malloc(9): introduce M_NEVERFREED

[bnovkov]

Summary

This patch adds an additional malloc(9) flag to distinguish UMA_ZONE_NOFREE allocations, as previously discussed in https://reviews.freebsd.org/D16620.

[kib]

Why not M_NOFREE?

I believe that the addition require some (perhaps relatively comprehensive) explanation where to use the flag, and where not to. E.g. is it available to the casual malloc(9) user?

[bnovkov]

In D45045#1027214, @kib wrote:

Why not M_NOFREE?

M_NOFREE was already taken by sys/mbuf.h, M_NEVERFREED is one of the alternative names proposed by @alc .

I believe that the addition require some (perhaps relatively comprehensive) explanation where to use the flag, and where not to. E.g. is it available to the casual malloc(9) user?

I see this as more of an internal flag meant to be used by allocators rather than users. At the moment passing it to malloc(9) would effectively be a no-op as its only used by small_alloc.
I refrained from modifying the manpages until we discussed this collectively, but I'll definitely add an explanation once we settle on something.

[kib]

Mentioning the supposed scope of use for flag in the review would be best. BTW if it is internal to vm allocators, why not call it M_VM_NOFREE?

[markj]

I'm fine with the name. It would indeed be good to document the flag further, at least in malloc.9.