[PATCH 2/2] mm/page_alloc: only skip kswapd nodes below their kswapd watermark

[Joshua Hahn <joshua.hahnjy@gmail.com>]

Commit 19c5fb83f2a4 ("mm: page_alloc: avoid kswapd thrashing due to
NUMA restrictions") makes get_page_from_freelist() skip any node whose
kswapd is active to avoid piling on an already-pressured node that is
constantly churning to make free space that is immediately consumed.

But kswapd can also be woken up for order 9 requests with THP defrag
set to defer. This means that a fragmented node with plenty of free
memory will have a kswapd thread continuously spinning and preventing
order 0 pages from allocating on the node. This causes the opposite
problem of piling on pressure on all other nodes.

This behavior was observed on some Meta production workloads running on
2-NUMA systems, where processes with a preference for node0 allocations
continuously allocated order 0 pages on node1, despite node0 having
hundreds of GB of free memory.

To prevent piling pressure on any one node, only skip a node during
page allocation if the zone also fails the watermark kswapd is
reclaiming it up to. This is the high watermark, or the promo watermark
if NUMA balancing memory tiering is enabled. Make sure to mask the
reserve flags so that __GFP_HIGH / atomic requests in the slowpath
do not get a relaxed version of the check.

This preserves the existing fix to prevent piling memory on the local
node, while also preventing memory pileups on non-local nodes when
the local node has memory.

On a 2-NUMA VM with a 60G node0 and a 20G node1, with THP enabled=always
and defrag=defer, I ran a process that prefers allocating on node0.
With a fragmented node0:
- Before the patch, 66% of faults were served from node0
- After the patch, 100% of faults were served from node0

Suggested-by: Daniel Hodges <hodgesd@meta.com>
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
---
 mm/page_alloc.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5423ff4805f84..8e8a2c8d6e6a4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3882,14 +3882,18 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
 		}
 
 		/*
-		 * If kswapd is already active on a node, keep looking
-		 * for other nodes that might be idle. This can happen
-		 * if another process has NUMA bindings and is causing
-		 * kswapd wakeups on only some nodes. Avoid accidental
-		 * "node_reclaim_mode"-like behavior in this case.
+		 * If kswapd is already active on a node and the zone is below
+		 * its kswapd watermark for this request, keep looking for other
+		 * nodes that might be idle. This can happen if another process
+		 * has NUMA bindings and is causing kswapd wakeups on only some
+		 * nodes. Avoid accidental "node_reclaim_mode" behavior.
 		 */
 		if (skip_kswapd_nodes &&
-		    !waitqueue_active(&zone->zone_pgdat->kswapd_wait)) {
+		    !waitqueue_active(&zone->zone_pgdat->kswapd_wait) &&
+		    !zone_watermark_fast(zone, order, kswapd_wmark_pages(zone),
+					 ac->highest_zoneidx,
+					 alloc_flags & ~ALLOC_RESERVES,
+					 gfp_mask)) {
 			skipped_kswapd_nodes = true;
 			continue;
 		}
-- 
2.53.0-Meta