* [RFC PATCH 6.12.y v2] mm/damon/core: error damos_commit_quota_goal() for zero target_value
@ 2026-10-26 13:55 SJ Park
2026-10-26 14:11 ` sashiko-bot
2026-10-26 22:55 ` Sasha Levin
0 siblings, 2 replies; 6+ messages in thread
From: SJ Park @ 2026-10-26 13:55 UTC (permalink / raw)
Cc: SJ Park, Andrew Morton, damon, linux-kernel, linux-mm, stable
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>
---
Changes from RFC
- RFC: https://lore.kernel.org/20261025182326.178326-1-sj@kernel.org
- Fix a typo and macro expansion build error for quota goal iteration.
- Destroy ctx in the before-damon_start() error path.
mm/damon/core.c | 18 +++++++++++++-----
mm/damon/sysfs.c | 18 ++++++++++++++++++
2 files changed, 31 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..2c8749b62c621 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->target_value)
+ return false;
+ }
+ }
+ return true;
+}
+
static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
{
struct damon_ctx *ctx;
@@ -1517,6 +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)) {
+ damon_destroy_ctx(ctx);
+ return -EINVAL;
+ }
err = damon_start(&ctx, 1, false);
if (err) {
damon_destroy_ctx(ctx);
base-commit: 85d82e0d7c2dfd3e75d34f35b7ebbf41f45c9c4e
--
2.47.3
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [RFC PATCH 6.12.y v2] mm/damon/core: error damos_commit_quota_goal() for zero target_value
2026-10-26 13:55 [RFC PATCH 6.12.y v2] mm/damon/core: error damos_commit_quota_goal() for zero target_value SJ Park
@ 2026-10-26 14:11 ` sashiko-bot
2026-10-26 15:08 ` SJ Park
2026-10-26 22:55 ` Sasha Levin
1 sibling, 1 reply; 6+ messages in thread
From: sashiko-bot @ 2026-10-26 14:11 UTC (permalink / raw)
To: SJ Park; +Cc: Andrew Morton, damon, linux-mm
Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] mm/damon/sysfs: pid leak in damon_sysfs_turn_damon_on() on invalid start ctx
--
commit dc23c827b1a01b5ed768b0a218bb6a67def40106
Author: SJ Park <sj@kernel.org>
mm/damon/core: error damos_commit_quota_goal() for zero target_value
This commit validates that DAMOS quota goal target_value is non-zero in
damos_commit_quota_goal() and damos_commit_quota_goals() to prevent a
division-by-zero error. For the stable backport, it also adds
damon_sysfs_valid_start_ctx() to reject zero target_value before starting
DAMON 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..2c8749b62c621 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->target_value)
> + return false;
> + }
> + }
> + return true;
> +}
> +
> static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
> {
> struct damon_ctx *ctx;
> @@ -1517,6 +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)) {
> + damon_destroy_ctx(ctx);
> + return -EINVAL;
> + }
[Severity: Medium]
Does this error path leak the target struct pid references (t->pid) when
damon_target_has_pid(ctx) is true?
When damon_sysfs_build_ctx() constructs ctx via damon_sysfs_apply_inputs() ->
damon_sysfs_add_targets() -> damon_sysfs_add_target(), it acquires a struct
pid reference for each target:
mm/damon/sysfs.c:damon_sysfs_add_target() {
...
if (damon_target_has_pid(ctx)) {
t->pid = find_get_pid(sys_target->pid);
if (!t->pid)
goto destroy_targets_out;
}
...
}
Because damon_sysfs_valid_start_ctx(ctx) fails before damon_start() is
called, kdamond never runs and ctx->callback.before_terminate
(damon_sysfs_before_terminate()) is never invoked to release those
references. Calling damon_destroy_ctx(ctx) invokes damon_destroy_targets(ctx),
which frees each target without calling put_pid(t->pid):
mm/damon/core.c:damon_destroy_targets() {
struct damon_target *t, *next_t;
if (ctx->ops.cleanup) {
ctx->ops.cleanup(ctx);
return;
}
damon_for_each_target_safe(t, next_t, ctx)
damon_destroy_target(t);
}
In comparison, damon_sysfs_commit_input() cleans up an unstarted context from
damon_sysfs_build_ctx() by calling damon_sysfs_destroy_targets() before
damon_destroy_ctx():
mm/damon/sysfs.c:damon_sysfs_commit_input() {
...
err = damon_commit_ctx(kdamond->damon_ctx, param_ctx);
damon_sysfs_destroy_targets(param_ctx);
damon_destroy_ctx(param_ctx);
return err;
}
Should damon_sysfs_turn_damon_on() also call damon_sysfs_destroy_targets(ctx)
before damon_destroy_ctx(ctx) here?
--
Sashiko AI review · https://sashiko.dev/#/patchset/20261026135555.149244-1-sj@kernel.org?part=1
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [RFC PATCH 6.12.y v2] mm/damon/core: error damos_commit_quota_goal() for zero target_value
2026-10-26 14:11 ` sashiko-bot
@ 2026-10-26 15:08 ` SJ Park
0 siblings, 0 replies; 6+ messages in thread
From: SJ Park @ 2026-10-26 15:08 UTC (permalink / raw)
To: sashiko-bot; +Cc: SJ Park, Andrew Morton, damon, linux-mm
On Mon, 26 Oct 2026 14:11:30 +0000 sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] mm/damon/sysfs: pid leak in damon_sysfs_turn_damon_on() on invalid start ctx
> --
>
> commit dc23c827b1a01b5ed768b0a218bb6a67def40106
> Author: SJ Park <sj@kernel.org>
>
> mm/damon/core: error damos_commit_quota_goal() for zero target_value
>
> This commit validates that DAMOS quota goal target_value is non-zero in
> damos_commit_quota_goal() and damos_commit_quota_goals() to prevent a
> division-by-zero error. For the stable backport, it also adds
> damon_sysfs_valid_start_ctx() to reject zero target_value before starting
> DAMON 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..2c8749b62c621 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->target_value)
> > + return false;
> > + }
> > + }
> > + return true;
> > +}
> > +
> > static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
> > {
> > struct damon_ctx *ctx;
> > @@ -1517,6 +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)) {
> > + damon_destroy_ctx(ctx);
> > + return -EINVAL;
> > + }
>
> [Severity: Medium]
> Does this error path leak the target struct pid references (t->pid) when
> damon_target_has_pid(ctx) is true?
>
> When damon_sysfs_build_ctx() constructs ctx via damon_sysfs_apply_inputs() ->
> damon_sysfs_add_targets() -> damon_sysfs_add_target(), it acquires a struct
> pid reference for each target:
>
> mm/damon/sysfs.c:damon_sysfs_add_target() {
> ...
> if (damon_target_has_pid(ctx)) {
> t->pid = find_get_pid(sys_target->pid);
> if (!t->pid)
> goto destroy_targets_out;
> }
> ...
> }
>
> Because damon_sysfs_valid_start_ctx(ctx) fails before damon_start() is
> called, kdamond never runs and ctx->callback.before_terminate
> (damon_sysfs_before_terminate()) is never invoked to release those
> references. Calling damon_destroy_ctx(ctx) invokes damon_destroy_targets(ctx),
> which frees each target without calling put_pid(t->pid):
>
> mm/damon/core.c:damon_destroy_targets() {
> struct damon_target *t, *next_t;
>
> if (ctx->ops.cleanup) {
> ctx->ops.cleanup(ctx);
> return;
> }
>
> damon_for_each_target_safe(t, next_t, ctx)
> damon_destroy_target(t);
> }
>
> In comparison, damon_sysfs_commit_input() cleans up an unstarted context from
> damon_sysfs_build_ctx() by calling damon_sysfs_destroy_targets() before
> damon_destroy_ctx():
>
> mm/damon/sysfs.c:damon_sysfs_commit_input() {
> ...
> err = damon_commit_ctx(kdamond->damon_ctx, param_ctx);
> damon_sysfs_destroy_targets(param_ctx);
> damon_destroy_ctx(param_ctx);
> return err;
> }
>
> Should damon_sysfs_turn_damon_on() also call damon_sysfs_destroy_targets(ctx)
> before damon_destroy_ctx(ctx) here?
Nice catch. I will fix this in the next revision.
FYI, this patch for 6.18 and later kernels doesn't have this problem, since
they have commit 7114bc5e01cf ("mm/damon/core: add cleanup_target() ops
callback"). The commit made damon_destroy_ctx() to do the pid reference drop
by itself, and merged into 6.17.
>
> --
> Sashiko AI review · https://sashiko.dev/#/patchset/20261026135555.149244-1-sj@kernel.org?part=1
Thanks,
SJ
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [RFC PATCH 6.12.y v2] mm/damon/core: error damos_commit_quota_goal() for zero target_value
2026-10-26 13:55 [RFC PATCH 6.12.y v2] mm/damon/core: error damos_commit_quota_goal() for zero target_value SJ Park
2026-10-26 14:11 ` sashiko-bot
@ 2026-10-26 22:55 ` Sasha Levin
2026-10-27 0:01 ` SJ Park
1 sibling, 1 reply; 6+ messages in thread
From: Sasha Levin @ 2026-10-26 22:55 UTC (permalink / raw)
To: SJ Park; +Cc: Sasha Levin, Andrew Morton, damon, linux-kernel, linux-mm, stable
> 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.
Queued for 6.12, thanks.
--
Thanks,
Sasha
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [RFC PATCH 6.12.y v2] mm/damon/core: error damos_commit_quota_goal() for zero target_value
2026-10-26 22:55 ` Sasha Levin
@ 2026-10-27 0:01 ` SJ Park
2026-10-27 0:06 ` Sasha Levin
0 siblings, 1 reply; 6+ messages in thread
From: SJ Park @ 2026-10-27 0:01 UTC (permalink / raw)
To: Sasha Levin; +Cc: SJ Park, Andrew Morton, damon, linux-kernel, linux-mm, stable
Hello Sasha,
On Mon, 26 Oct 2026 18:55:58 -0400 Sasha Levin <sashal@kernel.org> wrote:
> > 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.
>
> Queued for 6.12, thanks.
I'm sorry but could you please unqueue? Sashiko found [1] a bug that can leak
pids.
Also, could we avoid queueing DAMON patches that marked as RFC in general?
[1] https://lore.kernel.org/20261026150825.152576-1-sj@kernel.org
Thanks,
SJ
[...]
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [RFC PATCH 6.12.y v2] mm/damon/core: error damos_commit_quota_goal() for zero target_value
2026-10-27 0:01 ` SJ Park
@ 2026-10-27 0:06 ` Sasha Levin
0 siblings, 0 replies; 6+ messages in thread
From: Sasha Levin @ 2026-10-27 0:06 UTC (permalink / raw)
To: SJ Park; +Cc: Andrew Morton, damon, linux-kernel, linux-mm, stable
On Mon, Oct 26, 2026 at 05:01:20PM -0700, SJ Park wrote:
>Hello Sasha,
>
>On Mon, 26 Oct 2026 18:55:58 -0400 Sasha Levin <sashal@kernel.org> wrote:
>
>> > 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.
>>
>> Queued for 6.12, thanks.
>
>I'm sorry but could you please unqueue? Sashiko found [1] a bug that can leak
>pids.
No problem, now dropped.
>Also, could we avoid queueing DAMON patches that marked as RFC in general?
Sorry about this. We don't normally get RFC tags for backports so my brain
didn't register this at all :)
--
Thanks,
Sasha
^ permalink raw reply [flat|nested] 6+ messages in thread
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2026-10-26 13:55 [RFC PATCH 6.12.y v2] mm/damon/core: error damos_commit_quota_goal() for zero target_value SJ Park
2026-10-26 14:11 ` sashiko-bot
2026-10-26 15:08 ` SJ Park
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2026-10-27 0:01 ` SJ Park
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