[PATCH] reduce the dirty threshold when there's a lot of mapped

Dirty memory thresholds are currently set by /proc/sys/vm/dirty_ratio.

Background writeout levels are controlled by
/proc/sys/vm/dirty_background_ratio.

Problem is that these levels are hard to get right - they are too
static.  If there is a lot of mapped memory around then the 40%
clamping level causes too much dirty data.  We do lots of scanning in
page reclaim, and the VM generally starts getting into distress.  Extra
swapping, extra page unmapping.

It would be much better to simply tell the caller of write(2) to slow
down - to write out their dirty data sooner, to make those written
pages trivially reclaimable.  Penalise the offender, not the innocent
page allocators.

This patch changes the writer throttling code so that we clamp down
much harder on writers if there is a lot of mapped memory in the
machine.  We only permit memory dirtiers to dirty up to 50% of unmapped
memory before forcing them to clean their own pagecache.

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