[RFC PATCH 6.18.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value

[RFC PATCH 6.18.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value

[SJ Park]

Patch series "mm/damon: move zero damos quota target_value handling to the
core layer".

Having zero DAMOS quota target value can cause division by zero.  DAMON
API callers are checking the target value parameters to avoid that.  It is
easy to make mistakes in some of the multiple API callers.  Move that to
the core layer.

Patch 1 adds the corner case handling into the core layer DAMON parameters
validation logic.  Patches 2 and 3 remove no more needed DAMON API callers
side handling of the corner case in DAMON_LRU_SORT and DMON_SAMPLIE_MTIER,
respectively.

This patch (of 3):

If a DAMOS scheme has a damos_quota_goal of zero target_value,
damos_quota_goal() could trigger division-by-zero error.  Hence each DAMON
API callers should do the zero target_value validation.  It is easy to
make mistakes.  Actually such bugs in DAMON_LRU_SORT and
DAMON_SAMPLE_MTIER were found and fixed [1].

It is better to handle the corner case only once in the core layer,
instead of multiple places in all DAMON API callers.  One straightforward
option is using an alternative denominator for the corner case in the
damos_quota_goal().  However, the zero target_value is meaningless.  In
this case, the quota goal is always evaluated as achieved or
over-achieved.  The quota will only keep being reduced.

Simply avoid using zero target_value by adding a check in the core layer
DAMOS quota goal parameters validation/commit path,
damos_commit_quota_goal().  Update it to return an error in the case.
Also update its caller to propagate the error.

Notes for stable kernel porting.  The upstream commit doesn't have
Fixes: and therefore not ported to stable kernels.  It turned out it
fixes commit 8b549a4fd3c5 ("mm/damon/sysfs-schemes: commit damos quota
goals user input to DAMOS") which was merged into v6.8.  The user impact
is a race condition that can result in divide-by-zero.  Hence backport.
Reproducing the issue requires sysfs write permission, certain DAMON ABI
usages that are supposed to be uncommon, and a rare race timing, though.
The issue was discovered [2] by Sashiko as a part [3] of its bug report.

[2] https://sashiko.dev/#/bug/linux-7f814c9a-986f-4103-a55a-ef9537dcf62b
[3] https://lore.kernel.org/20261021005234.94107-1-sj@kernel.org

Link: https://lore.kernel.org/20260903010722.94244-1-sj@kernel.org
Link: https://lore.kernel.org/20260903010722.94244-2-sj@kernel.org
Link: https://lore.kernel.org/20260803134034.15217-1-sj@kernel.org [1]
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 610c4438fb66e06d9e693c0ad3b2c1be54c0d61f)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 4e6e3d7d0a5c0..8fa788b0d709c 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -807,15 +807,18 @@ static void damos_commit_quota_goal_union(
 	}
 }
 
-static void damos_commit_quota_goal(
+static int damos_commit_quota_goal(
 		struct damos_quota_goal *dst, struct damos_quota_goal *src)
 {
+	if (!src->target_value)
+		return  -EINVAL;
 	dst->metric = src->metric;
 	dst->target_value = src->target_value;
 	if (dst->metric == DAMOS_QUOTA_USER_INPUT)
 		dst->current_value = src->current_value;
 	/* keep last_psi_total as is, since it will be updated in next cycle */
 	damos_commit_quota_goal_union(dst, src);
+	return 0;
 }
 
 /**
@@ -833,14 +836,17 @@ static void damos_commit_quota_goal(
 int damos_commit_quota_goals(struct damos_quota *dst, struct damos_quota *src)
 {
 	struct damos_quota_goal *dst_goal, *next, *src_goal, *new_goal;
-	int i = 0, j = 0;
+	int i = 0, j = 0, err;
 
 	damos_for_each_quota_goal_safe(dst_goal, next, dst) {
 		src_goal = damos_nth_quota_goal(i++, src);
-		if (src_goal)
-			damos_commit_quota_goal(dst_goal, src_goal);
-		else
+		if (src_goal) {
+			err = damos_commit_quota_goal(dst_goal, src_goal);
+			if (err)
+				return err;
+		} else {
 			damos_destroy_quota_goal(dst_goal);
+		}
 	}
 	damos_for_each_quota_goal_safe(src_goal, next, src) {
 		if (j++ < i)
@@ -849,7 +855,11 @@ int damos_commit_quota_goals(struct damos_quota *dst, struct damos_quota *src)
 				src_goal->metric, src_goal->target_value);
 		if (!new_goal)
 			return -ENOMEM;
-		damos_commit_quota_goal(new_goal, src_goal);
+		err = damos_commit_quota_goal(new_goal, src_goal);
+		if (err) {
+			damos_free_quota_goal(new_goal);
+			return err;
+		}
 		damos_add_quota_goal(dst, new_goal);
 	}
 	return 0;

base-commit: 2926d88bb7fc4608fd942453d929c39e4e5ec006
-- 
2.47.3

Re: [RFC PATCH 6.18.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] mm/damon: incomplete zero target_value check in damos_commit_quota_goal()
--

commit ea716ea29282ef733822096401c5c8d92eaaef58
Author: SJ Park <sj@kernel.org>

mm/damon/core: error damos_commit_quota_goal() for zero target_value

This commit validates that a DAMOS quota goal's target_value is non-zero in
damos_commit_quota_goal(), returning -EINVAL and propagating the error through
damos_commit_quota_goals() to prevent a division-by-zero in
damos_quota_score().

Link: https://lore.kernel.org/20260903010722.94244-1-sj@kernel.org
Link: https://lore.kernel.org/20260903010722.94244-2-sj@kernel.org
Link: https://lore.kernel.org/20260803134034.15217-1-sj@kernel.org [1]

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 4e6e3d7d0a5c0..8fa788b0d709c 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -807,15 +807,18 @@ static void damos_commit_quota_goal_union(
>  	}
>  }
>  
> -static void damos_commit_quota_goal(
> +static int damos_commit_quota_goal(
>  		struct damos_quota_goal *dst, struct damos_quota_goal *src)
>  {
> +	if (!src->target_value)
> +		return  -EINVAL;

[Severity: High]
Does checking for a zero target_value only in damos_commit_quota_goal() cover
the path where DAMON is started via sysfs in this stable tree?

In mm/damon/sysfs-schemes.c, target_value_store() updates goal->target_value
without holding damon_sysfs_lock:

mm/damon/sysfs-schemes.c:target_value_store() {
	struct damos_sysfs_quota_goal *goal = container_of(kobj, struct
			damos_sysfs_quota_goal, kobj);
	int err = kstrtoul(buf, 0, &goal->target_value);

	return err ? err : count;
}

Meanwhile, damos_sysfs_add_quota_score() reads sysfs_goal->target_value twice
without synchronization against target_value_store():

mm/damon/sysfs-schemes.c:damos_sysfs_add_quota_score() {
		if (!sysfs_goal->target_value)
			continue;

		goal = damos_new_quota_goal(sysfs_goal->metric,
				sysfs_goal->target_value);
		...
		damos_add_quota_goal(quota, goal);
}

If target_value_store() writes 0 between those two reads,
damos_new_quota_goal() allocates a goal with target_value == 0 and attaches it
to quota->goals.

When starting DAMON via state_store() -> damon_sysfs_cmd_request_callback() ->
damon_sysfs_handle_cmd() -> damon_sysfs_turn_damon_on(), this branch builds
the context and starts kdamond directly without calling damon_commit_ctx() or
damos_commit_quota_goals():

mm/damon/sysfs.c:damon_sysfs_turn_damon_on() {
	ctx = damon_sysfs_build_ctx(kdamond->contexts->contexts_arr[0]);
	if (IS_ERR(ctx)) {
		kfree(repeat_call_control);
		return PTR_ERR(ctx);
	}
	err = damon_start(&ctx, 1, false);
}

Here damon_sysfs_build_ctx() populates scheme->quota.goals via
damon_sysfs_apply_inputs() -> damon_sysfs_add_schemes() ->
damon_sysfs_mk_scheme() -> damos_sysfs_add_quota_score(), bypassing
damos_commit_quota_goal() entirely.

When kdamond subsequently runs kdamond_fn() -> kdamond_apply_schemes() ->
damos_adjust_quota() -> damos_set_effective_quota() -> damos_quota_score():

mm/damon/core.c:damos_quota_score() {
		highest_score = max(highest_score,
				goal->current_value * 10000 /
				goal->target_value);
}

can this still divide by zero if the race occurs during
damon_sysfs_turn_damon_on()?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261025172712.164385-1-sj@kernel.org?part=1

Re: [RFC PATCH 6.18.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value

[SJ Park]

On Sun, 25 Oct 2026 17:37:55 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] mm/damon: incomplete zero target_value check in damos_commit_quota_goal()
> --
> 
> commit ea716ea29282ef733822096401c5c8d92eaaef58
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: error damos_commit_quota_goal() for zero target_value
> 
> This commit validates that a DAMOS quota goal's target_value is non-zero in
> damos_commit_quota_goal(), returning -EINVAL and propagating the error through
> damos_commit_quota_goals() to prevent a division-by-zero in
> damos_quota_score().
> 
> Link: https://lore.kernel.org/20260903010722.94244-1-sj@kernel.org
> Link: https://lore.kernel.org/20260903010722.94244-2-sj@kernel.org
> Link: https://lore.kernel.org/20260803134034.15217-1-sj@kernel.org [1]
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 4e6e3d7d0a5c0..8fa788b0d709c 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -807,15 +807,18 @@ static void damos_commit_quota_goal_union(
> >  	}
> >  }
> >  
> > -static void damos_commit_quota_goal(
> > +static int damos_commit_quota_goal(
> >  		struct damos_quota_goal *dst, struct damos_quota_goal *src)
> >  {
> > +	if (!src->target_value)
> > +		return  -EINVAL;
> 
> [Severity: High]
> Does checking for a zero target_value only in damos_commit_quota_goal() cover
> the path where DAMON is started via sysfs in this stable tree?
> 
> In mm/damon/sysfs-schemes.c, target_value_store() updates goal->target_value
> without holding damon_sysfs_lock:
> 
> mm/damon/sysfs-schemes.c:target_value_store() {
> 	struct damos_sysfs_quota_goal *goal = container_of(kobj, struct
> 			damos_sysfs_quota_goal, kobj);
> 	int err = kstrtoul(buf, 0, &goal->target_value);
> 
> 	return err ? err : count;
> }
> 
> Meanwhile, damos_sysfs_add_quota_score() reads sysfs_goal->target_value twice
> without synchronization against target_value_store():
> 
> mm/damon/sysfs-schemes.c:damos_sysfs_add_quota_score() {
> 		if (!sysfs_goal->target_value)
> 			continue;
> 
> 		goal = damos_new_quota_goal(sysfs_goal->metric,
> 				sysfs_goal->target_value);
> 		...
> 		damos_add_quota_goal(quota, goal);
> }
> 
> If target_value_store() writes 0 between those two reads,
> damos_new_quota_goal() allocates a goal with target_value == 0 and attaches it
> to quota->goals.
> 
> When starting DAMON via state_store() -> damon_sysfs_cmd_request_callback() ->
> damon_sysfs_handle_cmd() -> damon_sysfs_turn_damon_on(), this branch builds
> the context and starts kdamond directly without calling damon_commit_ctx() or
> damos_commit_quota_goals():
> 
> mm/damon/sysfs.c:damon_sysfs_turn_damon_on() {
> 	ctx = damon_sysfs_build_ctx(kdamond->contexts->contexts_arr[0]);
> 	if (IS_ERR(ctx)) {
> 		kfree(repeat_call_control);
> 		return PTR_ERR(ctx);
> 	}
> 	err = damon_start(&ctx, 1, false);
> }
> 
> Here damon_sysfs_build_ctx() populates scheme->quota.goals via
> damon_sysfs_apply_inputs() -> damon_sysfs_add_schemes() ->
> damon_sysfs_mk_scheme() -> damos_sysfs_add_quota_score(), bypassing
> damos_commit_quota_goal() entirely.
> 
> When kdamond subsequently runs kdamond_fn() -> kdamond_apply_schemes() ->
> damos_adjust_quota() -> damos_set_effective_quota() -> damos_quota_score():
> 
> mm/damon/core.c:damos_quota_score() {
> 		highest_score = max(highest_score,
> 				goal->current_value * 10000 /
> 				goal->target_value);
> }
> 
> can this still divide by zero if the race occurs during
> damon_sysfs_turn_damon_on()?

Nice catch.  I also just found this issue while working on 6.8.  Same issue in
the patch for 7.2.y [1].  I will withdraw this and that, and work on the next
spin.

[1] https://lore.kernel.org/20261025172434.161597-1-sj@kernel.org

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261025172712.164385-1-sj@kernel.org?part=1


Thanks,
SJ

Re: [RFC PATCH 6.18.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value

[SJ Park]

On Sun, 25 Oct 2026 10:27:10 -0700 SJ Park <sj@kernel.org> wrote:

> Patch series "mm/damon: move zero damos quota target_value handling to the
> core layer".
> 
> Having zero DAMOS quota target value can cause division by zero.  DAMON
> API callers are checking the target value parameters to avoid that.  It is
> easy to make mistakes in some of the multiple API callers.  Move that to
> the core layer.
> 
> Patch 1 adds the corner case handling into the core layer DAMON parameters
> validation logic.  Patches 2 and 3 remove no more needed DAMON API callers
> side handling of the corner case in DAMON_LRU_SORT and DMON_SAMPLIE_MTIER,
> respectively.
> 
> This patch (of 3):
> 
> If a DAMOS scheme has a damos_quota_goal of zero target_value,
> damos_quota_goal() could trigger division-by-zero error.  Hence each DAMON
> API callers should do the zero target_value validation.  It is easy to
> make mistakes.  Actually such bugs in DAMON_LRU_SORT and
> DAMON_SAMPLE_MTIER were found and fixed [1].
> 
> It is better to handle the corner case only once in the core layer,
> instead of multiple places in all DAMON API callers.  One straightforward
> option is using an alternative denominator for the corner case in the
> damos_quota_goal().  However, the zero target_value is meaningless.  In
> this case, the quota goal is always evaluated as achieved or
> over-achieved.  The quota will only keep being reduced.
> 
> Simply avoid using zero target_value by adding a check in the core layer
> DAMOS quota goal parameters validation/commit path,
> damos_commit_quota_goal().  Update it to return an error in the case.
> Also update its caller to propagate the error.
> 
> Notes for stable kernel porting.  The upstream commit doesn't have
> Fixes: and therefore not ported to stable kernels.  It turned out it
> fixes commit 8b549a4fd3c5 ("mm/damon/sysfs-schemes: commit damos quota
> goals user input to DAMOS") which was merged into v6.8.  The user impact
> is a race condition that can result in divide-by-zero.  Hence backport.
> Reproducing the issue requires sysfs write permission, certain DAMON ABI
> usages that are supposed to be uncommon, and a rare race timing, though.
> The issue was discovered [2] by Sashiko as a part [3] of its bug report.
> 
> [2] https://sashiko.dev/#/bug/linux-7f814c9a-986f-4103-a55a-ef9537dcf62b
> [3] https://lore.kernel.org/20261021005234.94107-1-sj@kernel.org

On v6.18, DAMON sysfs interface calls damon_commit_ctx() for validation
purpose, for parameters commit but initial DAMON start.  Hence this patch fixes
the issue in parmeters commit but initial DAMON start.  I will post a new
version to cover the start case.


Thanks,
SJ

[...]