[RFC PATCH 0/4] mm/damon: allow NULL or unstarted damon_ctx parameter for damon_call()

[RFC PATCH 0/4] mm/damon: allow NULL or unstarted damon_ctx parameter for damon_call()

[SJ Park]

Callers of damon_call() should validate the damon_ctx object parameter.
If it is NULL or never damon_start()-ed object, damon_call() could
dereference the NULL pointer or indefinitely hang.  Ensuring all callers
doing the validation correctly has turned out to be difficult.  Handle
the corner cases inside the core layer and remove callers' validations.

Patches 1 and 2 respectively allow passing NULL and not yet
damon_start()-ed ctx parameter to damon_call().  Patches 3 and 4 remove
the callers side validations in DMON_RECLAIM and DASMON_LRU_SORT,
respectively.

SJ Park (4):
  mm/damon/core: handle NULL ctx parameter in damon_call()
  mm/damon/core: set ctx->call_controls_obsolete in damon_new_ctx()
  mm/damon/reclaim: remove unnecessary damon_call() param validation
  mm/damon/lru_sort: remove unnecessary damon_call() param validation

 mm/damon/core.c     | 7 +++----
 mm/damon/lru_sort.c | 8 --------
 mm/damon/reclaim.c  | 8 --------
 3 files changed, 3 insertions(+), 20 deletions(-)


base-commit: c8520e181a2aaf85290d0447656de0df112dbbea
-- 
2.47.3

[RFC PATCH 1/4] mm/damon/core: handle NULL ctx parameter in damon_call()

[SJ Park]

When NULL damon_ctx pointer parameter is passed, damon_call() could do
NULL dereference.  The caller is responsible to avoid that.  It is easy
to forget, and there are many damon_call() callers.  Meanwhile,
damon_call() is never meant to be performance critical.  It uses mutex
and completion.  Add the NULL pointer check inside damon_call() so that
callers can pass the parameter without NULL checks.

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

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 92631a36d7b51..5882f9c94c47f 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2188,6 +2188,8 @@ int damon_kdamond_pid(struct damon_ctx *ctx)
  */
 int damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
 {
+	if (!ctx)
+		return -EINVAL;
 	if (!control->repeat)
 		init_completion(&control->completion);
 	control->canceled = false;
-- 
2.47.3

Re: [RFC PATCH 1/4] mm/damon/core: handle NULL ctx parameter in damon_call()

[Gutierrez Asier]

Hi SJ,

On 8/13/2026 6:49 AM, SJ Park wrote:
> When NULL damon_ctx pointer parameter is passed, damon_call() could do
> NULL dereference.  The caller is responsible to avoid that.  It is easy
> to forget, and there are many damon_call() callers.  Meanwhile,
> damon_call() is never meant to be performance critical.  It uses mutex
> and completion.  Add the NULL pointer check inside damon_call() so that
> callers can pass the parameter without NULL checks.
> 
> Signed-off-by: SJ Park <sj@kernel.org>
> ---
>  mm/damon/core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 92631a36d7b51..5882f9c94c47f 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2188,6 +2188,8 @@ int damon_kdamond_pid(struct damon_ctx *ctx)
>   */
>  int damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
>  {
> +	if (!ctx)
> +		return -EINVAL;
>  	if (!control->repeat)
>  		init_completion(&control->completion);
>  	control->canceled = false;

I know that this is a minor optimization, but how about moving INIT_LIST_HEAD(&control->list)
to right before list_add_tail?

It doesn't make sense to initialize the list before checking call_controls_obsolete.

Maybe this suggestion should go in a different patch.

-- 
Asier Gutierrez
Huawei

[RFC PATCH 2/4] mm/damon/core: set ctx->call_controls_obsolete in damon_new_ctx()

[SJ Park]

damon_ctx->call_controls_obsolete is used to disallow damon_call()
requests when the request cannot be served.  The field is unset and set
when the context execution is started and terminated, respectively.  The
intention is to allow damon_call() requests only while the context is
actively being executed.

damon_ctx constructor, damon_new_ctx() unsets the field, though.  As a
result, passing the damon_ctx parameter that never successfully
damon_start()-ed to damon_call() can indefinitely hang.  The callers
should ensure to avoid the case.  Such parameter validation is not
always simple.  Actually such bugs in DAMON_RECLAIM and DAMON_LRU_SORT
have been found and fixed [1].

Set the field in damon_new_ctx(), so that DAMON API callers can pass the
context parameter to damon_call() without the additional check.

[1] https://lore.kernel.org/20260803134646.16640-1-sj@kernel.org

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 5882f9c94c47f..5a92e4fac6d92 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -920,6 +920,7 @@ struct damon_ctx *damon_new_ctx(void)
 	INIT_LIST_HEAD(&ctx->adaptive_targets);
 	INIT_LIST_HEAD(&ctx->schemes);
 
+	ctx->call_controls_obsolete = true;
 	prandom_seed_state(&ctx->rnd_state, get_random_u64());
 
 	return ctx;
@@ -2178,10 +2179,6 @@ int damon_kdamond_pid(struct damon_ctx *ctx)
  * synchronization.  The return value of the function will be saved in
  * &damon_call_control->return_code.
  *
- * Note that this function should be called only after damon_start() with the
- * @ctx has succeeded.  Otherwise, this function could fall into an indefinite
- * wait.
- *
  * When this function is failed, the @ctx is guaranteed to be stopped.
  *
  * Return: 0 on success, negative error code otherwise.
-- 
2.47.3

[RFC PATCH 3/4] mm/damon/reclaim: remove unnecessary damon_call() param validation

[SJ Park]

DAMON_RECLAIM avoids passing NULL or unstarted damon_ctx to damon_call()
with its own validation.  The validation is no longer needed, because
the DAMON core layer now handles the corner cases itself.  Remove the
unnecessary check.

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/reclaim.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
index 45d5557cc575a..42a2c9cb13431 100644
--- a/mm/damon/reclaim.c
+++ b/mm/damon/reclaim.c
@@ -271,8 +271,6 @@ 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)
 {
@@ -293,10 +291,6 @@ static int damon_reclaim_commit_inputs_store(const char *val,
 	if (!commit_inputs_request)
 		return 0;
 
-	/* Skip damon_call() if ctx has not successfully started. */
-	if (!damon_reclaim_damon_has_started)
-		return -EINVAL;
-
 	err = damon_call(ctx, &control);
 
 	return err ? err : control.return_code;
@@ -343,8 +337,6 @@ 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 4/4] mm/damon/lru_sort: remove unnecessary damon_call() param validation

[SJ Park]

DAMON_LRU_SORT avoids passing NULL or unstarted damon_ctx to
damon_call() with its own validation.  The validation is no longer
needed, because the DAMON core layer now handles the corner cases
itself.  Remove the unnecessary check.

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/lru_sort.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
index bd847829a9907..f25ee7326e87c 100644
--- a/mm/damon/lru_sort.c
+++ b/mm/damon/lru_sort.c
@@ -346,8 +346,6 @@ 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)
 {
@@ -368,10 +366,6 @@ static int damon_lru_sort_commit_inputs_store(const char *val,
 	if (!commit_inputs_request)
 		return 0;
 
-	/* Skip damon_call() if ctx has not successfully started. */
-	if (!damon_lru_sort_damon_has_started)
-		return -EINVAL;
-
 	err = damon_call(ctx, &control);
 
 	return err ? err : control.return_code;
@@ -422,8 +416,6 @@ 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