas <me at acm.jhu.edu> added the comment:

While we will survive fsstress, since the name zone hits its limit before the 
dirent one under fsstress, the basic problem (that limits can be reached and 
tmpfs wasn't counting resources from each zone) remains for dirent structures. A 
well-written test that makes many symlinks while minimizing name zone usage 
would still panic the kernel.

Before I close this bug, it'd be nice if more people could confirm/deny that 
fsstress can run on tmpfs.

Things to do to make the world better: 
1) Move the tmpfs name zone from a global malloc zone to a per-mount zone
2) Convert dirent allocations from M_WAITOK to M_WAITOK | M_NULLOK and handle 
the null return case; there are only two places that dirents are allocated, so 
this wouldn't be too bad.

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