[RFC PATCH 0/2] mm/damon/{reclaim,lru_sort}: fix commit_inputs infinite hang

[RFC PATCH 0/2] mm/damon/{reclaim,lru_sort}: fix commit_inputs infinite hang

[SJ Park]

Writing 'Y' to commit_inputs parameters of DAMON_RECLAIM and
DAMON_LRU_SORT before the modules were ever turned on causes infinite
hang.  Fix those.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260802173021.762-1-sj@kernel.org

SJ Park (2):
  mm/damon/reclaim: skip damon_call() if ctx has not started
  mm/damon/lru_sort: skip damon_call() if ctx has not started

 mm/damon/lru_sort.c | 11 ++++++-----
 mm/damon/reclaim.c  | 11 ++++++-----
 2 files changed, 12 insertions(+), 10 deletions(-)


base-commit: d6580a342bc8c459628d2b68b313e9c0668bca59
-- 
2.47.3

[RFC PATCH 1/2] mm/damon/reclaim: skip damon_call() if ctx has not started

[SJ Park]

DAMON_RECLAIM 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 commit_inputs on a
system that DAMON_RECLAIM was 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/reclaim.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
index 09e941d75f67d..45d5557cc575a 100644
--- a/mm/damon/reclaim.c
+++ b/mm/damon/reclaim.c
@@ -271,6 +271,8 @@ static int damon_reclaim_commit_inputs_fn(void *arg)
 	return damon_reclaim_apply_parameters();
 }
 
+static bool damon_reclaim_damon_has_started;
+
 static int damon_reclaim_commit_inputs_store(const char *val,
 					     const struct kernel_param *kp)
 {
@@ -291,11 +293,8 @@ static int damon_reclaim_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_reclaim_damon_has_started)
 		return -EINVAL;
 
 	err = damon_call(ctx, &control);
@@ -344,6 +343,8 @@ static int damon_reclaim_turn(bool on)
 	err = damon_start(&ctx, 1, true);
 	if (err)
 		return err;
+	if (!damon_reclaim_damon_has_started)
+		damon_reclaim_damon_has_started = true;
 	return damon_call(ctx, &call_control);
 }
 
-- 
2.47.3

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

[SJ Park]

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