From 613c2d5552d2f5f631a6738e01eb95f660854590 Mon Sep 17 00:00:00 2001 From: Jiahui Zhang Date: Tue, 21 Jul 2026 06:56:05 -0700 Subject: [PATCH] mm/damon/ops-common: prevent migration fallback to non-target nodes 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. Link: https://lore.kernel.org/20260721135607.251869-1-sj@kernel.org Signed-off-by: Jiahui Zhang Reviewed-by: SJ Park Signed-off-by: SJ Park Cc: Honggyu Kim Signed-off-by: Andrew Morton --- 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 d7d7f100389b..e59f77eca83b 100644 --- a/mm/damon/ops-common.c +++ b/mm/damon/ops-common.c @@ -312,7 +312,7 @@ 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, };