[Bug 248008] Post r358097 an i386 system can hang with many processes sleeping on btalloc

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=248008

            Bug ID: 248008
           Summary: Post r358097 an i386 system can hang with many
                    processes sleeping on btalloc
           Product: Base System
           Version: CURRENT
          Hardware: i386
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: rmacklem at FreeBSD.org

I think the patch is not complete.  It looks like the problem is that
for systems that do not have UMA_MD_SMALL_ALLOC, we do
        uma_zone_set_allocf(vmem_bt_zone, vmem_bt_alloc);
but we haven't set an appropriate free function.  This is probably why
UMA_ZONE_NOFREE was originally there.  When NOFREE was removed, it was
appropriate for systems with uma_small_alloc.

So by default we get page_free as our free function.  That calls
kmem_free, which calls vmem_free ... but we do our allocs with
vmem_xalloc.  I'm not positive, but I think the problem is that in
effect we vmem_xalloc -> vmem_free, not vmem_xfree.

Three possible fixes:
 1: The one you tested, but this is not best for systems with
    uma_small_alloc.
 2: Pass UMA_ZONE_NOFREE conditional on UMA_MD_SMALL_ALLOC.
 3: Actually provide an appropriate vmem_bt_free function.

I think we should just do option 2 with a comment, it's simple and it's
what we used to do.  I'm not sure how much benefit we would see from
option 3, but it's more work.

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