[PATCH 3/8] mm/damon/core: cancel damos_walk() before damon_ctx->kdamond reset

[SeongJae Park <sj@kernel.org>]

damos_walk() request is canceled after damon_ctx->kdamond is reset.
This can make weird situations where damon_is_running() returns false
but the DAMON context has the damos_walk() request linked.  There was a
similar situation for damon_call() requests handling [1], which _was_
able to cause a racy use-after-free bug.  Unlike the case of
damon_call(), because damos_walk() is always synchronously handled and
allows only single request at time, there is no such problematic race
cases.  But, keeping it as is could stem another subtle race condition
bug in future.

Avoid that by cancelling the requests before the ->kdamond reset.  Note
that this change also makes all damon_ctx dependent resource cleanups
consistently done before the damon_ctx->kdamond reset.

[1] https://lore.kernel.org/20251230014532.47563-1-sj@kernel.org

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 0c8ac11a49f9..0bed937b1dce 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2856,14 +2856,13 @@ static int kdamond_fn(void *data)
 
 	kfree(ctx->regions_score_histogram);
 	kdamond_call(ctx, true);
+	damos_walk_cancel(ctx);
 
 	pr_debug("kdamond (%d) finishes\n", current->pid);
 	mutex_lock(&ctx->kdamond_lock);
 	ctx->kdamond = NULL;
 	mutex_unlock(&ctx->kdamond_lock);
 
-	damos_walk_cancel(ctx);
-
 	mutex_lock(&damon_lock);
 	nr_running_ctxs--;
 	if (!nr_running_ctxs && running_exclusive_ctxs)
-- 
2.47.3