[PATCH v6] mm, vmscan: retry kswapd's priority loop with cache_trim_mode off on failure

[Byungchul Park]

Changes from v5:
	1. Make it retry the kswapd's scan priority loop with
	   cache_trim_mode off *only if* the mode didn't work in the
	   previous loop. (feedbacked by Huang Ying)
	2. Take into account 'break's from the priority loop when making
	   the decision whether to retry. (feedbacked by Huang Ying)
	3. Update the test result in the commit message.

Changes from v4:
	1. Make other scans start with may_cache_trim_mode = 1.

Changes from v3:
	1. Update the test result in the commit message with v4.
	2. Retry the whole priority loop with cache_trim_mode off again,
	   rather than forcing the mode off at the highest priority,
	   when the mode doesn't work. (feedbacked by Johannes Weiner)

Changes from v2:
	1. Change the condition to stop cache_trim_mode.

	   From - Stop it if it's at high scan priorities, 0 or 1.
	   To   - Stop it if it's at high scan priorities, 0 or 1, and
	          the mode didn't work in the previous turn.

	   (feedbacked by Huang Ying)

	2. Change the test result in the commit message after testing
	   with the new logic.

Changes from v1:
	1. Add a comment describing why this change is necessary in code
	   and rewrite the commit message with how to reproduce and what
	   the result is using vmstat. (feedbacked by Andrew Morton and
	   Yu Zhao)
	2. Change the condition to avoid cache_trim_mode from
	   'sc->priority != 1' to 'sc->priority > 1' to reflect cases
	   where the priority goes to zero all the way. (feedbacked by
	   Yu Zhao)

--->8---