[RFC PATCH 2/2] mm/damon/lru_sort: skip damon_call() if ctx has not started

[SJ Park <sj@kernel.org>]

DAMON_LRU_SORT calls damon_call() for commit_inputs parameter user input
if the DAMON context is initialized.  The context could be initialized,
but not yet successfully started.  In the case, damon_call() could
indefinitely hang.  Read the comment on damon_call() for more detail.
Fix the problem by memorizing if the DAMON context has ever successfully
started, and skip damon_call() if it has not.

This issue can easily be reproduced by writing Y to the commit_inputs
parameter file on a system that DAMON_LRU_SORT has not turned on before.

Fixes: de3c60e1c831 ("mm/damon: add synchronous commit for commit_inputs")
Cc: <stable@vger.kernel.org> # 7.2.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/lru_sort.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
index 3920d5042f109..bd847829a9907 100644
--- a/mm/damon/lru_sort.c
+++ b/mm/damon/lru_sort.c
@@ -346,6 +346,8 @@ static int damon_lru_sort_commit_inputs_fn(void *arg)
 	return damon_lru_sort_apply_parameters();
 }
 
+static bool damon_lru_sort_damon_has_started;
+
 static int damon_lru_sort_commit_inputs_store(const char *val,
 					      const struct kernel_param *kp)
 {
@@ -366,11 +368,8 @@ static int damon_lru_sort_commit_inputs_store(const char *val,
 	if (!commit_inputs_request)
 		return 0;
 
-	/*
-	 * Skip damon_call() if ctx is not initialized to avoid
-	 * NULL pointer dereference.
-	 */
-	if (!ctx)
+	/* Skip damon_call() if ctx has not successfully started. */
+	if (!damon_lru_sort_damon_has_started)
 		return -EINVAL;
 
 	err = damon_call(ctx, &control);
@@ -423,6 +422,8 @@ static int damon_lru_sort_turn(bool on)
 	err = damon_start(&ctx, 1, true);
 	if (err)
 		return err;
+	if (!damon_lru_sort_damon_has_started)
+		damon_lru_sort_damon_has_started = true;
 	return damon_call(ctx, &call_control);
 }
 
-- 
2.47.3