When suffering from memory fragmentation due to unfreeable pages,
THP page faults will repeatedly try to compact memory.  Due to
the unfreeable pages, compaction fails.

Needless to say, at that point page reclaim also fails to create
free contiguous 2MB areas.  However, that doesn't stop the current
code from trying, over and over again, and freeing a minimum of
4MB (2UL << sc->order pages) at every single invocation.

This resulted in my 12GB system having 2-3GB free memory, a
corresponding amount of used swap and very sluggish response times.

This can be avoided by having the direct reclaim code not reclaim
from zones that already have plenty of free memory available for
compaction.

If compaction still fails due to unmovable memory, doing additional
reclaim will only hurt the system, not help.

Signed-off-by: Rik van Riel <redacted>

---
-v2: shrink_zones now uses the same thresholds as used by compaction itself,
     not only is this conceptually nicer, it also results in kswapd doing
     some actual work; before all the page freeing work was done by THP
     allocators, I seem to see fewer application stalls after this change.

 mm/vmscan.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b7719ec..117eb4d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -2083,6 +2083,16 @@ static void shrink_zones(int priority, struct zonelist *zonelist,
 				continue;
 			if (zone->all_unreclaimable && priority != DEF_PRIORITY)
 				continue;	/* Let kswapd poll it */
+			if (COMPACTION_BUILD) {
+				/*
+				 * If we already have plenty of memory free
+				 * for compaction, don't free any more.
+				 */
+				if (sc->order > PAGE_ALLOC_COSTLY_ORDER &&
+					(compaction_suitable(zone, sc->order) ||
+					 compaction_deferred(zone)))
+					continue;
+			}