[RFC PATCH 6.12.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value

[RFC PATCH 6.12.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.

Also, the upstream commit on this stable kernel can fix the issue for
parameters commit paths but DAMON start path.  That's because DAMON
start path does not invoke damon_commit_quota_goals().  To fix the path
with minimum change, add a new small validation of quota goal target
value before damon_start() call.

[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  | 18 +++++++++++++-----
 mm/damon/sysfs.c | 16 ++++++++++++++++
 2 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 81d44e41de210..ed32855c16ed8 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -698,14 +698,17 @@ static struct damos_quota_goal *damos_nth_quota_goal(
 	return NULL;
 }
 
-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 */
+	return 0;
 }
 
 /**
@@ -723,18 +726,23 @@ 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)
 			continue;
+		if (!src_goal->target_value)
+			return -EINVAL;
 		new_goal = damos_new_quota_goal(
 				src_goal->metric, src_goal->target_value);
 		if (!new_goal)
diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 5aa23c6545cb8..7895b11555d06 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1497,6 +1497,20 @@ static struct damon_ctx *damon_sysfs_build_ctx(
 	return ctx;
 }
 
+static bool damon_sysfs_valid_start_ctx(struct damon_ctx *ctx)
+{
+	struct damos *s;
+	struct damos_quota_goal *g;
+
+	damon_for_each_scheme(s, ctx) {
+		damos_for_each_quota_goal(g, &s->quota) {
+			if (!g->targe_value)
+				return false;
+		}
+	}
+	return true;
+}
+
 static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 {
 	struct damon_ctx *ctx;
@@ -1517,6 +1531,8 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 	ctx = damon_sysfs_build_ctx(kdamond->contexts->contexts_arr[0]);
 	if (IS_ERR(ctx))
 		return PTR_ERR(ctx);
+	if (!damon_sysfs_valid_start_ctx(ctx))
+		return -EINVAL;
 	err = damon_start(&ctx, 1, false);
 	if (err) {
 		damon_destroy_ctx(ctx);

base-commit: 85d82e0d7c2dfd3e75d34f35b7ebbf41f45c9c4e
-- 
2.47.3

Re: [RFC PATCH 6.12.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Medium] mm/damon/sysfs: ctx memory leak in damon_sysfs_turn_damon_on()
- [Low] mm/damon/sysfs: unknown member targe_value in damon_sysfs_valid_start_ctx()
--

commit 1635d9f393d64d22688141e9540fde6f98f5d6bb
Author: SJ Park <sj@kernel.org>

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

This patch rejects zero target_value in DAMOS quota goals during parameter
commit in damos_commit_quota_goal() and damos_commit_quota_goals() to avoid
division-by-zero errors. For this stable backport, it also adds a validation
helper before damon_start() in damon_sysfs_turn_damon_on().

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/sysfs.c b/mm/damon/sysfs.c
> index 5aa23c6545cb8..7895b11555d06 100644
> --- a/mm/damon/sysfs.c
> +++ b/mm/damon/sysfs.c
> @@ -1497,6 +1497,20 @@ static struct damon_ctx *damon_sysfs_build_ctx(
>  	return ctx;
>  }
>  
> +static bool damon_sysfs_valid_start_ctx(struct damon_ctx *ctx)
> +{
> +	struct damos *s;
> +	struct damos_quota_goal *g;
> +
> +	damon_for_each_scheme(s, ctx) {
> +		damos_for_each_quota_goal(g, &s->quota) {
> +			if (!g->targe_value)
> +				return false;
> +		}
> +	}
> +	return true;
> +}

[Severity: Low]
Will mm/damon/sysfs.c fail to build in damon_sysfs_valid_start_ctx() due to
referencing g->targe_value instead of g->target_value?

In include/linux/damon.h, struct damos_quota_goal defines this field as:

struct damos_quota_goal {
	enum damos_quota_goal_metric metric;
	unsigned long target_value;
	unsigned long current_value;

> +
>  static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
>  {
>  	struct damon_ctx *ctx;
> @@ -1517,6 +1531,8 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
>  	ctx = damon_sysfs_build_ctx(kdamond->contexts->contexts_arr[0]);
>  	if (IS_ERR(ctx))
>  		return PTR_ERR(ctx);
> +	if (!damon_sysfs_valid_start_ctx(ctx))
> +		return -EINVAL;

[Severity: Medium]
Does damon_sysfs_turn_damon_on() leak ctx and its associated allocations when
damon_sysfs_valid_start_ctx(ctx) returns false?

When userspace writes "on" to the kdamond state sysfs file with a scheme
quota goal whose target_value is zero, damon_sysfs_build_ctx() allocates ctx
along with its targets, regions, schemes, quota goals, filters, and any
target struct pid references. Returning -EINVAL directly here skips
damon_destroy_ctx(ctx) (and target pid cleanup via
damon_sysfs_destroy_targets(ctx)), whereas the damon_start() error path right
below destroys ctx before returning:

	err = damon_start(&ctx, 1, false);
	if (err) {
		damon_destroy_ctx(ctx);
		return err;
	}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261025182326.178326-1-sj@kernel.org?part=1

Re: [RFC PATCH 6.12.y] mm/damon/core: error damos_commit_quota_goal() for zero target_value

[SJ Park]

On Sun, 25 Oct 2026 18:33:53 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> - [Medium] mm/damon/sysfs: ctx memory leak in damon_sysfs_turn_damon_on()
> - [Low] mm/damon/sysfs: unknown member targe_value in damon_sysfs_valid_start_ctx()
> --
> 
> commit 1635d9f393d64d22688141e9540fde6f98f5d6bb
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: error damos_commit_quota_goal() for zero target_value
> 
> This patch rejects zero target_value in DAMOS quota goals during parameter
> commit in damos_commit_quota_goal() and damos_commit_quota_goals() to avoid
> division-by-zero errors. For this stable backport, it also adds a validation
> helper before damon_start() in damon_sysfs_turn_damon_on().
> 
> 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/sysfs.c b/mm/damon/sysfs.c
> > index 5aa23c6545cb8..7895b11555d06 100644
> > --- a/mm/damon/sysfs.c
> > +++ b/mm/damon/sysfs.c
> > @@ -1497,6 +1497,20 @@ static struct damon_ctx *damon_sysfs_build_ctx(
> >  	return ctx;
> >  }
> >  
> > +static bool damon_sysfs_valid_start_ctx(struct damon_ctx *ctx)
> > +{
> > +	struct damos *s;
> > +	struct damos_quota_goal *g;
> > +
> > +	damon_for_each_scheme(s, ctx) {
> > +		damos_for_each_quota_goal(g, &s->quota) {
> > +			if (!g->targe_value)
> > +				return false;
> > +		}
> > +	}
> > +	return true;
> > +}
> 
> [Severity: Low]
> Will mm/damon/sysfs.c fail to build in damon_sysfs_valid_start_ctx() due to
> referencing g->targe_value instead of g->target_value?

Nice catch.  My build test was somehow configured to disable DAMON_SYSFS.  I
will fix this in the next spin.

Also, my build test found an issue in macro expansion after fixing the config.
I will add below change for the two build issues.

'''
@@ -1503,8 +1503,8 @@ static bool damon_sysfs_valid_start_ctx(struct damon_ctx *ctx)
        struct damos_quota_goal *g;

        damon_for_each_scheme(s, ctx) {
-               damos_for_each_quota_goal(g, &s->quota) {
-                       if (!g->targe_value)
+               damos_for_each_quota_goal(g, (&s->quota)) {
+                       if (!g->target_value)
                                return false;
                }
        }
'''

> 
> In include/linux/damon.h, struct damos_quota_goal defines this field as:
> 
> struct damos_quota_goal {
> 	enum damos_quota_goal_metric metric;
> 	unsigned long target_value;
> 	unsigned long current_value;
> 
> > +
> >  static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
> >  {
> >  	struct damon_ctx *ctx;
> > @@ -1517,6 +1531,8 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
> >  	ctx = damon_sysfs_build_ctx(kdamond->contexts->contexts_arr[0]);
> >  	if (IS_ERR(ctx))
> >  		return PTR_ERR(ctx);
> > +	if (!damon_sysfs_valid_start_ctx(ctx))
> > +		return -EINVAL;
> 
> [Severity: Medium]
> Does damon_sysfs_turn_damon_on() leak ctx and its associated allocations when
> damon_sysfs_valid_start_ctx(ctx) returns false?

Good catch.  I will destroy the ctx in the error case in the next version, like
below:

'''
@@ -1531,8 +1531,10 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
        ctx = damon_sysfs_build_ctx(kdamond->contexts->contexts_arr[0]);
        if (IS_ERR(ctx))
                return PTR_ERR(ctx);
-       if (!damon_sysfs_valid_start_ctx(ctx))
+       if (!damon_sysfs_valid_start_ctx(ctx)) {
+               damon_destroy_ctx(ctx);
                return -EINVAL;
+       }
        err = damon_start(&ctx, 1, false);
        if (err) {
                damon_destroy_ctx(ctx);
'''

> 
> When userspace writes "on" to the kdamond state sysfs file with a scheme
> quota goal whose target_value is zero, damon_sysfs_build_ctx() allocates ctx
> along with its targets, regions, schemes, quota goals, filters, and any
> target struct pid references. Returning -EINVAL directly here skips
> damon_destroy_ctx(ctx) (and target pid cleanup via
> damon_sysfs_destroy_targets(ctx)), whereas the damon_start() error path right
> below destroys ctx before returning:
> 
> 	err = damon_start(&ctx, 1, false);
> 	if (err) {
> 		damon_destroy_ctx(ctx);
> 		return err;
> 	}
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261025182326.178326-1-sj@kernel.org?part=1


Thanks,
SJ