mm: remove use-once cache bias from LRU balancing

When the splitlru patches divided page cache and swap-backed pages into
separate LRU lists, the pressure balance between the lists was biased to
account for the fact that streaming IO can cause memory pressure with a
flood of pages that are used only once.  New page cache additions would
tip the balance toward the file LRU, and repeat access would neutralize
that bias again.  This ensured that page reclaim would always go for
used-once cache first.

Since e9868505987a ("mm,vmscan: only evict file pages when we have
plenty"), page reclaim generally skips over swap-backed memory entirely as
long as there is used-once cache present, and will apply the LRU balancing
when only repeatedly accessed cache pages are left - at which point the
previous use-once bias will have been neutralized.  This makes the
use-once cache balancing bias unnecessary.

Signed-off-by: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Rik van Riel <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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