[RFC PATCH 6.6.y 0/3] mm/damon: porting fixes that cherry-pick failed on 2026-11-07

[RFC PATCH 6.6.y 0/3] mm/damon: porting fixes that cherry-pick failed on 2026-11-07

[SJ Park]

Manually port three DAMON fixes that cherry-pick failed [1-3] and turned
out to really need to be picked.  Note that seven patches [4-10] that
also failed in the same day were having wrong Cc:stable@ tags, so
ignored.

Patches that this series is porting.

[1] https://lore.kernel.org/20261107053211-failed-6.6-90c2fa6adfed@kernel.org
[2] https://lore.kernel.org/20261107053907-failed-6.6-bbeec2d8324d@kernel.org
[3] https://lore.kernel.org/20261107053915-failed-6.6-5695abc4e742@kernel.org

Patches that also cherry-pick failed but deliverately ignored.

[4] https://lore.kernel.org/20261107053348-failed-6.6-6f276422aded@kernel.org
[5] https://lore.kernel.org/20261107053350-failed-6.6-cec08aad3def@kernel.org
[6] https://lore.kernel.org/20261107053353-failed-6.6-409d2ab6d982@kernel.org
[7] https://lore.kernel.org/20261107053355-failed-6.6-1bfb9384ae0e@kernel.org
[8] https://lore.kernel.org/20261107053357-failed-6.6-a1c7fdaf1657@kernel.org
[9] https://lore.kernel.org/20261107053400-failed-6.6-1d2397b2c414@kernel.org
[10] https://lore.kernel.org/20261107053402-failed-6.6-1c3352282063@kernel.org

SJ Park (3):
  mm/damon/core: skip applying scheme if region split for quota fails
  mm/damon/core: filter out damos address filter-split failed region
  mm/damon/core: remove unreachable damos charge_{target,addr}_from

 mm/damon/core-test.h | 11 +++++-----
 mm/damon/core.c      | 48 ++++++++++++++++++++++++++++++++++++--------
 2 files changed, 46 insertions(+), 13 deletions(-)


base-commit: ba98b310288deb9f3814fc8f80d8a33ae0f59850
-- 
2.47.3

[RFC PATCH 6.6.y 1/3] mm/damon/core: skip applying scheme if region split for quota fails

[SJ Park]

Patch series "mm/damon: fix DAMOS bugs in core, paddr and vaddr".

Fix misc bugs of DAMOS.  Patch 1 makes DAMOS less stress memory allocator
under extreme situation.  Patches 2 and 3 fix wrong folios walking in
DAMON_PADDR.  Patches 4 and 5 fix wrong folios walking in DAMON_VADDR.
Patches 6-8 handle extreme and unlikely memory situations that can cause
divide by zero and underflow.

All bugs are discovered by Sashiko.

This patch (of 8):

damos_apply_scheme() splits a region and apply the action to the subregion
if it is needed for not violating the quota.  The split operation
(damon_split_region_at()) could fail for allocation failure.  In the case,
the quota could be violated.  From the user's perspective, DAMOS becomes
more aggressive than expected under the extreme situation.  Handle the
failure.

The user impact is not critical.  The failure of damon_split_region_at()
is unlikely since it is arguably too small to fail.  Also DAMOS being
aggressive is limited to the single region.  Users can set min_nr_regions
to set the maximum size of each region.  If it is reasonably set, the
transient overhead shouldn't be critical.

The issue was discovered [1] by Sashiko.

6.6.y porting notes.  Modify damon_split_region_at() to return failures.
It is a part of commit c60874860762 ("mm/damon/core: handle region split
failure in apply_min_nr_regions()").  Without it, build fails.

Link: https://lore.kernel.org/20260901131850.98037-1-sj@kernel.org
Link: https://lore.kernel.org/20260901131850.98037-2-sj@kernel.org
Link: https://lore.kernel.org/20260718171523.87547-1-sj@kernel.org [1]
Fixes: 2b8a248d5873 ("mm/damon/schemes: implement size quota for schemes application speed control")
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: <stable@vger.kernel.org> # 5.16.x
(cherry picked from commit 90c2fa6adfed7abe0935506a43cdff3a05c1fff0)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index b5b6d00d7a3b5..9d4a78deaa06a 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -828,7 +828,7 @@ static void kdamond_reset_aggregated(struct damon_ctx *c)
 	}
 }
 
-static void damon_split_region_at(struct damon_target *t,
+static int damon_split_region_at(struct damon_target *t,
 				  struct damon_region *r, unsigned long sz_r);
 
 static bool __damos_valid_target(struct damon_region *r, struct damos *s)
@@ -1004,7 +1004,8 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
 					DAMON_MIN_REGION);
 			if (!sz)
 				goto update_stat;
-			damon_split_region_at(t, r, sz);
+			if (damon_split_region_at(t, r, sz))
+				goto update_stat;
 		}
 		if (damos_filter_out(c, t, r, s))
 			return;
@@ -1266,15 +1267,17 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold,
  *
  * r		the region to be split
  * sz_r		size of the first sub-region that will be made
+ *
+ * Return: 0 on success, negative error code otherwise.
  */
-static void damon_split_region_at(struct damon_target *t,
+static int damon_split_region_at(struct damon_target *t,
 				  struct damon_region *r, unsigned long sz_r)
 {
 	struct damon_region *new;
 
 	new = damon_new_region(r->ar.start + sz_r, r->ar.end);
 	if (!new)
-		return;
+		return -ENOMEM;
 
 	r->ar.end = new->ar.start;
 
@@ -1283,6 +1286,7 @@ static void damon_split_region_at(struct damon_target *t,
 	new->nr_accesses = r->nr_accesses;
 
 	damon_insert_region(new, r, damon_next_region(r), t);
+	return 0;
 }
 
 /* Split every region in the given target into 'nr_subs' regions */
-- 
2.47.3

[RFC PATCH 6.6.y 2/3] mm/damon/core: filter out damos address filter-split failed region

[SJ Park]

When an address type DAMOS filter is configured, DAMOS tries to split the
region for the filter range, and apply DAMOS action to the filter-in
region.  The split operation (damon_split_region_at()) could fail from
allocation failure under extreme memory pressure.  In the case, DAMOS
action could be applied more or less than expected, ignoring the address
filter.

The issue only rarely happens under extreme memory pressure.  No crash or
leak happens.  Occasionally not applying the action to a few regions
shouldn't be a real problem.  But applying the action to a region that the
user has excluded via address filter could be a surprise depending on the
use case.  It is only theoretical.  No such real case is found yet.

Filter out the entire region if the split is failed.  Update
damos_filter_match() signature to receive the error from split.  Also
update kunit tests for the updated signature.

The issue was discovered [1] by sashiko.

Link: https://lore.kernel.org/20261020004119.106690-1-sj@kernel.org
Link: https://lore.kernel.org/20260801203121.117449-1-sj@kernel.org [1]
Fixes: ab9bda001b68 ("mm/damon/core: introduce address range type damos filter")
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: <stable@vger.kernel.org> # 6.6.x
(cherry picked from commit bbeec2d8324d7aac438bb12dd2d22b184e0a8d43)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core-test.h | 11 ++++++-----
 mm/damon/core.c      | 13 +++++++++----
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/mm/damon/core-test.h b/mm/damon/core-test.h
index dd91b0eea61ff..1988b7dd29e96 100644
--- a/mm/damon/core-test.h
+++ b/mm/damon/core-test.h
@@ -461,6 +461,7 @@ static void damos_test_filter_out(struct kunit *test)
 	struct damon_target *t;
 	struct damon_region *r, *r2;
 	struct damos_filter *f;
+	int err;
 
 	f = damos_new_filter(DAMOS_FILTER_TYPE_ADDR, true);
 	if (!f)
@@ -482,25 +483,25 @@ static void damos_test_filter_out(struct kunit *test)
 	damon_add_region(r, t);
 
 	/* region in the range */
-	KUNIT_EXPECT_TRUE(test, __damos_filter_out(NULL, t, r, f));
+	KUNIT_EXPECT_TRUE(test, __damos_filter_out(NULL, t, r, f, &err));
 	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 1);
 
 	/* region before the range */
 	r->ar.start = DAMON_MIN_REGION * 1;
 	r->ar.end = DAMON_MIN_REGION * 2;
-	KUNIT_EXPECT_FALSE(test, __damos_filter_out(NULL, t, r, f));
+	KUNIT_EXPECT_FALSE(test, __damos_filter_out(NULL, t, r, f, &err));
 	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 1);
 
 	/* region after the range */
 	r->ar.start = DAMON_MIN_REGION * 6;
 	r->ar.end = DAMON_MIN_REGION * 8;
-	KUNIT_EXPECT_FALSE(test, __damos_filter_out(NULL, t, r, f));
+	KUNIT_EXPECT_FALSE(test, __damos_filter_out(NULL, t, r, f, &err));
 	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 1);
 
 	/* region started before the range */
 	r->ar.start = DAMON_MIN_REGION * 1;
 	r->ar.end = DAMON_MIN_REGION * 4;
-	KUNIT_EXPECT_FALSE(test, __damos_filter_out(NULL, t, r, f));
+	KUNIT_EXPECT_FALSE(test, __damos_filter_out(NULL, t, r, f, &err));
 	/* filter should have split the region */
 	KUNIT_EXPECT_EQ(test, r->ar.start, DAMON_MIN_REGION * 1);
 	KUNIT_EXPECT_EQ(test, r->ar.end, DAMON_MIN_REGION * 2);
@@ -515,7 +516,7 @@ static void damos_test_filter_out(struct kunit *test)
 	/* region started in the range */
 	r->ar.start = DAMON_MIN_REGION * 2;
 	r->ar.end = DAMON_MIN_REGION * 8;
-	KUNIT_EXPECT_TRUE(test, __damos_filter_out(NULL, t, r, f));
+	KUNIT_EXPECT_TRUE(test, __damos_filter_out(NULL, t, r, f, &err));
 	/* filter should have split the region */
 	KUNIT_EXPECT_EQ(test, r->ar.start, DAMON_MIN_REGION * 2);
 	KUNIT_EXPECT_EQ(test, r->ar.end, DAMON_MIN_REGION * 6);
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 9d4a78deaa06a..99e934a6d1ce9 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -930,13 +930,14 @@ static void damos_update_stat(struct damos *s,
 }
 
 static bool __damos_filter_out(struct damon_ctx *ctx, struct damon_target *t,
-		struct damon_region *r, struct damos_filter *filter)
+		struct damon_region *r, struct damos_filter *filter, int *err)
 {
 	bool matched = false;
 	struct damon_target *ti;
 	int target_idx = 0;
 	unsigned long start, end;
 
+	*err = 0;
 	switch (filter->type) {
 	case DAMOS_FILTER_TYPE_TARGET:
 		damon_for_each_target(ti, ctx) {
@@ -962,12 +963,13 @@ static bool __damos_filter_out(struct damon_ctx *ctx, struct damon_target *t,
 		}
 		/* start before the range and overlap */
 		if (r->ar.start < start) {
-			damon_split_region_at(t, r, start - r->ar.start);
+			*err = damon_split_region_at(t, r,
+					start - r->ar.start);
 			matched = false;
 			break;
 		}
 		/* start inside the range */
-		damon_split_region_at(t, r, end - r->ar.start);
+		*err = damon_split_region_at(t, r, end - r->ar.start);
 		matched = true;
 		break;
 	default:
@@ -981,9 +983,12 @@ static bool damos_filter_out(struct damon_ctx *ctx, struct damon_target *t,
 		struct damon_region *r, struct damos *s)
 {
 	struct damos_filter *filter;
+	bool matching;
+	int err;
 
 	damos_for_each_filter(filter, s) {
-		if (__damos_filter_out(ctx, t, r, filter))
+		matching = __damos_filter_out(ctx, t, r, filter, &err);
+		if (matching || err)
 			return true;
 	}
 	return false;
-- 
2.47.3

[RFC PATCH 6.6.y 3/3] mm/damon/core: remove unreachable damos charge_{target,addr}_from

[SJ Park]

damos_quota->charge_{target,addr}_from saves the target and region the
scheme's quota limit is reached.  damos_skip_charged_region() skips the
target and regions before the point.  The target that was assigned to
charge_target_from could be dynamically removed from the context by the
runtime parameters update (aka commit) feature.  In this case,
damos_skip_charged_region() cannot find if the point is passed or not,
so the scheme is effectively disabled.

Similarly, the user could also remove all the regions of the target
using the runtime parameters update feature.  It would be uncommon to do
that, but it is a supported feature.  Because the skip pointer detection
logic (damos_skip_charged_region()) works for each region of targets,
this result in the logic is entirely skipped and the scheme is
effectively disabled again.

The user impact is that the DAMOS scheme could suddenly stop working
under certain uses of the runtime parameters update feature.  It could
surprise users. No crash or leak happens though.  Users could also
simply work around by re-installing the scheme.

Catch the corner cases in damos_adjust_quota() and reset the pointers.

Link: https://lore.kernel.org/20261020023958.108276-1-sj@kernel.org
Fixes: da87878010e5 ("mm/damon/sysfs: support online inputs update")
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Closes: https://sashiko.dev/#/bug/linux-cd0c55fc-0cc8-45d8-9637-3334811c49b8
Cc: <stable@vger.kernel.org> # 5.19.x
(cherry picked from commit 5695abc4e742b947f586bfb6a080a80ad9957e5d)
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 99e934a6d1ce9..61d45d40fc1d3 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1088,6 +1088,27 @@ static void damos_set_effective_quota(struct damos_quota *quota)
 	quota->esz = esz;
 }
 
+
+static void damos_fix_stale_charge_pointers(struct damon_ctx *c,
+		struct damos *s)
+{
+	struct damos_quota *quota;
+	struct damon_target *t;
+
+	quota = &s->quota;
+	if (!quota->charge_target_from)
+		return;
+	damon_for_each_target(t, c) {
+		if (t == quota->charge_target_from) {
+			if (damon_nr_regions(t) > 0)
+				return;
+			break;
+		}
+	}
+	quota->charge_target_from = NULL;
+	quota->charge_addr_from = 0;
+}
+
 static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 {
 	struct damos_quota *quota = &s->quota;
@@ -1115,6 +1136,8 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 		damos_set_effective_quota(quota);
 	}
 
+	damos_fix_stale_charge_pointers(c, s);
+
 	if (!c->ops.get_scheme_score)
 		return;
 
-- 
2.47.3

Re: [RFC PATCH 6.6.y 1/3] mm/damon/core: skip applying scheme if region split for quota fails

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] mm/damon: false tried stats on split failure in
  damos_apply_scheme()
--

commit ae5a2312a946a6108ba6414ca8a1f6fb984dd078
Author: SJ Park <sj@kernel.org>

mm/damon/core: skip applying scheme if region split for quota fails

This commit updates damon_split_region_at() to return an error code on
allocation failure and skips applying the DAMOS scheme in
damos_apply_scheme() when splitting a region to fit within the remaining
size quota fails.

Link: https://lore.kernel.org/20260901131850.98037-1-sj@kernel.org
Link: https://lore.kernel.org/20260901131850.98037-2-sj@kernel.org
Link: https://lore.kernel.org/20260718171523.87547-1-sj@kernel.org [1]

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index b5b6d00d7a3b5..9d4a78deaa06a 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -1004,7 +1004,8 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
>  					DAMON_MIN_REGION);
>  			if (!sz)
>  				goto update_stat;
> -			damon_split_region_at(t, r, sz);
> +			if (damon_split_region_at(t, r, sz))
> +				goto update_stat;
>  		}
>  		if (damos_filter_out(c, t, r, s))
>  			return;

[Severity: Medium]
Does jumping to update_stat when damon_split_region_at() fails inflate
s->stat.nr_tried and s->stat.sz_tried?

At this point sz is non-zero, so damos_update_stat(s, sz, sz_applied)
increments s->stat.nr_tried and adds the remaining quota size sz to
s->stat.sz_tried even though scheme application was skipped before
damos_filter_out() or c->ops.apply_scheme() ran.  Because quota->charged_sz
is not updated either, every subsequent matching region in the same walk
that fails to split will add sz to s->stat.sz_tried again.  Returning
directly instead of jumping to update_stat would avoid accounting the
skipped region as tried.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261107222202.331476-1-sj@kernel.org?part=1

Re: [RFC PATCH 6.6.y 3/3] mm/damon/core: remove unreachable damos charge_{target,addr}_from

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] mm/damon/core: dangling pointer ABA match in
  damos_fix_stale_charge_pointers()
--

commit f7e621ee98abae5b2fb7828ae72ea3e96d1173fd
Author: SJ Park <sj@kernel.org>

mm/damon/core: remove unreachable damos charge_{target,addr}_from

This commit adds damos_fix_stale_charge_pointers() to damos_adjust_quota() to
reset quota->charge_target_from and quota->charge_addr_from when the target is
no longer in the context or has no regions.

Link: https://lore.kernel.org/20261020023958.108276-1-sj@kernel.org

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 99e934a6d1ce9..61d45d40fc1d3 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -1088,6 +1088,27 @@ static void damos_set_effective_quota(struct damos_quota *quota)
>  	quota->esz = esz;
>  }
>  
> +
> +static void damos_fix_stale_charge_pointers(struct damon_ctx *c,
> +		struct damos *s)
> +{
> +	struct damos_quota *quota;
> +	struct damon_target *t;
> +
> +	quota = &s->quota;
> +	if (!quota->charge_target_from)
> +		return;
> +	damon_for_each_target(t, c) {
> +		if (t == quota->charge_target_from) {
> +			if (damon_nr_regions(t) > 0)
> +				return;
> +			break;
> +		}
> +	}
> +	quota->charge_target_from = NULL;
> +	quota->charge_addr_from = 0;
> +}

[Severity: Medium]
Can t == quota->charge_target_from falsely match a newly allocated
damon_target if the old one was freed and its slab slot reused before
damos_adjust_quota() runs?

When an online commit removes a target in damon_sysfs_apply_inputs(),
damon_sysfs_set_targets() frees the damon_target via damon_destroy_target(),
leaving quota->charge_target_from dangling, while damon_set_attrs() advances
s->next_apply_sis past ctx->passed_sample_intervals so kdamond_apply_schemes()
skips damos_adjust_quota() on that cycle.  If another target is added before
damos_adjust_quota() next runs, kmalloc() can allocate the new damon_target at
the same address, causing damos_fix_stale_charge_pointers() to return early
and leave the stale charge_target_from and charge_addr_from values active in
damos_skip_charged_region().  Clearing charge_target_from and charge_addr_from
when the target is destroyed during commit would avoid keeping a dangling
pointer.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261107222202.331476-1-sj@kernel.org?part=3

Re: [RFC PATCH 6.6.y 1/3] mm/damon/core: skip applying scheme if region split for quota fails

[SJ Park]

On Sat, 07 Nov 2026 22:33:38 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential
> issue to consider:
> - [Medium] mm/damon: false tried stats on split failure in
>   damos_apply_scheme()
> --
> 
> commit ae5a2312a946a6108ba6414ca8a1f6fb984dd078
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: skip applying scheme if region split for quota fails
> 
> This commit updates damon_split_region_at() to return an error code on
> allocation failure and skips applying the DAMOS scheme in
> damos_apply_scheme() when splitting a region to fit within the remaining
> size quota fails.
> 
> Link: https://lore.kernel.org/20260901131850.98037-1-sj@kernel.org
> Link: https://lore.kernel.org/20260901131850.98037-2-sj@kernel.org
> Link: https://lore.kernel.org/20260718171523.87547-1-sj@kernel.org [1]
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index b5b6d00d7a3b5..9d4a78deaa06a 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -1004,7 +1004,8 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
> >  					DAMON_MIN_REGION);
> >  			if (!sz)
> >  				goto update_stat;
> > -			damon_split_region_at(t, r, sz);
> > +			if (damon_split_region_at(t, r, sz))
> > +				goto update_stat;
> >  		}
> >  		if (damos_filter_out(c, t, r, s))
> >  			return;
> 
> [Severity: Medium]
> Does jumping to update_stat when damon_split_region_at() fails inflate
> s->stat.nr_tried and s->stat.sz_tried?
> 
> At this point sz is non-zero, so damos_update_stat(s, sz, sz_applied)
> increments s->stat.nr_tried and adds the remaining quota size sz to
> s->stat.sz_tried even though scheme application was skipped before
> damos_filter_out() or c->ops.apply_scheme() ran.  Because quota->charged_sz
> is not updated either, every subsequent matching region in the same walk
> that fails to split will add sz to s->stat.sz_tried again.  Returning
> directly instead of jumping to update_stat would avoid accounting the
> skipped region as tried.

Good finding, but no blocker.  The user impact is trivial.  Also same issue
exists in the mainline.  I will think about whether to fix it in mainline and
backport to stable.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261107222202.331476-1-sj@kernel.org?part=1
> 


Thanks,
SJ

Re: [RFC PATCH 6.6.y 3/3] mm/damon/core: remove unreachable damos charge_{target,addr}_from

[SJ Park]

On Sat, 07 Nov 2026 22:34:54 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential
> issue to consider:
> - [Medium] mm/damon/core: dangling pointer ABA match in
>   damos_fix_stale_charge_pointers()
> --
> 
> commit f7e621ee98abae5b2fb7828ae72ea3e96d1173fd
> Author: SJ Park <sj@kernel.org>
> 
> mm/damon/core: remove unreachable damos charge_{target,addr}_from
> 
> This commit adds damos_fix_stale_charge_pointers() to damos_adjust_quota() to
> reset quota->charge_target_from and quota->charge_addr_from when the target is
> no longer in the context or has no regions.
> 
> Link: https://lore.kernel.org/20261020023958.108276-1-sj@kernel.org
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 99e934a6d1ce9..61d45d40fc1d3 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> > @@ -1088,6 +1088,27 @@ static void damos_set_effective_quota(struct damos_quota *quota)
> >  	quota->esz = esz;
> >  }
> >  
> > +
> > +static void damos_fix_stale_charge_pointers(struct damon_ctx *c,
> > +		struct damos *s)
> > +{
> > +	struct damos_quota *quota;
> > +	struct damon_target *t;
> > +
> > +	quota = &s->quota;
> > +	if (!quota->charge_target_from)
> > +		return;
> > +	damon_for_each_target(t, c) {
> > +		if (t == quota->charge_target_from) {
> > +			if (damon_nr_regions(t) > 0)
> > +				return;
> > +			break;
> > +		}
> > +	}
> > +	quota->charge_target_from = NULL;
> > +	quota->charge_addr_from = 0;
> > +}
> 
> [Severity: Medium]
> Can t == quota->charge_target_from falsely match a newly allocated
> damon_target if the old one was freed and its slab slot reused before
> damos_adjust_quota() runs?
> 
> When an online commit removes a target in damon_sysfs_apply_inputs(),
> damon_sysfs_set_targets() frees the damon_target via damon_destroy_target(),
> leaving quota->charge_target_from dangling, while damon_set_attrs() advances
> s->next_apply_sis past ctx->passed_sample_intervals so kdamond_apply_schemes()
> skips damos_adjust_quota() on that cycle.  If another target is added before
> damos_adjust_quota() next runs, kmalloc() can allocate the new damon_target at
> the same address, causing damos_fix_stale_charge_pointers() to return early
> and leave the stale charge_target_from and charge_addr_from values active in
> damos_skip_charged_region().  Clearing charge_target_from and charge_addr_from
> when the target is destroyed during commit would avoid keeping a dangling
> pointer.

Good finding, but no real issue.  Occasionally starting quota charging from
unexpected target is no real issue in real DAMOS use case in my opinion.  Also,
same behavior in the mainline.  No blocker for this patch.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261107222202.331476-1-sj@kernel.org?part=3
> 


Thanks,
SJ