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: [email protected]
Reporter: [email protected]
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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