upon pmap_enter() to create a mapping within a different address space,
i.e., not the thread's own address space. On i386, this entails the
creation of a temporary mapping to the affected page table page (PTP). In
general, pmap_enter() will read from this PTP, allocate a PV entry, and
write to this PTP. The trouble comes when the system is short of memory.
In order to allocate a new PV entry, an older PV entry has to be
reclaimed. Reclaiming a PV entry involves destroying a mapping, which
requires access to the affected PTP. Thus, the PTP mapped at the
beginning of pmap_enter() is no longer mapped at the end of pmap_enter(),
which leads to pmap_enter() modifying the wrong PTP. To address this
problem, pmap_pv_reclaim() is changed to use an alternate method of
mapping PTPs.
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Reported by: pho
Diagnosed by: kib
MFC after: 5 days
41b1f8207c