[PATCH v1 0/2] mm/damon/core: fix the size charged for a filter-trimmed region

[PATCH v1 0/2] mm/damon/core: fix the size charged for a filter-trimmed region

[Donggeun Yoo]

An address range DAMOS filter that partially overlaps a monitoring region
splits it, so the scheme action reaches only one side of the boundary.
damos_apply_scheme() reads the region size before that split and never
re-reads it, so the quota charge and schemes/<S>/stats/sz_tried account for
the whole original region.

Patch 1 re-reads the size after the core filters have run.  Patch 2 adds
KUnit coverage for what the function charges and reports, for the trimmed
cases and for the cases that must not change.

Donggeun Yoo (2):
  mm/damon/core: charge only the part of a region the filter left
  mm/damon/tests/core-kunit: test the size charged for a filter-trimmed
    region

 mm/damon/core.c             |   1 +
 mm/damon/tests/core-kunit.h | 316 ++++++++++++++++++++++++++++++++++++
 2 files changed, 317 insertions(+)


base-commit: 892f5b3b07e5b07f2f29c87750ff94fa58dcab73
-- 
2.53.0

[PATCH v1 1/2] mm/damon/core: charge only the part of a region the filter left

[Donggeun Yoo]

An address range DAMOS filter that partially overlaps a monitoring
region splits the region at the filter boundary, so the scheme action is
applied to only one side of it.  damos_apply_scheme() reads the region
size once on entry, before damos_core_filter_out() performs that split.

The quota charge and the statistics update therefore account for the
region as it was before the trim.  schemes/<S>/stats/sz_tried counts
memory the filter excluded, which Documentation/mm/damon/design.rst says
is not counted as tried, and with quotas/bytes set the excluded part is
charged against the budget, throttling the scheme to a fraction of what
was configured.

Re-read the region size after the core filters have run.

Fixes: ab9bda001b68 ("mm/damon/core: introduce address range type damos filter")
Cc: <stable@vger.kernel.org> # 6.6.x
Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
---
KUnit, mm/damon/tests, x86_64, with patch 2 applied to both trees:

  without patch 1  36/38  damos_test_apply_scheme_filtered_sz and
                          damos_test_apply_scheme_filter_sz_unchanged fail
  with patch 1     38/38

  Expected s->stat.sz_tried == 2 * min_sz, but
      s->stat.sz_tried == 16384 (0x4000)
      2 * min_sz == 8192 (0x2000)

All 15 DAMON selftests pass with the series applied, in a QEMU guest booted
with slab_nomerge and CONFIG_DEBUG_INFO, the latter so drgn could run two of
them.  sysfs_update_schemes_tried_regions_wss_estimation.py and
damos_quota.py are timing sensitive and have each failed intermittently on
both patched and unpatched trees over 15 runs.

One path is not covered by patch 2: the quota split failing at
damon_split_region_at().  A KUnit case cannot make that allocation fail.  The
sibling exit, a quota remainder below one region, is covered and reaches the
same update_stat label.

 mm/damon/core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index dd4317df42ff..68f1749ded43 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2784,6 +2784,7 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
 		}
 		if (damos_core_filter_out(c, t, r, s))
 			return;
+		sz = damon_sz_region(r);
 		ktime_get_coarse_ts64(&begin);
 		trace_damos_before_apply(cidx, sidx, tidx, r, nr_accesses,
 				damon_nr_regions(t), do_trace);
-- 
2.53.0

Re: [PATCH v1 1/2] mm/damon/core: charge only the part of a region the filter left

[SJ Park]

On Thu, 17 Sep 2026 09:35:52 +0900 Donggeun Yoo <donggeunyoo.kernel@gmail.com> wrote:

> An address range DAMOS filter that partially overlaps a monitoring
> region splits the region at the filter boundary, so the scheme action is
> applied to only one side of it.  damos_apply_scheme() reads the region
> size once on entry, before damos_core_filter_out() performs that split.
> 
> The quota charge and the statistics update therefore account for the
> region as it was before the trim.  schemes/<S>/stats/sz_tried counts
> memory the filter excluded, which Documentation/mm/damon/design.rst says
> is not counted as tried, and with quotas/bytes set the excluded part is
> charged against the budget, throttling the scheme to a fraction of what
> was configured.

The user impact is that DAMOS could work unexpectedly slow, due to over-charged
quota.  Also DAMOS stat could be confusing.  No critical events such as crashes
or leask happen.

If above is not incorrect, I will add that to the commit message when I add
this patch to damon/next tree.  Let me know if anything is incorrect.

> 
> Re-read the region size after the core filters have run.

Thank you for finding and fixing this bug!

> 
> Fixes: ab9bda001b68 ("mm/damon/core: introduce address range type damos filter")
> Cc: <stable@vger.kernel.org> # 6.6.x
> Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>

Reviewed-by: SJ Park <sj@kernel.org>


Thanks,
SJ

[...]

Re: [PATCH v1 1/2] mm/damon/core: charge only the part of a region the filter left

[Donggeun Yoo]

On Wed, 16 Sep 2026 18:24:19 -0700, SJ Park wrote:
> The user impact is that DAMOS could work unexpectedly slow, due to
> over-charged quota.  Also DAMOS stat could be confusing.  No critical
> events such as crashes or leask happen.
>
> If above is not incorrect, I will add that to the commit message when I
> add this patch to damon/next tree.

That is correct, please add it.

The stale size is only ever arithmetic: damos_charge_quota() and
damos_update_stat() take it, and nothing dereferences or frees against
it, so there is no memory-safety consequence.

Thanks for the review.

[PATCH v1 2/2] mm/damon/tests/core-kunit: test the size charged for a filter-trimmed region

[Donggeun Yoo]

damos_test_filter_out() checks that an address range filter splits a
region at the filter boundary, and stops there.  Nothing checks what
damos_apply_scheme() then charges to the quota and reports as tried.

damos_test_apply_scheme_filtered_sz() covers the two ways such a filter
trims a region: one that starts before the filter's range, and one that
starts inside it.

damos_test_apply_scheme_filter_sz_unchanged() and
damos_test_apply_scheme_quota_sz() cover the cases whose accounting must
not change: a region wholly inside a reject range, one wholly inside an
allow range, two filters trimming in a single call, a filter type that
never splits, no filter at all, a region the quota trims, and a quota
remainder too small for one region.

Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>
---
 mm/damon/tests/core-kunit.h | 316 ++++++++++++++++++++++++++++++++++++
 1 file changed, 316 insertions(+)

diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index 5da84caf4124..82c542afaa60 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -1676,6 +1676,319 @@ static void damos_test_filter_out(struct kunit *test)
 	damos_free_filter(f);
 }
 
+static unsigned long damos_test_apply_scheme_stub(struct damon_ctx *c,
+		struct damon_target *t, struct damon_region *r,
+		struct damos *s, unsigned long *sz_filter_passed)
+{
+	return 0;
+}
+
+static void damos_test_apply_scheme_filtered_sz(struct kunit *test)
+{
+	struct damos_access_pattern pattern = {
+		.min_sz_region = 0,
+		.max_sz_region = ULONG_MAX,
+		.min_nr_accesses = 0,
+		.max_nr_accesses = UINT_MAX,
+		.min_age_region = 0,
+		.max_age_region = UINT_MAX,
+	};
+	unsigned long min_sz = DAMON_MIN_REGION_SZ;
+	struct damos_watermarks wmarks = {};
+	struct damos_quota quota = {};
+	struct damon_ctx *c;
+	struct damon_target *t;
+	struct damon_region *r;
+	struct damos_filter *f;
+	struct damos *s;
+
+	c = damon_new_ctx();
+	if (!c)
+		kunit_skip(test, "ctx alloc fail");
+	c->ops.apply_scheme = damos_test_apply_scheme_stub;
+
+	s = damon_new_scheme(&pattern, DAMOS_STAT, 0, &quota, &wmarks,
+			NUMA_NO_NODE);
+	if (!s) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "scheme alloc fail");
+	}
+	damon_add_scheme(c, s);
+
+	t = damon_new_target();
+	if (!t) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "target alloc fail");
+	}
+	damon_add_target(c, t);
+
+	f = damos_new_filter(DAMOS_FILTER_TYPE_ADDR, true, false);
+	if (!f) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "filter alloc fail");
+	}
+	f->addr_range = (struct damon_addr_range){
+		.start = 2 * min_sz,
+		.end = 8 * min_sz
+	};
+	damos_add_filter(s, f);
+	damos_set_filters_default_reject(s);
+
+	/* reject filter, region starting before the range */
+	r = damon_new_region(0, 4 * min_sz);
+	if (!r) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "region alloc fail");
+	}
+	damon_add_region(r, t);
+
+	damos_apply_scheme(c, t, r, s);
+	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 2);
+	KUNIT_EXPECT_EQ(test, r->ar.end, 2 * min_sz);
+	KUNIT_EXPECT_EQ(test, s->stat.sz_tried, 2 * min_sz);
+
+	if (damon_nr_regions(t) != 2)
+		goto out;
+	damon_destroy_region(damon_next_region(r), t);
+	damon_destroy_region(r, t);
+	s->stat = (struct damos_stat){};
+
+	/* allow filter, region starting inside the range */
+	f->allow = true;
+	damos_set_filters_default_reject(s);
+	r = damon_new_region(2 * min_sz, 10 * min_sz);
+	if (!r) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "region alloc fail");
+	}
+	damon_add_region(r, t);
+
+	damos_apply_scheme(c, t, r, s);
+	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 2);
+	KUNIT_EXPECT_EQ(test, r->ar.end, 8 * min_sz);
+	KUNIT_EXPECT_EQ(test, s->stat.sz_tried, 6 * min_sz);
+
+out:
+	damon_destroy_ctx(c);
+}
+
+static void damos_test_apply_scheme_filter_sz_unchanged(struct kunit *test)
+{
+	struct damos_access_pattern pattern = {
+		.min_sz_region = 0,
+		.max_sz_region = ULONG_MAX,
+		.min_nr_accesses = 0,
+		.max_nr_accesses = UINT_MAX,
+		.min_age_region = 0,
+		.max_age_region = UINT_MAX,
+	};
+	unsigned long min_sz = DAMON_MIN_REGION_SZ;
+	struct damos_watermarks wmarks = {};
+	struct damos_quota quota = {};
+	struct damos_filter *f, *f2;
+	struct damon_ctx *c;
+	struct damon_target *t;
+	struct damon_region *r;
+	struct damos *s;
+
+	c = damon_new_ctx();
+	if (!c)
+		kunit_skip(test, "ctx alloc fail");
+	c->ops.apply_scheme = damos_test_apply_scheme_stub;
+
+	s = damon_new_scheme(&pattern, DAMOS_STAT, 0, &quota, &wmarks,
+			NUMA_NO_NODE);
+	if (!s) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "scheme alloc fail");
+	}
+	damon_add_scheme(c, s);
+
+	t = damon_new_target();
+	if (!t) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "target alloc fail");
+	}
+	damon_add_target(c, t);
+
+	f = damos_new_filter(DAMOS_FILTER_TYPE_ADDR, true, false);
+	if (!f) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "filter alloc fail");
+	}
+	f->addr_range = (struct damon_addr_range){
+		.start = 2 * min_sz, .end = 8 * min_sz};
+	damos_add_filter(s, f);
+	damos_set_filters_default_reject(s);
+
+	/* wholly inside a reject range: not counted at all */
+	r = damon_new_region(4 * min_sz, 6 * min_sz);
+	if (!r) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "region alloc fail");
+	}
+	damon_add_region(r, t);
+
+	damos_apply_scheme(c, t, r, s);
+	KUNIT_EXPECT_EQ(test, s->stat.nr_tried, 0);
+	KUNIT_EXPECT_EQ(test, s->stat.sz_tried, 0);
+	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 1);
+
+	/* wholly inside an allow range: counted whole, not split */
+	f->allow = true;
+	damos_set_filters_default_reject(s);
+
+	damos_apply_scheme(c, t, r, s);
+	KUNIT_EXPECT_EQ(test, s->stat.sz_tried, 2 * min_sz);
+	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 1);
+
+	/* two filters, each trimming: the size left after both is counted */
+	damon_destroy_region(r, t);
+	s->stat = (struct damos_stat){};
+	f->allow = false;
+	f->addr_range = (struct damon_addr_range){
+		.start = 4 * min_sz, .end = 12 * min_sz};
+	f2 = damos_new_filter(DAMOS_FILTER_TYPE_ADDR, true, false);
+	if (!f2) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "filter alloc fail");
+	}
+	f2->addr_range = (struct damon_addr_range){
+		.start = 2 * min_sz, .end = 3 * min_sz};
+	damos_add_filter(s, f2);
+	damos_set_filters_default_reject(s);
+
+	r = damon_new_region(0, 8 * min_sz);
+	if (!r) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "region alloc fail");
+	}
+	damon_add_region(r, t);
+
+	damos_apply_scheme(c, t, r, s);
+	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 3);
+	KUNIT_EXPECT_EQ(test, r->ar.end, 2 * min_sz);
+	KUNIT_EXPECT_EQ(test, s->stat.sz_tried, 2 * min_sz);
+
+	/* a core filter that never splits: counted whole */
+	damos_destroy_filter(f2);
+	damos_destroy_filter(f);
+	f = damos_new_filter(DAMOS_FILTER_TYPE_TARGET, true, true);
+	if (!f) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "filter alloc fail");
+	}
+	f->target_idx = 0;
+	damos_add_filter(s, f);
+	damos_set_filters_default_reject(s);
+	s->stat = (struct damos_stat){};
+
+	damos_apply_scheme(c, t, r, s);
+	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 3);
+	KUNIT_EXPECT_EQ(test, r->ar.end, 2 * min_sz);
+	KUNIT_EXPECT_EQ(test, s->stat.sz_tried, 2 * min_sz);
+
+	damon_destroy_ctx(c);
+}
+
+static void damos_test_apply_scheme_quota_sz(struct kunit *test)
+{
+	struct damos_access_pattern pattern = {
+		.min_sz_region = 0,
+		.max_sz_region = ULONG_MAX,
+		.min_nr_accesses = 0,
+		.max_nr_accesses = UINT_MAX,
+		.min_age_region = 0,
+		.max_age_region = UINT_MAX,
+	};
+	unsigned long min_sz = DAMON_MIN_REGION_SZ;
+	struct damos_watermarks wmarks = {};
+	struct damos_quota quota = {};
+	struct damon_region *r, *next;
+	struct damon_ctx *c;
+	struct damon_target *t;
+	struct damos *s;
+
+	c = damon_new_ctx();
+	if (!c)
+		kunit_skip(test, "ctx alloc fail");
+
+	s = damon_new_scheme(&pattern, DAMOS_STAT, 0, &quota, &wmarks,
+			NUMA_NO_NODE);
+	if (!s) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "scheme alloc fail");
+	}
+	damon_add_scheme(c, s);
+	damos_set_filters_default_reject(s);
+
+	t = damon_new_target();
+	if (!t) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "target alloc fail");
+	}
+	damon_add_target(c, t);
+
+	/* no apply_scheme operation: the whole region is counted */
+	r = damon_new_region(0, 4 * min_sz);
+	if (!r) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "region alloc fail");
+	}
+	damon_add_region(r, t);
+
+	damos_apply_scheme(c, t, r, s);
+	KUNIT_EXPECT_EQ(test, s->stat.sz_tried, 4 * min_sz);
+	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 1);
+
+	/* no filter and no quota: the whole region is counted */
+	c->ops.apply_scheme = damos_test_apply_scheme_stub;
+	s->stat = (struct damos_stat){};
+
+	damos_apply_scheme(c, t, r, s);
+	KUNIT_EXPECT_EQ(test, s->stat.sz_tried, 4 * min_sz);
+	KUNIT_EXPECT_EQ(test, damon_nr_regions(t), 1);
+
+	/* the quota trims the region: the trimmed size is counted */
+	damon_for_each_region_safe(r, next, t)
+		damon_destroy_region(r, t);
+	s->stat = (struct damos_stat){};
+	s->quota.esz = 2 * min_sz;
+	s->quota.charged_sz = 0;
+
+	r = damon_new_region(0, 8 * min_sz);
+	if (!r) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "region alloc fail");
+	}
+	damon_add_region(r, t);
+
+	damos_apply_scheme(c, t, r, s);
+	KUNIT_EXPECT_EQ(test, r->ar.end, 2 * min_sz);
+	KUNIT_EXPECT_EQ(test, s->stat.sz_tried, 2 * min_sz);
+
+	/* quota remainder below one region: tried, but nothing counted */
+	damon_for_each_region_safe(r, next, t)
+		damon_destroy_region(r, t);
+	s->stat = (struct damos_stat){};
+	s->quota.esz = 1;
+	s->quota.charged_sz = 0;
+
+	r = damon_new_region(0, 4 * min_sz);
+	if (!r) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "region alloc fail");
+	}
+	damon_add_region(r, t);
+
+	damos_apply_scheme(c, t, r, s);
+	KUNIT_EXPECT_EQ(test, s->stat.nr_tried, 1);
+	KUNIT_EXPECT_EQ(test, s->stat.sz_tried, 0);
+	KUNIT_EXPECT_EQ(test, r->ar.end, 4 * min_sz);
+
+	damon_destroy_ctx(c);
+}
+
 static void damon_test_feed_loop_next_input(struct kunit *test)
 {
 	unsigned long last_input = 900000, current_score = 200;
@@ -1929,6 +2242,9 @@ static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_commit_ctx),
 	KUNIT_CASE(damon_test_valid_probe_params),
 	KUNIT_CASE(damos_test_filter_out),
+	KUNIT_CASE(damos_test_apply_scheme_filtered_sz),
+	KUNIT_CASE(damos_test_apply_scheme_filter_sz_unchanged),
+	KUNIT_CASE(damos_test_apply_scheme_quota_sz),
 	KUNIT_CASE(damon_test_feed_loop_next_input),
 	KUNIT_CASE(damon_test_set_filters_default_reject),
 	KUNIT_CASE(damon_test_apply_min_nr_regions),
-- 
2.53.0

Re: [PATCH v1 2/2] mm/damon/tests/core-kunit: test the size charged for a filter-trimmed region

[SJ Park]

On Thu, 17 Sep 2026 09:35:53 +0900 Donggeun Yoo <donggeunyoo.kernel@gmail.com> wrote:

> damos_test_filter_out() checks that an address range filter splits a
> region at the filter boundary, and stops there.  Nothing checks what
> damos_apply_scheme() then charges to the quota and reports as tried.
> 
> damos_test_apply_scheme_filtered_sz() covers the two ways such a filter
> trims a region: one that starts before the filter's range, and one that
> starts inside it.
> 
> damos_test_apply_scheme_filter_sz_unchanged() and
> damos_test_apply_scheme_quota_sz() cover the cases whose accounting must
> not change: a region wholly inside a reject range, one wholly inside an
> allow range, two filters trimming in a single call, a filter type that
> never splits, no filter at all, a region the quota trims, and a quota
> remainder too small for one region.

Thank you for adding these tests!  I to be honestly would prefer making the
code shorter, but I wouldn't block this.  We could cleanup later if it turns
out causing real readability issues.

> 
> Signed-off-by: Donggeun Yoo <donggeunyoo.kernel@gmail.com>

Reviewed-by: SJ Park <sj@kernel.org>


Thanks,
SJ

[...]

Re: [PATCH v1 0/2] mm/damon/core: fix the size charged for a filter-trimmed region

[SJ Park]

Hello Donggeun,

On Thu, 17 Sep 2026 09:35:51 +0900 Donggeun Yoo <donggeunyoo.kernel@gmail.com> wrote:

> An address range DAMOS filter that partially overlaps a monitoring region
> splits it, so the scheme action reaches only one side of the boundary.
> damos_apply_scheme() reads the region size before that split and never
> re-reads it, so the quota charge and schemes/<S>/stats/sz_tried account for
> the whole original region.
> 
> Patch 1 re-reads the size after the core filters have run.  Patch 2 adds
> KUnit coverage for what the function charges and reports, for the trimmed
> cases and for the cases that must not change.

This series is applied to damon/next [1] tree.  If this patch is not added to
mm.git in short term (~1 week?), I will ask mm.git maintainer (Andrew Morton)
to pick this.  So, no action from your side is needed for now.  If it seems I
also forgot doing that or you cannot wait for my action, please feel free to
ping me or directly ask that to Andrew.

[1] https://origin.kernel.org/doc/html/latest/mm/damon/maintainer-profile.html#scm-trees


Thanks,
SJ

[...]