[Bug 235747] vmem_bt_alloc() may leak reserved boundary tags

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235747

            Bug ID: 235747
           Summary: vmem_bt_alloc() may leak reserved boundary tags
           Product: Base System
           Version: CURRENT
          Hardware: powerpc
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: markj at FreeBSD.org

vmem_bt_alloc() is somewhat magical in that it must use the vmem allocator to
allocate boundary tags for the vmem allocator.  It uses a reserve of boundary
tags and a global mutex to avoid infinite recursion.  If the reserve somehow
gets depleted, vmem_bt_alloc() effectively gets stuck and the system will grind
to a halt; jhibbits has observed this happening on a powerpc64 platform.

I think this can happen in the following scenario: vmem_bt_alloc() calls
vmem_xalloc() on the per-domain kernel arena and puts some reserved boundary
tags into the arena's pool.  The attempt to allocate a KVA range subsequently
fails because the per-domain arena has no free ranges and we cannot import from
kernel_arena because it is too fragmented to satisfy a KVA_QUANTUM-sized
allocation.  So, vmem_xalloc() fails, leaving the reserved boundary tags up for
grabs.  Later, even if the per-domain kernel arena can satisfy an allocation
for vmem_bt_alloc(), we may not have any boundary tags left to actually perform
the allocation, and we're stuck.

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