[Devel] [PATCH rh7] vmscan: don't report reclaim progress if there was no progress.

[Andrey Ryabinin]

__alloc_pages_slowpath relies on the direct reclaim and did_some_progress
as an indicator that it makes sense to retry allocation rather than
declaring OOM. shrink_zones checks if all zones reclaimable and if
shrink_zone didn't make any progress it prevents from a premature OOM
killer invocation by reporting the progress.
This might happen if the LRU is full of dirty or writeback pages
and direct reclaim cannot clean those up.

zone_reclaimable allows to rescan the reclaimable lists several times
and restart if a page is freed.  This is really subtle behavior and it
might lead to a livelock when a single freed page keeps allocator
looping but the current task will not be able to allocate that single
page.  OOM killer would be more appropriate than looping without any
progress for unbounded amount of time.

Report no progress even if zones are reclaimable as OOM is more appropiate
in that case.

https://jira.sw.ru/browse/PSBM-104900
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 mm/vmscan.c | 24 ------------------------
 1 file changed, 24 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 13ae9bd1e92e..85622f235e78 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2952,26 +2952,6 @@ static void snapshot_refaults(struct mem_cgroup *root_memcg, struct zone *zone)
        } while ((memcg = mem_cgroup_iter(root_memcg, memcg, NULL)));
 }
 
-/* All zones in zonelist are unreclaimable? */
-static bool all_unreclaimable(struct zonelist *zonelist,
-		struct scan_control *sc)
-{
-	struct zoneref *z;
-	struct zone *zone;
-
-	for_each_zone_zonelist_nodemask(zone, z, zonelist,
-			gfp_zone(sc->gfp_mask), sc->nodemask) {
-		if (!populated_zone(zone))
-			continue;
-		if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
-			continue;
-		if (zone_reclaimable(zone))
-			return false;
-	}
-
-	return true;
-}
-
 static void shrink_tcrutches(struct scan_control *scan_ctrl)
 {
 	int nid;
@@ -3097,10 +3077,6 @@ out:
 		goto retry;
 	}
 
-	/* top priority shrink_zones still had more to do? don't OOM, then */
-	if (global_reclaim(sc) && !all_unreclaimable(zonelist, sc))
-		return 1;
-
 	return 0;
 }
 
-- 
2.26.2