Introduce pmap_try_insert_pv_entry(), a function that conditionally
creates a pv entry if the number of entries is below the high water mark
for pv entries.
Introduce pmap_enter_quick_locked() and use it to reimplement
pmap_enter_object(). The old implementation was broken. For example,
it could block while holding a mutex lock.
Change pmap_enter_quick_locked() to fail rather than wait if it is
unable to allocate a page table page. This prevents a race between
pmap_enter_object() and the page daemon. Specifically, an inactive
page that is a successor to the page that was given to
pmap_enter_quick_locked() might become a cache page while
pmap_enter_quick_locked() waits and later pmap_enter_object() maps
the cache page violating the invariant that cache pages are never
mapped. Similarly, change
pmap_enter_quick_locked() to call pmap_try_insert_pv_entry() rather
than pmap_insert_entry(). Generally speaking,
pmap_enter_quick_locked() is used to create speculative mappings. So,
it should not try hard to allocate memory if free memory is scarce.
Tested by: gonzo
1c951556bc