1. Remove a race whereby contigmalloc() would deadlock against the
running processes in the system if they kept reinstantiating
the memory on the active and inactive page queues that it was
trying to flush out. The process doing the contigmalloc() would
sit in "swwrt" forever and the swap pager would be going at full
force, but never get anywhere. Instead of doing it until the
queues are empty, launder for as many iterations as there are
pages in the queue.
2. Do all laundering to swap synchronously; previously, the vnode
laundering was synchronous and the swap laundering not.
3. Increase the number of launder-or-allocate passes to three, from
two, while failing without bothering to do all the laundering on
the third pass if allocation was not possible. This effectively
gives exactly two chances to launder enough contiguous memory,
helpful with high memory churn where a lot of memory from one pass
to the next (and during a single laundering loop) becomes dirtied
again.
I can now reliably hot-plug hardware requiring a 256KB contigmalloc()
without having the kldload/cbb ithread sit around failing to make
progress, while running a busy X session. Previously, it took killing
X to get contigmalloc() to get further (that is, quiescing the system),
and even then contigmalloc() returned failure.
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