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 <jiahuitry@outlook.com>
Reviewed-by: SJ Park <sj@kernel.org>
Signed-off-by: SJ Park <sj@kernel.org>
Cc: Honggyu Kim <honggyu.kim@sk.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

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,
 	};