[PATCH] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable zones

[Nishanth Aravamudan]

Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc
reserves if node has no ZONE_NORMAL") from Mel.

We have a system with the following topology:

(0) root @ br30p03: /root
# numactl -H
available: 3 nodes (0,2-3)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
23 24 25 26 27 28 29 30 31
node 0 size: 28273 MB
node 0 free: 27323 MB
node 2 cpus:
node 2 size: 16384 MB
node 2 free: 0 MB
node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
node 3 size: 30533 MB
node 3 free: 13273 MB
node distances:
node   0   2   3 
  0:  10  20  20 
  2:  20  10  20 
  3:  20  20  10 

Node 2 has no free memory, because:

# cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages 
1

This leads to the following zoneinfo:

Node 2, zone      DMA
  pages free     0
        min      1840
        low      2300
        high     2760
        scanned  0
        spanned  262144
        present  262144
        managed  262144
...
  all_unreclaimable: 1

If one then attempts to allocate some normal 16M hugepages:

echo 37 > /proc/sys/vm/nr_hugepages

The echo enver returns and kswapd2 consumes CPU cycles.

This is because throttle_direct_reclaim ends up calling
wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...).
pfmemalloc_watermark_ok() in turn checks all zones on the node and see
if the there are any reserves, and if so, then indicates the watermarks
are ok, by seeing if there are sufficient free pages.

675becce15 added a condition already for memoryless nodes. In this case,
though, the node has memory, it is just all consumed (and not
recliamable). Effectively, though, the result is the same on this
call to pfmemalloc_watermark_ok() and thus seems like a reasonable
additional condition.

With this change, the afore-mentioned 16M hugepage allocation succeeds
and correctly round-robins between Nodes 1 and 3.

Signed-off-by: Nishanth Aravamudan <nacc at linux.vnet.ibm.com>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index dcd90c8..033c2b7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
 
        for (i = 0; i <= ZONE_NORMAL; i++) {
                zone = &pgdat->node_zones[i];
-               if (!populated_zone(zone))
+               if (!populated_zone(zone) || !zone_reclaimable(zone))
                        continue;
 
                pfmemalloc_reserve += min_wmark_pages(zone);