Place VM objects on the object list when created and never remove them.

[jhb]

Summary

This is ok since objects come from a NOFREE zone and allows objects to
be locked while traversing the object list without triggering a LOR.

Ensure that objects on the list are marked DEAD while free or stillborn,
and that they have a refcount of zero.

Test Plan

• booted a kernel with INVARIANTS and WITNESS

[alc]

sys/vm/vm_object.c

191 ↗

(On Diff #5140)

I don't think that it is correct to remove this because of:

obj_zone = uma_zcreate("VM OBJECT", sizeof (struct vm_object), NULL,

#ifdef INVARIANTS

vm_object_zdtor,

#else

NULL,

#endif

vm_object_zinit, NULL, UMA_ALIGN_PTR, UMA_ZONE_NOFREE);

191 ↗

(On Diff #5140)

I would suggest KASSERT()ing this here and moving the actual assignment to the backing object scan below.

1802 ↗

(On Diff #5140)

Add:
backing_object->type = OBJT_DEAD;

[jhb]

Ok. I had actually meant to enable the zone dtor unconditionally (oops), but I'm fine with fixing it as you suggest instead.

[jhb]

Hmm, one other note is that most objects that are destroyed when the ref count drops to zero do not set OBJT_DEAD in their pager's dealloc routine. In fact, only the vnode pager seems to do this, so I think I will need to explicitly set OBJT_DEAD in vm_object_deallocate(). I should perhaps clear the handle to NULL as well?

[alc]

Ya, you're right. Add the OBJT_DEAD assignment to the other pagers so that we're handling this in a uniform way.

[jhb]

• Restore the zdtor to INVARIANTS only and set OBJT_DEAD in callers.
• Clear object handle and set type to DEAD in all pagers.
• Move the KASSERT to check for credentials before clearing object type.