[PATCH v2] mm/damon/ops-common: prevent migration fallback to non-target nodes

[Jiahui Zhang]

DAMOS_MIGRATE_{HOT,COLD} passes a target NUMA node to migrate_pages().
But alloc_migration_target() only treats mtc->nid as a preferred node
unless __GFP_THISNODE is set.  Hence target allocation can fall back
to another node, and migrate_pages() can report success without placing
the folio on the requested target node.

Consider a two-node tiered system where node 0 is a fast tier and node 1
is a CPU-less slow tier such as CXL memory, and the user wants to promote
hot regions from node 1 to node 0 with a command like:

  sudo damo start --ops vaddr --target_pid ${workload_pid} \
      --damos_action migrate_hot 0 \
      --damos_access_rate 70% max

Without the __GFP_THISNODE flag, when the memory allocator finds that
node 0 is nearly full, it can fall back to node 1 without waking up
kswapd.  Then the pages allocated for migrate_pages() are still on node 1,
and the regions that are expected to be promoted to node 0 are only moved
to different physical pages on node 1.

Meanwhile, both the mm_migrate_pages tracepoint and DAMOS's own
sz_applied statistics (reported via the damos_stat_after_apply_interval
tracepoint) show the migrations as successful, which makes the failure
practically invisible and hard to investigate.

Running a demotion-purpose DAMOS scheme alongside the promotion scheme
does not fully avoid this either. If demotion cannot keep up with the
promotion rate, allocation can still fall back to node 1 during
promotion, and the same misleading statistics show up.

Make DAMON's migration target allocation strict by setting
__GFP_THISNODE, so that a failed allocation on the target node is
reported as a failure instead of silently landing on a different node.
This is consistent with alloc_misplaced_dst_folio(), alloc_demote_folio(),
and with do_move_pages_to_node(), which all use __GFP_THISNODE for
migrations to an explicit destination node.

Signed-off-by: Jiahui Zhang <jiahuitry@xxxxxxxxxxx>
---
Changes since v1:
- Use the mm/damon/ops-common subject prefix.
- Describe the observed misleading migration accounting and how strict
  target-node allocation resolves it.
- Use the formal author name.
- No functional/code change from v1
- Link to v1: https://lore.kernel.org/damon/BY5PR02MB6642F4D1A8BA92BFF3A034B4B5FF2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

 mm/damon/ops-common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index 6bdd1cfd3863..d21b106b81ca 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -311,7 +311,7 @@ static unsigned int __damon_migrate_folio_list(
 		 * instead of migrated.
 		 */
 		.gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
-			__GFP_NOMEMALLOC | GFP_NOWAIT,
+			__GFP_NOMEMALLOC | GFP_NOWAIT | __GFP_THISNODE,
 		.nid = target_nid,
 	};
 
-- 
2.53.0