[RFC PATCH v1.1 1/7] mm/damon/core: initialize damos->last_applied

[SJ Park]

Multiple DAMON regions could exist across a folio.  If they fulfill the
condition to apply a DAMOS scheme, the scheme could be applied multiple
times to the folio.  To avoid this, each DAMOS scheme stores the folio
that the scheme was applied to last time in the damos->last_applied
field and skips repeatedly applying the same scheme to the same folio.

The field is being used without initialization, though.  Hence, the
mechanism could wrongly skip applying a scheme to a folio at the very
first time of DAMOS run.

The user impact is trivial.  DAMON might unexpectedly skip applying
DAMOS action for one folio for the first time per scheme.  In the
DAMON's best-effort world, this is never a real problem.  No critical
consequences such as kernel panic or memory corruption happen.

It is a clear bug, though, and the fix is straightforward.  Fix the
issue by initializing the field in DAMOS scheme creation function,
damon_new_scheme().

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260714055436.120034-1-sj@xxxxxxxxxx

Fixes: 94ba17adaba0 ("mm/damon: avoid applying DAMOS action to same entity multiple times")
Cc: <stable@xxxxxxxxxxxxxxx> # 6.15.x
Signed-off-by: SJ Park <sj@xxxxxxxxxx>
---
 mm/damon/core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index f464b4f0976c3..60255f5cd715e 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -705,6 +705,7 @@ struct damos *damon_new_scheme(struct damos_access_pattern *pattern,
 	INIT_LIST_HEAD(&scheme->ops_filters);
 	scheme->stat = (struct damos_stat){};
 	scheme->max_nr_snapshots = 0;
+	scheme->last_applied = NULL;
 	INIT_LIST_HEAD(&scheme->list);
 
 	scheme->quota = *(damos_quota_init(quota));
-- 
2.47.3