[RFC PATCH 0/2] mm/damon: fix damon_call()-related leak and deadlock

[RFC PATCH 0/2] mm/damon: fix damon_call()-related leak and deadlock

[SeongJae Park]

DAMON_SYSFS can leak memory due to improper handling of damon_call()
failure.  It can also cause a deadlock due to a race between
damon_call() and kdamond_fn() termination.  Fix those.

SeongJae Park (2):
  mm/damon/sysfs: dealloc repeat_call_control if damon_call() fails
  mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock

 include/linux/damon.h |  1 +
 mm/damon/core.c       | 41 ++++++++++-------------------------------
 mm/damon/sysfs.c      |  3 ++-
 3 files changed, 13 insertions(+), 32 deletions(-)


base-commit: 2b3fbc1796d335685d9b7a825c621914a1c97d1d
-- 
2.47.3

[RFC PATCH 2/2] mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock

[SeongJae Park]

When kdamond_fn() main loop is finished, the function cancels all
remaining damon_call() requests and unset the damon_ctx->kdamond so that
API callers can show the context is terminated.  damon_call() adds the
caller's request to the queue first.  After that, it shows if the
kdamond is of the damon_ctx is still running (damon_ctx->kdamond is
set).  Only if the kdamond is running, damon_call() starts waiting for
the kdamond's handling of the newly added request.

The damon_call() requests registration and damon_ctx->kdamond unset are
protected by different mutexes, though.  Hence, damon_call() could race
with damon_ctx->kdamond unset, and results in deadlocks.

For example, let's suppose kdamond successfully finished the
damon_call() requests cancelling.  Right after that, damon_call() is
called for the context.  It registers the new request, and show the
context is still running, because damon_ctx->kdamond unset is not yet
done.  Hence the damon_call() caller starts waiting for the handling of
the request.  However, the kdamond is already on the termination steps,
so never handles the new request.  As a result, the damon_call() caller
threads infinitely waits.

Fix this by introducing another damon_ctx field, namely
call_controls_obsolete.  It is protected by the
damon_ctx->call_controls_lock, which protects damon_call() registration.
Initialize (unset) it in kdamond_init_ctx() and set just before the
cancelling of remaining damon_call() is executed.  damon_call() reads
the obsolete field under the lock and avoid adding a new request.

After this change, only requests that guaranteed to be handled or
cancelled are registered.  Hence the after-registration DAMON context
termination check is no more needed.  Remove it together.

The issue is found by sashiko [1].

[1] https://lore.kernel.org/20260325141956.87144-1-sj@kernel.org

Fixes: 42b7491af14c ("mm/damon/core: introduce damon_call()")
Cc: <stable@vger.kernel.org> # 6.14.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/damon.h |  1 +
 mm/damon/core.c       | 41 ++++++++++-------------------------------
 2 files changed, 11 insertions(+), 31 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index d9a3babbafc16..5129de70e7b70 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -818,6 +818,7 @@ struct damon_ctx {
 
 	/* lists of &struct damon_call_control */
 	struct list_head call_controls;
+	bool call_controls_obsolete;
 	struct mutex call_controls_lock;
 
 	struct damos_walk_control *walk_control;
diff --git a/mm/damon/core.c b/mm/damon/core.c
index db6c67e52d2b8..a2b553e2c5a81 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1573,35 +1573,6 @@ int damon_kdamond_pid(struct damon_ctx *ctx)
 	return pid;
 }
 
-/*
- * damon_call_handle_inactive_ctx() - handle DAMON call request that added to
- *				      an inactive context.
- * @ctx:	The inactive DAMON context.
- * @control:	Control variable of the call request.
- *
- * This function is called in a case that @control is added to @ctx but @ctx is
- * not running (inactive).  See if @ctx handled @control or not, and cleanup
- * @control if it was not handled.
- *
- * Returns 0 if @control was handled by @ctx, negative error code otherwise.
- */
-static int damon_call_handle_inactive_ctx(
-		struct damon_ctx *ctx, struct damon_call_control *control)
-{
-	struct damon_call_control *c;
-
-	mutex_lock(&ctx->call_controls_lock);
-	list_for_each_entry(c, &ctx->call_controls, list) {
-		if (c == control) {
-			list_del(&control->list);
-			mutex_unlock(&ctx->call_controls_lock);
-			return -EINVAL;
-		}
-	}
-	mutex_unlock(&ctx->call_controls_lock);
-	return 0;
-}
-
 /**
  * damon_call() - Invoke a given function on DAMON worker thread (kdamond).
  * @ctx:	DAMON context to call the function for.
@@ -1629,10 +1600,12 @@ int damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
 	INIT_LIST_HEAD(&control->list);
 
 	mutex_lock(&ctx->call_controls_lock);
+	if (ctx->call_controls_obsolete) {
+		mutex_unlock(&ctx->call_controls_lock);
+		return -ECANCELED;
+	}
 	list_add_tail(&control->list, &ctx->call_controls);
 	mutex_unlock(&ctx->call_controls_lock);
-	if (!damon_is_running(ctx))
-		return damon_call_handle_inactive_ctx(ctx, control);
 	if (control->repeat)
 		return 0;
 	wait_for_completion(&control->completion);
@@ -2939,6 +2912,9 @@ static void kdamond_init_ctx(struct damon_ctx *ctx)
 		damos_set_next_apply_sis(scheme, ctx);
 		damos_set_filters_default_reject(scheme);
 	}
+	mutex_lock(&ctx->call_controls_lock);
+	ctx->call_controls_obsolete = false;
+	mutex_unlock(&ctx->call_controls_lock);
 }
 
 /*
@@ -3062,6 +3038,9 @@ static int kdamond_fn(void *data)
 	damon_destroy_targets(ctx);
 
 	kfree(ctx->regions_score_histogram);
+	mutex_lock(&ctx->call_controls_lock);
+	ctx->call_controls_obsolete = true;
+	mutex_unlock(&ctx->call_controls_lock);
 	kdamond_call(ctx, true);
 	damos_walk_cancel(ctx);
 
-- 
2.47.3

Re: (sashiko status) [RFC PATCH 0/2] mm/damon: fix damon_call()-related leak and deadlock

[SeongJae Park]

Forwarding sashiko.dev review status for this thread.

# review url: https://sashiko.dev/#/patchset/20260326062347.88569-1-sj@kernel.org

- [RFC PATCH 1/2] mm/damon/sysfs: dealloc repeat_call_control if damon_call() fails
  - status: Reviewed
  - review: No issues found.
- [RFC PATCH 2/2] mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock
  - status: Reviewed
  - review: ISSUES MAY FOUND

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Re: (sashiko review) [RFC PATCH 2/2] mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock

[SeongJae Park]

Forwarding Sashiko review for doing discussions via mails.

# review url: https://sashiko.dev/#/patchset/20260326062347.88569-3-sj@kernel.org
# start of sashiko.dev inline review
commit b886bf00a72b1462f29533c91c431cd2ba3fa3a6
Author: SeongJae Park <sj@kernel.org>
mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock

This commit introduces a call_controls_obsolete flag to protect damon_call()
from racing with the kdamond termination sequence. This replaces the previous
after-registration damon_is_running() check.

Link: https://lore.kernel.org/20260325141956.87144-1-sj@kernel.org

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index db6c67e52d2b8..a2b553e2c5a81 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
[ ... ]
> @@ -1629,10 +1600,12 @@ int damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
>  	INIT_LIST_HEAD(&control->list);
>  
>  	mutex_lock(&ctx->call_controls_lock);
> +	if (ctx->call_controls_obsolete) {
> +		mutex_unlock(&ctx->call_controls_lock);
> +		return -ECANCELED;
> +	}
>  	list_add_tail(&control->list, &ctx->call_controls);
>  	mutex_unlock(&ctx->call_controls_lock);
> -	if (!damon_is_running(ctx))
> -		return damon_call_handle_inactive_ctx(ctx, control);

Does this removal cause an indefinite wait if damon_call() is invoked on an
unstarted context?

For a newly allocated damon_ctx, call_controls_obsolete is zero-initialized
to false. Without the damon_is_running() check, if a caller invokes
damon_call() before damon_start(), it appears the request will be queued and
block on wait_for_completion() indefinitely since no kdamond thread is
running to handle or cancel it.

>  	if (control->repeat)
>  		return 0;
>  	wait_for_completion(&control->completion);
[ ... ]
> @@ -3062,6 +3038,9 @@ static int kdamond_fn(void *data)
>  	damon_destroy_targets(ctx);
>  
>  	kfree(ctx->regions_score_histogram);
> +	mutex_lock(&ctx->call_controls_lock);
> +	ctx->call_controls_obsolete = true;
> +	mutex_unlock(&ctx->call_controls_lock);
>  	kdamond_call(ctx, true);
>  	damos_walk_cancel(ctx);

Can this sequence also cause a spurious -ECANCELED failure when a context is
restarted?

When restarting a context, kdamond_fn() signals kdamond_started before
calling kdamond_init_ctx():

        complete(&ctx->kdamond_started);
        kdamond_init_ctx(ctx);

If damon_start() unblocks and immediately invokes damon_call(), couldn't it
observe the stale call_controls_obsolete == true from the previous run and
return -ECANCELED, since kdamond_init_ctx() has not yet reset the flag?

Also, is damos_walk() vulnerable to the same termination race being fixed
here for damon_call()?

In kdamond_fn(), damos_walk_cancel() processes cancellations before
ctx->kdamond is set to NULL. If damos_walk() is called during this window,
it checks damon_is_running() (which still returns true), sets the walk
control, and blocks on wait_for_completion().

Since cancellations are already processed, will it wait indefinitely?


# end of sashiko.dev inline review
# review url: https://sashiko.dev/#/patchset/20260326062347.88569-3-sj@kernel.org
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Re: (sashiko review) [RFC PATCH 2/2] mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock

[SeongJae Park]

On Thu, 26 Mar 2026 06:43:29 -0700 SeongJae Park <sj@kernel.org> wrote:

> Forwarding Sashiko review for doing discussions via mails.
> 
> # review url: https://sashiko.dev/#/patchset/20260326062347.88569-3-sj@kernel.org
> # start of sashiko.dev inline review
> commit b886bf00a72b1462f29533c91c431cd2ba3fa3a6
> Author: SeongJae Park <sj@kernel.org>
> mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock
> 
> This commit introduces a call_controls_obsolete flag to protect damon_call()
> from racing with the kdamond termination sequence. This replaces the previous
> after-registration damon_is_running() check.
> 
> Link: https://lore.kernel.org/20260325141956.87144-1-sj@kernel.org
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index db6c67e52d2b8..a2b553e2c5a81 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> [ ... ]
> > @@ -1629,10 +1600,12 @@ int damon_call(struct damon_ctx *ctx, struct damon_call_control *control)
> >  	INIT_LIST_HEAD(&control->list);
> >  
> >  	mutex_lock(&ctx->call_controls_lock);
> > +	if (ctx->call_controls_obsolete) {
> > +		mutex_unlock(&ctx->call_controls_lock);
> > +		return -ECANCELED;
> > +	}
> >  	list_add_tail(&control->list, &ctx->call_controls);
> >  	mutex_unlock(&ctx->call_controls_lock);
> > -	if (!damon_is_running(ctx))
> > -		return damon_call_handle_inactive_ctx(ctx, control);
> 
> Does this removal cause an indefinite wait if damon_call() is invoked on an
> unstarted context?
> 
> For a newly allocated damon_ctx, call_controls_obsolete is zero-initialized
> to false. Without the damon_is_running() check, if a caller invokes
> damon_call() before damon_start(), it appears the request will be queued and
> block on wait_for_completion() indefinitely since no kdamond thread is
> running to handle or cancel it.

Yes.  But it is clearly wrong usage of this function.  I will add a comment
clarifying this, e.g., "this function shouldn't be called for unstarted DAMON
context.  In the case, it could be indefinitely sleep."

> 
> >  	if (control->repeat)
> >  		return 0;
> >  	wait_for_completion(&control->completion);
> [ ... ]
> > @@ -3062,6 +3038,9 @@ static int kdamond_fn(void *data)
> >  	damon_destroy_targets(ctx);
> >  
> >  	kfree(ctx->regions_score_histogram);
> > +	mutex_lock(&ctx->call_controls_lock);
> > +	ctx->call_controls_obsolete = true;
> > +	mutex_unlock(&ctx->call_controls_lock);
> >  	kdamond_call(ctx, true);
> >  	damos_walk_cancel(ctx);
> 
> Can this sequence also cause a spurious -ECANCELED failure when a context is
> restarted?
> 
> When restarting a context, kdamond_fn() signals kdamond_started before
> calling kdamond_init_ctx():
> 
>         complete(&ctx->kdamond_started);
>         kdamond_init_ctx(ctx);
> 
> If damon_start() unblocks and immediately invokes damon_call(), couldn't it
> observe the stale call_controls_obsolete == true from the previous run and
> return -ECANCELED, since kdamond_init_ctx() has not yet reset the flag?

Good catch, I will do the call_controls_obsolete unset before the complete()
call.

> 
> Also, is damos_walk() vulnerable to the same termination race being fixed
> here for damon_call()?
> 
> In kdamond_fn(), damos_walk_cancel() processes cancellations before
> ctx->kdamond is set to NULL. If damos_walk() is called during this window,
> it checks damon_is_running() (which still returns true), sets the walk
> control, and blocks on wait_for_completion().
> 
> Since cancellations are already processed, will it wait indefinitely?

Yes.  I'm working on it for another patch.


Thanks,
SJ

[...]

Re: (sashiko status) [RFC PATCH 0/2] mm/damon: fix damon_call()-related leak and deadlock

[SeongJae Park]

On Thu, 26 Mar 2026 06:42:08 -0700 SeongJae Park <sj@kernel.org> wrote:

> Forwarding sashiko.dev review status for this thread.
> 
> # review url: https://sashiko.dev/#/patchset/20260326062347.88569-1-sj@kernel.org
> 
> - [RFC PATCH 1/2] mm/damon/sysfs: dealloc repeat_call_control if damon_call() fails
>   - status: Reviewed
>   - review: No issues found.
> - [RFC PATCH 2/2] mm/damon/core: fix damon_call() vs kdamond_fn() exit race deadlock
>   - status: Reviewed
>   - review: ISSUES MAY FOUND

The patches are hotfixes, and have no reason to be one series.

I will send the first patch as an individual one without RFC, soon.

For the second patch, I will go one more RFC round.


Thanks,
SJ

[...]