[PATCH v3 0/2] mm/damon/core: detect internal variation above max_nr_regions/2

[PATCH v3 0/2] mm/damon/core: detect internal variation above max_nr_regions/2

[SJ Park]

kdamond_split_regions() bails out early when nr_regions is already
above max_nr_regions / 2.  A large region that picks up new internal
variation after that point never gets split, so we lose visibility
into its hot/cold structure.

We hit this with damon-paddr on hugepage workloads and damon-vaddr
on processes that mmap a large anonymous range.

Example with max_nr_regions == 1500.  A target ends up with 799
small hot/cold regions plus one big region (an earlier merge
collapsed a uniformly-accessed range into a single piece):

H:hot
C:cold

      r1     r2     r3                 r800
    HHHHHH|CCCCCC|HHHHHH|...|HHHHHH..........................|

    nr_regions = 800  >  max_nr_regions / 2 = 750

Now a cold subarea shows up inside r800:

      r1     r2     r3                 r800
    HHHHHH|CCCCCC|HHHHHH|...|HHHHHH........CCCCCC.............|

The small regions can't merge with each other (their access counts
differ), so budget never frees up.  r800 can't be split because
nr_regions > max_nr_regions / 2 returns early.  The cold subarea
stays invisible.

Patch 1 keeps refining on this path: when nr_regions is above
max_nr_regions / 2 but still under the maximum, it splits a fraction
of the regions instead of returning.  The fraction shrinks as the
remaining budget shrinks, so the count approaches max_nr_regions
smoothly.  A useless split is undone by the next merge cycle.

Patch 2 adds a KUnit test for the case where nr_regions is already
above max_nr_regions / 2.

Thanks to SJ for the suggestion to drive the split fraction
from the remaining budget rather than an age-based filter.

Changes from v2
- v2: https://lore.kernel.org/20260626085851.70754-1-jiayuan.chen@linux.dev
- Collect R-b: from SJ.
- Rebase to latest mm-new.
Changes from v1
- v1: https://lore.kernel.org/damon/20260521045236.115749-1-jiayuan.chen@linux.dev/
- Some feedback from SJ.

Jiayuan Chen (2):
  mm/damon/core: split a fraction of regions when nr_regions exceeds
    max/2
  mm/damon/tests/core-kunit: test split above max_nr_regions/2

 mm/damon/core.c             | 49 +++++++++++++++++++++++++---
 mm/damon/tests/core-kunit.h | 64 +++++++++++++++++++++++++++++++++++++
 2 files changed, 108 insertions(+), 5 deletions(-)


base-commit: 86100fb7e27ebb5da22fc8a2810eebcf8cc897e8
-- 
2.47.3

[PATCH v3 1/2] mm/damon/core: split a fraction of regions when nr_regions exceeds max/2

[SJ Park]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

kdamond_split_regions() returns early when nr_regions is above
max_nr_regions / 2, leaving internal access variation inside a large
region undetected.

Such a layout is common with damon-paddr on hugepage workloads or
damon-vaddr on processes with a large anonymous mmap.

For example, with max_nr_regions == 1500, a target may end up with
799 small alternating-temperature regions plus one large region that
absorbed a uniformly-accessed range during an earlier merge:

H:hot
C:cold

      r1     r2     r3                 r800
    HHHHHH|CCCCCC|HHHHHH|...|HHHHHH..........................|

    nr_regions = 800  >  max_nr_regions / 2 = 750

If a cold subarea later emerges inside r800:

      r1     r2     r3                 r800
    HHHHHH|CCCCCC|HHHHHH|...|HHHHHH........CCCCCC.............|

The small regions cannot merge with each other (different access
counts), so the budget stays full.  r800 cannot be split because
nr_regions > max_nr_regions / 2 causes an early return.  The cold
subarea is never discovered.

When nr_regions is above max_nr_regions / 2 but still under the
maximum, split only a fraction of the regions instead of returning.
One region in every 'max_nr_regions / budget' regions is split, where
budget is the remaining room (max_nr_regions - nr_regions), starting
from a rotating offset so different regions get picked over time.  The
fraction shrinks as the budget shrinks, so the region count keeps
refining while approaching max_nr_regions smoothly rather than
overshooting it.  An unnecessary split is reverted by the next
kdamond_merge_regions().

Link: https://lore.kernel.org/20260626085851.70754-2-jiayuan.chen@linux.dev
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Reviewed-by: SJ Park <sj@kernel.org>
Cc: SJ Park <sj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Shu Anzai <shu17az@gmail.com>
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 49 ++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 44 insertions(+), 5 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index ded76719e8a14..972a19fcee3ec 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3240,6 +3240,37 @@ static void damon_split_regions_of(struct damon_ctx *ctx,
 	}
 }
 
+/* Split one in every @split_step regions into two, from a rotating offset */
+static void damon_split_some_regions(struct damon_ctx *ctx,
+				     unsigned long split_step)
+{
+	static unsigned long rotation;
+	struct damon_target *t;
+	struct damon_region *r, *next;
+	unsigned long offset = rotation++ % split_step;
+	unsigned long idx = 0;
+
+	damon_for_each_target(t, ctx) {
+		damon_for_each_region_safe(r, next, t) {
+			unsigned long sz_region, sz_sub;
+
+			if (idx++ % split_step != offset)
+				continue;
+			sz_region = damon_sz_region(r);
+			if (sz_region < 2 * ctx->min_region_sz)
+				continue;
+
+			sz_sub = ALIGN_DOWN(damon_rand(ctx, 1, 10) *
+					sz_region / 10, ctx->min_region_sz);
+			/* Do not allow blank region */
+			if (sz_sub == 0 || sz_sub >= sz_region)
+				continue;
+
+			damon_split_region_at(t, r, sz_sub);
+		}
+	}
+}
+
 /*
  * Split every target region into randomly-sized small regions
  *
@@ -3253,25 +3284,33 @@ static void damon_split_regions_of(struct damon_ctx *ctx,
 static void kdamond_split_regions(struct damon_ctx *ctx)
 {
 	struct damon_target *t;
-	unsigned int nr_regions = 0;
-	static unsigned int last_nr_regions;
+	unsigned long nr_regions = 0;
+	unsigned long max_nr_regions = ctx->attrs.max_nr_regions;
+	static unsigned long last_nr_regions;
 	int nr_subregions = 2;
 
 	damon_for_each_target(t, ctx)
 		nr_regions += damon_nr_regions(t);
 
-	if (nr_regions > ctx->attrs.max_nr_regions / 2)
-		return;
+	if (nr_regions >= max_nr_regions)
+		goto done;
+
+	if (nr_regions > max_nr_regions / 2) {
+		damon_split_some_regions(ctx,
+			max_nr_regions / (max_nr_regions - nr_regions));
+		goto done;
+	}
 
 	/* Maybe the middle of the region has different access frequency */
 	if (last_nr_regions == nr_regions &&
-			nr_regions < ctx->attrs.max_nr_regions / 3)
+			nr_regions < max_nr_regions / 3)
 		nr_subregions = 3;
 
 	damon_for_each_target(t, ctx)
 		damon_split_regions_of(ctx, t, nr_subregions,
 				       ctx->min_region_sz);
 
+done:
 	last_nr_regions = nr_regions;
 }
 
-- 
2.47.3

[PATCH v3 2/2] mm/damon/tests/core-kunit: test split above max_nr_regions/2

[SJ Park]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

Add a test that exercises kdamond_split_regions() when the total
region count is already above max_nr_regions / 2, asserting that the
function still splits a fraction of the regions (makes progress) and
does not overshoot max_nr_regions.

The region size and min_region_sz are picked so the split arithmetic
does not depend on the page size.

All tests pass:
  damon: pass:31 fail:0 skip:0 total:31
  Totals: pass:31 fail:0 skip:0 total:31

Link: https://lore.kernel.org/20260626085851.70754-3-jiayuan.chen@linux.dev
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Reviewed-by: SJ Park <sj@kernel.org>
Cc: SJ Park <sj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Shu Anzai <shu17az@gmail.com>
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/tests/core-kunit.h | 64 +++++++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)

diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index c5f5124c3d1f4..a00168730445d 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -335,6 +335,69 @@ static void damon_test_split_regions_of(struct kunit *test)
 	damon_destroy_ctx(c);
 }
 
+/*
+ * When the total region count is already above max_nr_regions / 2,
+ * kdamond_split_regions() must keep refining the resolution by splitting a
+ * fraction of the regions (making progress), without exceeding
+ * max_nr_regions.
+ */
+static void damon_test_split_above_half_progresses(struct kunit *test)
+{
+	struct damon_ctx *c;
+	struct damon_target *t;
+	struct damon_region *r;
+	unsigned long start;
+	unsigned int nr_before, nr_after, i;
+	const unsigned int nr_init = 760;
+	const unsigned long region_sz = 100;
+
+	c = damon_new_ctx();
+	if (!c)
+		kunit_skip(test, "ctx alloc fail");
+
+	/* Keep the split arithmetic independent of the page size */
+	c->min_region_sz = 1;
+	c->attrs.min_nr_regions = 10;
+	c->attrs.max_nr_regions = 1500;
+
+	t = damon_new_target();
+	if (!t) {
+		damon_destroy_ctx(c);
+		kunit_skip(test, "target alloc fail");
+	}
+
+	for (i = 0; i < nr_init; i++) {
+		start = i * region_sz;
+		r = damon_new_region(start, start + region_sz);
+		if (!r) {
+			damon_free_target(t);
+			damon_destroy_ctx(c);
+			kunit_skip(test, "region alloc fail");
+		}
+		r->nr_accesses = (i & 1) ? 0 : 100;
+		r->age = 5;
+		damon_add_region(r, t);
+	}
+
+	damon_add_target(c, t);
+
+	nr_before = damon_nr_regions(t);
+	/* Above max_nr_regions / 2, so the blanket-split path is skipped */
+	KUNIT_EXPECT_GT(test, (unsigned long)nr_before,
+			c->attrs.max_nr_regions / 2);
+
+	kdamond_split_regions(c);
+
+	nr_after = damon_nr_regions(t);
+	/* Still made progress ... */
+	KUNIT_EXPECT_GT(test, nr_after, nr_before);
+	/* ... but did not overshoot the configured maximum */
+	KUNIT_EXPECT_LE(test, (unsigned long)nr_after,
+			c->attrs.max_nr_regions);
+
+	damon_destroy_ctx(c);
+}
+
 static void damon_test_ops_registration(struct kunit *test)
 {
 	struct damon_ctx *c = damon_new_ctx();
@@ -1491,6 +1554,7 @@ static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_merge_two),
 	KUNIT_CASE(damon_test_merge_regions_of),
 	KUNIT_CASE(damon_test_split_regions_of),
+	KUNIT_CASE(damon_test_split_above_half_progresses),
 	KUNIT_CASE(damon_test_ops_registration),
 	KUNIT_CASE(damon_test_set_regions),
 	KUNIT_CASE(damon_test_nr_accesses_to_accesses_bp),
-- 
2.47.3