[PATCH 00/10] mm/damon: let DAMON be paused and resumed

[PATCH 00/10] mm/damon: let DAMON be paused and resumed

[SeongJae Park]

DAMON utilizes a few mechanisms that enhance itself over time. Adaptive
regions adjustment, goal-based DAMOS quota auto-tuning and monitoring
intervals auto-tuning like self-training mechanisms are such examples.
It also adds access frequency stability information (age) to the
monitoring results, which makes it enhanced over time.

Sometimes users have to stop DAMON.  In this case, DAMON internal state
that enhanced over the time of the last execution simply goes away.
Restarted DAMON have to train itself and enhance its output from the
scratch.  This makes DAMON less useful in such cases.  Introducing three
such use cases below.

Investigation of DAMON.  It is best to do the investigation online,
especially when it is a production environment.  DAMON therefore
provides features for such online investigations, including DAMOS stats,
monitoring result snapshot exposure, and multiple tracepoints.  When
those are insufficient, and there are additional clues that could be
interfered by DAMON, users have to temporarily stop DAMON to collect the
additional clues.  It is not very useful since many of DAMON internal
clues are gone when DAMON is stopped.  The loss of the monitoring
results that improved over time is also problematic, especially in
production environments.

Monitoring of workloads that have different user-known phases.  For
example, in Android, applications are known to have very different
access patterns and behaviors when they are running on the foreground
and the background.  It can therefore be useful to separate monitoring
of apps based on whether they are running on the foreground and on the
background.  Having two DAMON threads per application that paused and
resumed for the apps foreground/background switches can be useful for
the purpose.  But such pause/resume of the execution is not supported.

Tests of DAMON.  A few DAMON selftests are using drgn to dump the
internal DAMON status.  The tests show if the dumped status is the same
as what the test code expected.  Because DAMON keeps running and
modifying its internal status, there are chances of data races that can
cause false test results.  Stopping DAMON can avoid the race.  But,
since the internal state of DAMON is dropped, the test coverage will be
limited.

Let DAMON execution be paused and resumed without loss of the internal
state, to overhaul the limitations.  For this, introduce a new DAMON
context parameter, namely 'pause'.  API callers can update it while the
context is running, using the online parameters update functions
(damon_commit_ctx() and damon_call()).  Once it is set, kdamond_fn()
main loop will do only limited works excluding the monitoring and DAMOS
works, while sleeping sampling intervals per the work.  The limited
works include handling of the online parameters update.  Hence users can
unset the 'pause' parameter again.  Once it is unset, kdamond_fn() main
loop will do all the work again (resumed).  Under the paused state, it
also does stop condition checks and handling of it, so that paused DAMON
can also be stopped if needed.  Expose the feature to the user space via
DAMON sysfs interface.  Also, update existing drgn-based tests to test
and use the feature.

Tests
=====

I confirmed the feature functionality using real time tracing ('perf
trace' or 'trace-cmd stream') of damon:damon_aggregated DAMON
tracepoint.  By pausing and resuming the DAMON execution, I was able to
see the trace stops and continued as expected.  Note that the pause
feature support is added to DAMON user-space tool (damo) after v3.1.9.
Users can use '--pause_ctx' command line option of damo for that, and I
actually used it for my test.  The extended drgn-based selftests are
also testing a part of the functionality.

Patches Sequence
================

Patch 1 introduces the new core API for the pause feature.  Patch 2
extend DAMON sysfs interface for the new parameter.  Patches 3-5 update
design, usage and ABI documents for the new sysfs file, respectively.
The following five patches are for tests.  Patch 6 implements a new
kunit test for the pause parameter online commitment.  Patches 7 and 8
extend DAMON selftest helpers to support the new feature.  Patch 9
extends selftest to test the commitment of the feature.  Finally, patch
10 updates existing selftest to be safe from the race condition using
the pause/resume feature.

Changelog
=========

Changes from RFC v2
(https://lore.kernel.org/20260319052157.99433-1-sj@kernel.org)
- Move damon_ctx->pause to public fields section.
- Wordsmith design doc change.
- Fix unintended resume of contexts in multiple contexts use case.
- Rebase to latest mm-new.
Changes from RFC v1
(https://lore.kernel.org/20260315210012.94846-1-sj@kernel.org)
- Continuously cancel new damos_walk() requests when paused.
- Initialize damon_sysfs_context->pause.
- Make sysfs.py dump-purpose pausing to work for all contexts.

SeongJae Park (10):
  mm/damon/core: introduce damon_ctx->paused
  mm/damon/sysfs: add pause file under context dir
  Docs/mm/damon/design: update for context pause/resume feature
  Docs/admin-guide/mm/damon/usage: update for pause file
  Docs/ABI/damon: update for pause sysfs file
  mm/damon/tests/core-kunit: test pause commitment
  selftests/damon/_damon_sysfs: support pause file staging
  selftests/damon/drgn_dump_damon_status: dump pause
  selftests/damon/sysfs.py: check pause on assert_ctx_committed()
  selftets/damon/sysfs.py: pause DAMON before dumping status

 .../ABI/testing/sysfs-kernel-mm-damon         |  7 ++++
 Documentation/admin-guide/mm/damon/usage.rst  | 12 ++++--
 Documentation/mm/damon/design.rst             |  7 ++++
 include/linux/damon.h                         |  2 +
 mm/damon/core.c                               |  9 +++++
 mm/damon/sysfs.c                              | 31 ++++++++++++++++
 mm/damon/tests/core-kunit.h                   |  4 ++
 tools/testing/selftests/damon/_damon_sysfs.py | 10 ++++-
 .../selftests/damon/drgn_dump_damon_status.py |  1 +
 tools/testing/selftests/damon/sysfs.py        | 37 +++++++++++++++++++
 10 files changed, 115 insertions(+), 5 deletions(-)


base-commit: 7580a23dc3d9af23b4911f9cc1d9da2f519d63a5
-- 
2.47.3

[PATCH 01/10] mm/damon/core: introduce damon_ctx->paused

[SeongJae Park]

DAMON supports only start and stop of the execution.  When it is
stopped, its internal data that it self-trained goes away.  It will be
useful if the execution can be paused and resumed with the previous
self-trained data.

Introduce per-context API parameter, 'paused', for the purpose.  The
parameter can be set and unset while DAMON is running and paused, using
the online parameters commit helper functions (damon_commit_ctx() and
damon_call()).  Once 'paused' is set, the kdamond_fn() main loop does
only limited works with sampling interval sleep during the works.  The
limited works include the handling of the online parameters update, so
that users can unset the 'pause' and resume the execution when they
want.  It also keep checking DAMON stop conditions and handling of it,
so that DAMON can be stopped while paused if needed.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/damon.h | 2 ++
 mm/damon/core.c       | 9 +++++++++
 2 files changed, 11 insertions(+)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index d9a3babbafc16..ea1649a09395d 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -787,6 +787,7 @@ struct damon_attrs {
  * @ops:	Set of monitoring operations for given use cases.
  * @addr_unit:	Scale factor for core to ops address conversion.
  * @min_region_sz:	Minimum region size.
+ * @pause:	Pause kdamond main loop.
  * @adaptive_targets:	Head of monitoring targets (&damon_target) list.
  * @schemes:		Head of schemes (&damos) list.
  */
@@ -838,6 +839,7 @@ struct damon_ctx {
 	struct damon_operations ops;
 	unsigned long addr_unit;
 	unsigned long min_region_sz;
+	bool pause;
 
 	struct list_head adaptive_targets;
 	struct list_head schemes;
diff --git a/mm/damon/core.c b/mm/damon/core.c
index db6c67e52d2b8..ce010009ca136 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1349,6 +1349,7 @@ int damon_commit_ctx(struct damon_ctx *dst, struct damon_ctx *src)
 		if (err)
 			return err;
 	}
+	dst->pause = src->pause;
 	dst->ops = src->ops;
 	dst->addr_unit = src->addr_unit;
 	dst->min_region_sz = src->min_region_sz;
@@ -3003,6 +3004,14 @@ static int kdamond_fn(void *data)
 		kdamond_call(ctx, false);
 		if (ctx->maybe_corrupted)
 			break;
+		while (ctx->pause) {
+			if (kdamond_need_stop(ctx))
+				goto done;
+			kdamond_usleep(ctx->attrs.sample_interval);
+			/* allow caller unset pause via damon_call() */
+			kdamond_call(ctx, false);
+			damos_walk_cancel(ctx);
+		}
 		if (!list_empty(&ctx->schemes))
 			kdamond_apply_schemes(ctx);
 		else
-- 
2.47.3

[PATCH 02/10] mm/damon/sysfs: add pause file under context dir

[SeongJae Park]

Add pause DAMON sysfs file under the context directory.  It exposes the
damon_ctx->pause API parameter to the users so that they can use the
pause/resume feature.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 6a44a2f3d8fc9..51893abd09472 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -866,6 +866,7 @@ struct damon_sysfs_context {
 	struct damon_sysfs_attrs *attrs;
 	struct damon_sysfs_targets *targets;
 	struct damon_sysfs_schemes *schemes;
+	bool pause;
 };
 
 static struct damon_sysfs_context *damon_sysfs_context_alloc(
@@ -878,6 +879,7 @@ static struct damon_sysfs_context *damon_sysfs_context_alloc(
 	context->kobj = (struct kobject){};
 	context->ops_id = ops_id;
 	context->addr_unit = 1;
+	context->pause = false;
 	return context;
 }
 
@@ -1053,6 +1055,30 @@ static ssize_t addr_unit_store(struct kobject *kobj,
 	return count;
 }
 
+static ssize_t pause_show(struct kobject *kobj, struct kobj_attribute *attr,
+		char *buf)
+{
+	struct damon_sysfs_context *context = container_of(kobj,
+			struct damon_sysfs_context, kobj);
+
+	return sysfs_emit(buf, "%c\n", context->pause ? 'Y' : 'N');
+}
+
+static ssize_t pause_store(struct kobject *kobj, struct kobj_attribute *attr,
+		const char *buf, size_t count)
+{
+	struct damon_sysfs_context *context = container_of(kobj,
+			struct damon_sysfs_context, kobj);
+	bool pause;
+	int err = kstrtobool(buf, &pause);
+
+	if (err)
+		return err;
+	context->pause = pause;
+	return count;
+}
+
+
 static void damon_sysfs_context_release(struct kobject *kobj)
 {
 	kfree(container_of(kobj, struct damon_sysfs_context, kobj));
@@ -1067,10 +1093,14 @@ static struct kobj_attribute damon_sysfs_context_operations_attr =
 static struct kobj_attribute damon_sysfs_context_addr_unit_attr =
 		__ATTR_RW_MODE(addr_unit, 0600);
 
+static struct kobj_attribute damon_sysfs_context_pause_attr =
+		__ATTR_RW_MODE(pause, 0600);
+
 static struct attribute *damon_sysfs_context_attrs[] = {
 	&damon_sysfs_context_avail_operations_attr.attr,
 	&damon_sysfs_context_operations_attr.attr,
 	&damon_sysfs_context_addr_unit_attr.attr,
+	&damon_sysfs_context_pause_attr.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(damon_sysfs_context);
@@ -1470,6 +1500,7 @@ static int damon_sysfs_apply_inputs(struct damon_ctx *ctx,
 	if (sys_ctx->ops_id == DAMON_OPS_PADDR)
 		ctx->min_region_sz = max(
 				DAMON_MIN_REGION_SZ / sys_ctx->addr_unit, 1);
+	ctx->pause = sys_ctx->pause;
 	err = damon_sysfs_set_attrs(ctx, sys_ctx->attrs);
 	if (err)
 		return err;
-- 
2.47.3

[PATCH 03/10] Docs/mm/damon/design: update for context pause/resume feature

[SeongJae Park]

Update DAMON design document for the context execution pause/resume
feature.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/mm/damon/design.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index afc7d52bda2f7..510ec6375178d 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -19,6 +19,13 @@ types of monitoring.
 To know how user-space can do the configurations and start/stop DAMON, refer to
 :ref:`DAMON sysfs interface <sysfs_interface>` documentation.
 
+Users can also request each context execution to be paused and resumed.  When
+it is paused, the kdamond does nothing other than applying online parameter
+update.
+
+To know how user-space can pause/resume each context, refer to :ref:`DAMON
+sysfs context <sysfs_context>` usage documentation.
+
 
 Overall Architecture
 ====================
-- 
2.47.3

[PATCH 04/10] Docs/admin-guide/mm/damon/usage: update for pause file

[SeongJae Park]

Update DAMON usage document for the DAMON context execution pause/resume
feature.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/admin-guide/mm/damon/usage.rst | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst
index 534e1199cf091..bfdb717441f05 100644
--- a/Documentation/admin-guide/mm/damon/usage.rst
+++ b/Documentation/admin-guide/mm/damon/usage.rst
@@ -66,7 +66,8 @@ comma (",").
     │ :ref:`kdamonds <sysfs_kdamonds>`/nr_kdamonds
     │ │ :ref:`0 <sysfs_kdamond>`/state,pid,refresh_ms
     │ │ │ :ref:`contexts <sysfs_contexts>`/nr_contexts
-    │ │ │ │ :ref:`0 <sysfs_context>`/avail_operations,operations,addr_unit
+    │ │ │ │ :ref:`0 <sysfs_context>`/avail_operations,operations,addr_unit,
+    │ │ │ │   pause
     │ │ │ │ │ :ref:`monitoring_attrs <sysfs_monitoring_attrs>`/
     │ │ │ │ │ │ intervals/sample_us,aggr_us,update_us
     │ │ │ │ │ │ │ intervals_goal/access_bp,aggrs,min_sample_us,max_sample_us
@@ -194,9 +195,9 @@ details).  At the moment, only one context per kdamond is supported, so only
 contexts/<N>/
 -------------
 
-In each context directory, three files (``avail_operations``, ``operations``
-and ``addr_unit``) and three directories (``monitoring_attrs``, ``targets``,
-and ``schemes``) exist.
+In each context directory, four files (``avail_operations``, ``operations``,
+``addr_unit`` and ``pause``) and three directories (``monitoring_attrs``,
+``targets``, and ``schemes``) exist.
 
 DAMON supports multiple types of :ref:`monitoring operations
 <damon_design_configurable_operations_set>`, including those for virtual address
@@ -214,6 +215,9 @@ reading from the ``operations`` file.
 ``addr_unit`` file is for setting and getting the :ref:`address unit
 <damon_design_addr_unit>` parameter of the operations set.
 
+``pause`` file is for setting and getting the :ref:`pause request
+<damon_design_execution_model_and_data_structures>` parameter of the context.
+
 .. _sysfs_monitoring_attrs:
 
 contexts/<N>/monitoring_attrs/
-- 
2.47.3

[PATCH 05/10] Docs/ABI/damon: update for pause sysfs file

[SeongJae Park]

Update DAMON ABI document for the DAMON context execution pause/resume
feature.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/ABI/testing/sysfs-kernel-mm-damon | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-damon b/Documentation/ABI/testing/sysfs-kernel-mm-damon
index 2424237ebb105..7059f540940f0 100644
--- a/Documentation/ABI/testing/sysfs-kernel-mm-damon
+++ b/Documentation/ABI/testing/sysfs-kernel-mm-damon
@@ -84,6 +84,13 @@ Description:	Writing an integer to this file sets the 'address unit'
 		parameter of the given operations set of the context.  Reading
 		the file returns the last-written 'address unit' value.
 
+What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/pause
+Date:		Mar 2026
+Contact:	SeongJae Park <sj@kernel.org>
+Description:	Writing a boolean keyword to this file sets the 'pause' request
+		parameter for the context.  Reading the file returns the
+		last-written 'pause' value.
+
 What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/monitoring_attrs/intervals/sample_us
 Date:		Mar 2022
 Contact:	SeongJae Park <sj@kernel.org>
-- 
2.47.3

[PATCH 06/10] mm/damon/tests/core-kunit: test pause commitment

[SeongJae Park]

Add a kunit test for commitment of damon_ctx->pause parameter that can
be done using damon_commit_ctx().

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/tests/core-kunit.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index 9e5904c2beeb2..0030f682b23b7 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -1077,6 +1077,10 @@ static void damon_test_commit_ctx(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, damon_commit_ctx(dst, src), 0);
 	src->min_region_sz = 4095;
 	KUNIT_EXPECT_EQ(test, damon_commit_ctx(dst, src), -EINVAL);
+	src->min_region_sz = 4096;
+	src->pause = true;
+	KUNIT_EXPECT_EQ(test, damon_commit_ctx(dst, src), 0);
+	KUNIT_EXPECT_TRUE(test, dst->pause);
 	damon_destroy_ctx(src);
 	damon_destroy_ctx(dst);
 }
-- 
2.47.3

[PATCH 07/10] selftests/damon/_damon_sysfs: support pause file staging

[SeongJae Park]

DAMON test-purpose sysfs interface control Python module, _damon_sysfs,
is not supporting the newly added pause file.  Add the support of the
file, for future test and use of the feature.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 tools/testing/selftests/damon/_damon_sysfs.py | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/damon/_damon_sysfs.py b/tools/testing/selftests/damon/_damon_sysfs.py
index 2b4df655d9fd0..120b96ecbd741 100644
--- a/tools/testing/selftests/damon/_damon_sysfs.py
+++ b/tools/testing/selftests/damon/_damon_sysfs.py
@@ -604,10 +604,11 @@ class DamonCtx:
     targets = None
     schemes = None
     kdamond = None
+    pause = None
     idx = None
 
     def __init__(self, ops='paddr', monitoring_attrs=DamonAttrs(), targets=[],
-            schemes=[]):
+            schemes=[], pause=False):
         self.ops = ops
         self.monitoring_attrs = monitoring_attrs
         self.monitoring_attrs.context = self
@@ -622,6 +623,8 @@ class DamonCtx:
             scheme.idx = idx
             scheme.context = self
 
+        self.pause=pause
+
     def sysfs_dir(self):
         return os.path.join(self.kdamond.sysfs_dir(), 'contexts',
                 '%d' % self.idx)
@@ -662,6 +665,11 @@ class DamonCtx:
             err = scheme.stage()
             if err is not None:
                 return err
+
+        err = write_file(os.path.join(self.sysfs_dir(), 'pause'), self.pause)
+        if err is not None:
+            return err
+
         return None
 
 class Kdamond:
-- 
2.47.3

[PATCH 08/10] selftests/damon/drgn_dump_damon_status: dump pause

[SeongJae Park]

drgn_dump_damon_status is not dumping the damon_ctx->pause parameter
value, so it cannot be tested.  Dump it for future tests.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 tools/testing/selftests/damon/drgn_dump_damon_status.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/damon/drgn_dump_damon_status.py b/tools/testing/selftests/damon/drgn_dump_damon_status.py
index af99b07a4f565..5b90eb8e7ef88 100755
--- a/tools/testing/selftests/damon/drgn_dump_damon_status.py
+++ b/tools/testing/selftests/damon/drgn_dump_damon_status.py
@@ -200,6 +200,7 @@ def damon_ctx_to_dict(ctx):
         ['attrs', attrs_to_dict],
         ['adaptive_targets', targets_to_list],
         ['schemes', schemes_to_list],
+        ['pause', bool],
         ])
 
 def main():
-- 
2.47.3

[PATCH 09/10] selftests/damon/sysfs.py: check pause on assert_ctx_committed()

[SeongJae Park]

Extend sysfs.py tests to confirm damon_ctx->pause can be set using the
pause sysfs file.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 tools/testing/selftests/damon/sysfs.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/damon/sysfs.py b/tools/testing/selftests/damon/sysfs.py
index 3aa5c91548a53..e6d34ba05893f 100755
--- a/tools/testing/selftests/damon/sysfs.py
+++ b/tools/testing/selftests/damon/sysfs.py
@@ -190,6 +190,7 @@ def assert_ctx_committed(ctx, dump):
     assert_monitoring_attrs_committed(ctx.monitoring_attrs, dump['attrs'])
     assert_monitoring_targets_committed(ctx.targets, dump['adaptive_targets'])
     assert_schemes_committed(ctx.schemes, dump['schemes'])
+    assert_true(dump['pause'] == ctx.pause, 'pause', dump)
 
 def assert_ctxs_committed(kdamonds):
     status, err = dump_damon_status_dict(kdamonds.kdamonds[0].pid)
-- 
2.47.3

[PATCH 10/10] selftets/damon/sysfs.py: pause DAMON before dumping status

[SeongJae Park]

The sysfs.py test commits DAMON parameters, dump the internal DAMON
state, and show if the parameters are committed as expected using the
dumped state.  While the dumping is ongoing, DAMON is alive.  It can
make internal changes including addition and removal of regions.  It can
therefore make a race that can result in false test results.  Pause
DAMON execution during the state dumping to avoid such races.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 tools/testing/selftests/damon/sysfs.py | 36 ++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/tools/testing/selftests/damon/sysfs.py b/tools/testing/selftests/damon/sysfs.py
index e6d34ba05893f..7a4cd915d5bc9 100755
--- a/tools/testing/selftests/damon/sysfs.py
+++ b/tools/testing/selftests/damon/sysfs.py
@@ -193,18 +193,53 @@ def assert_ctx_committed(ctx, dump):
     assert_true(dump['pause'] == ctx.pause, 'pause', dump)
 
 def assert_ctxs_committed(kdamonds):
+    ctxs_paused_for_dump = []
+    # pause for safe state dumping
+    for kd in kdamonds.kdamonds:
+        for ctx in kd.contexts:
+            if ctx.pause is False:
+                ctx.pause = True
+                ctxs_paused_for_dump.append(ctx)
+        if len(ctxs_paused_for_dump) > 0:
+            err = kd.commit()
+            if err is not None:
+                print('pause fail (%s)' % err)
+                kdamonds.stop()
+                exit(1)
+
     status, err = dump_damon_status_dict(kdamonds.kdamonds[0].pid)
     if err is not None:
         print(err)
         kdamonds.stop()
         exit(1)
 
+    # resume contexts paused for safe state dumping
+    for kd in kdamonds.kdamonds:
+        for ctx in ctxs_paused_for_dump:
+            ctx.pause = False
+        if len(ctxs_paused_for_dump) > 0:
+            err = kd.commit()
+            if err is not None:
+                print('resume fail (%s)' % err)
+                kdamonds.stop()
+                exit(1)
+
+    # restore for comparison
+    for ctx in ctxs_paused_for_dump:
+        ctx.pause = True
+
     ctxs = kdamonds.kdamonds[0].contexts
     dump = status['contexts']
     assert_true(len(ctxs) == len(dump), 'ctxs length', dump)
     for idx, ctx in enumerate(ctxs):
         assert_ctx_committed(ctx, dump[idx])
 
+    # restore for the caller
+    for kd in kdamonds.kdamonds:
+        for ctx in kd.contexts:
+            if ctx in ctxs_paused_for_dump:
+                ctx.pause = False
+
 def main():
     kdamonds = _damon_sysfs.Kdamonds(
             [_damon_sysfs.Kdamond(
@@ -302,6 +337,7 @@ def main():
         print('kdamond start failed: %s' % err)
         exit(1)
     kdamonds.kdamonds[0].contexts[0].targets[1].obsolete = True
+    kdamonds.kdamonds[0].contexts[0].pause = True
     kdamonds.kdamonds[0].commit()
     del kdamonds.kdamonds[0].contexts[0].targets[1]
     assert_ctxs_committed(kdamonds)
-- 
2.47.3

Re: [PATCH 00/10] mm/damon: let DAMON be paused and resumed

[SeongJae Park]

Forwarding sashiko.dev review status for this thread.

# review url: https://sashiko.dev/#/patchset/20260321181343.93971-1-sj@kernel.org

- [PATCH 01/10] mm/damon/core: introduce damon_ctx->paused
  - status: Reviewed
- [PATCH 02/10] mm/damon/sysfs: add pause file under context dir
  - status: Reviewed
  - review: No issues found.
- [PATCH 03/10] Docs/mm/damon/design: update for context pause/resume feature
  - status: Reviewed
  - review: No issues found.
- [PATCH 04/10] Docs/admin-guide/mm/damon/usage: update for pause file
  - status: Reviewed
  - review: No issues found.
- [PATCH 05/10] Docs/ABI/damon: update for pause sysfs file
  - status: Reviewed
  - review: No issues found.
- [PATCH 06/10] mm/damon/tests/core-kunit: test pause commitment
  - status: Reviewed
  - review: No issues found.
- [PATCH 07/10] selftests/damon/_damon_sysfs: support pause file staging
  - status: Reviewed
  - review: No issues found.
- [PATCH 08/10] selftests/damon/drgn_dump_damon_status: dump pause
  - status: Reviewed
  - review: No issues found.
- [PATCH 09/10] selftests/damon/sysfs.py: check pause on assert_ctx_committed()
  - status: Reviewed
  - review: No issues found.
- [PATCH 10/10] selftets/damon/sysfs.py: pause DAMON before dumping status
  - status: Reviewed

# hkml [1] generated a draft of this mail.  It can be regenerated
# using below command:
#
#     hkml patch sashiko_dev --thread_status --for_forwarding \
#             20260321181343.93971-1-sj@kernel.org
#
# [1] https://github.com/sjp38/hackermail

Re: (sashiko) [PATCH 01/10] mm/damon/core: introduce damon_ctx->paused

[SeongJae Park]

Forwarding Sashiko review for doing discussions via mails.

# review url: https://sashiko.dev/#/patchset/20260321181343.93971-2-sj@kernel.org
# start of sashiko.dev inline review
commit b56d17c42c1dc6d883f3a75d785a05ba53b5c35b
Author: SeongJae Park <sj@kernel.org>

mm/damon/core: introduce damon_ctx->paused

This commit introduces a paused parameter to the DAMON context API.
It allows users to pause and resume kdamond execution while preserving
self-trained data. The main loop is updated to perform limited work
and sleep when the context is paused.

> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index db6c67e52d2b..ce010009ca13 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c

[ ... ]

> @@ -3003,6 +3004,14 @@ static int kdamond_fn(void *data)
>  		kdamond_call(ctx, false);
>  		if (ctx->maybe_corrupted)
>  			break;
> +		while (ctx->pause) {
> +			if (kdamond_need_stop(ctx))
> +				goto done;
> +			kdamond_usleep(ctx->attrs.sample_interval);

This isn't a bug, but polling with kdamond_usleep() using the sample
interval could cause many wakeups per second even while the context is
paused. Would it be more efficient to use a waitqueue here to consume
near-zero CPU until the thread is explicitly woken?

> +			/* allow caller unset pause via damon_call() */
> +			kdamond_call(ctx, false);

If a parameter update fails during kdamond_call() and sets
ctx->maybe_corrupted to true, the loop will continue to run or exit the
pause loop and proceed to kdamond_apply_schemes() with corrupted
structures.

Is it possible to bypass the corruption safeguard here? Should we check
ctx->maybe_corrupted inside this loop to safely terminate the thread?

> +			damos_walk_cancel(ctx);

If kdamond_call() processes a request that unpauses the context, this
will immediately cancel any pending or concurrent damos_walk() requests
before the loop condition is re-evaluated.

Could this cause spurious cancellations of walk requests when unpausing
the context?

> +		}
>  		if (!list_empty(&ctx->schemes))
>  			kdamond_apply_schemes(ctx);
>  		else


# end of sashiko.dev inline review
# review url: https://sashiko.dev/#/patchset/20260321181343.93971-2-sj@kernel.org
#
# hkml [1] generated a draft of this mail.  It can be regenerated
# using below command:
#
#     hkml patch sashiko_dev --for_forwarding \
#             20260321181343.93971-2-sj@kernel.org
#
# [1] https://github.com/sjp38/hackermail

Re: (sashiko) [PATCH 10/10] selftets/damon/sysfs.py: pause DAMON before dumping status

[SeongJae Park]

Forwarding Sashiko review for doing discussions via mails.

# review url: https://sashiko.dev/#/patchset/20260321181343.93971-11-sj@kernel.org
# start of sashiko.dev inline review
commit 019bf04e4448bbce4f6d092792a1b959f718f39a
Author: SeongJae Park <sj@kernel.org>

selftets/damon/sysfs.py: pause DAMON before dumping status

This isn't a bug, but there is a typo (selftets) in the subject line.

This commit updates the DAMON sysfs selftest to pause DAMON execution
during state dumping. This prevents potential races caused by internal
DAMON changes, such as the addition or removal of regions, while the
dump is ongoing.

> diff --git a/tools/testing/selftests/damon/sysfs.py b/tools/testing/selftests/damon/sysfs.py
> --- a/tools/testing/selftests/damon/sysfs.py
> +++ b/tools/testing/selftests/damon/sysfs.py
> @@ -193,18 +193,53 @@ def assert_ctx_committed(ctx, dump):
>      assert_true(dump['pause'] == ctx.pause, 'pause', dump)
>  
>  def assert_ctxs_committed(kdamonds):
> +    ctxs_paused_for_dump = []
> +    # pause for safe state dumping
> +    for kd in kdamonds.kdamonds:
> +        for ctx in kd.contexts:
> +            if ctx.pause is False:
> +                ctx.pause = True
> +                ctxs_paused_for_dump.append(ctx)
> +        if len(ctxs_paused_for_dump) > 0:
> +            err = kd.commit()

Since ctxs_paused_for_dump is initialized outside the outer loop, if the
first kdamond appends a context to this list, won't len(ctxs_paused_for_dump)
> 0 evaluate to true for all subsequent kdamonds?

This seems like it would trigger kd.commit() on every subsequent kdamond
even if none of its contexts were actually modified.

[ ... ]

> +    # resume contexts paused for safe state dumping
> +    for kd in kdamonds.kdamonds:
> +        for ctx in ctxs_paused_for_dump:
> +            ctx.pause = False
> +        if len(ctxs_paused_for_dump) > 0:
> +            err = kd.commit()

Does this inner loop repeatedly modify contexts across all kdamonds on
every outer iteration?

Because ctxs_paused_for_dump is shared across all kdamonds, this appears
to redundantly modify and commit contexts for every kdamond instance.
Could ctxs_paused_for_dump be scoped per-kdamond instead to avoid this?

[ ... ]

> +    # restore for comparison
> +    for ctx in ctxs_paused_for_dump:
> +        ctx.pause = True

Since the context was forced to pause before the status dump, and the
python object is forced to pause right here before the comparison,
doesn't this mean the test only ever asserts that True == True?

It appears this entirely skips verifying whether the originally requested
unpaused state was successfully committed to the kernel before the dump
procedure began. Any kernel bug where the pause parameter is stuck as
True might silently pass this test now.

Should the original state be verified by reading the specific sysfs file
before overriding it?

[ ... ]


# end of sashiko.dev inline review
# review url: https://sashiko.dev/#/patchset/20260321181343.93971-11-sj@kernel.org
#
# hkml [1] generated a draft of this mail.  It can be regenerated
# using below command:
#
#     hkml patch sashiko_dev --for_forwarding \
#             20260321181343.93971-11-sj@kernel.org
#
# [1] https://github.com/sjp38/hackermail

Re: (sashiko) [PATCH 01/10] mm/damon/core: introduce damon_ctx->paused

[SeongJae Park]

TL; DR: I think this needs a few changes.  That can be a fixup (picked by
Andrew) or respin for v2.

On Sat, 21 Mar 2026 13:12:04 -0700 SeongJae Park <sj@kernel.org> wrote:

> Forwarding Sashiko review for doing discussions via mails.
> 
> # review url: https://sashiko.dev/#/patchset/20260321181343.93971-2-sj@kernel.org
> # start of sashiko.dev inline review
> commit b56d17c42c1dc6d883f3a75d785a05ba53b5c35b
> Author: SeongJae Park <sj@kernel.org>
> 
> mm/damon/core: introduce damon_ctx->paused
> 
> This commit introduces a paused parameter to the DAMON context API.
> It allows users to pause and resume kdamond execution while preserving
> self-trained data. The main loop is updated to perform limited work
> and sleep when the context is paused.
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index db6c67e52d2b..ce010009ca13 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -3003,6 +3004,14 @@ static int kdamond_fn(void *data)
> >  		kdamond_call(ctx, false);
> >  		if (ctx->maybe_corrupted)
> >  			break;
> > +		while (ctx->pause) {
> > +			if (kdamond_need_stop(ctx))
> > +				goto done;
> > +			kdamond_usleep(ctx->attrs.sample_interval);
> 
> This isn't a bug, but polling with kdamond_usleep() using the sample
> interval could cause many wakeups per second even while the context is
> paused. Would it be more efficient to use a waitqueue here to consume
> near-zero CPU until the thread is explicitly woken?

sample internval is 5ms by default and recommended auto-tuning setup makes it
hundreds of milliseconds.  So I don't think such change is required.

> 
> > +			/* allow caller unset pause via damon_call() */
> > +			kdamond_call(ctx, false);
> 
> If a parameter update fails during kdamond_call() and sets
> ctx->maybe_corrupted to true, the loop will continue to run or exit the
> pause loop and proceed to kdamond_apply_schemes() with corrupted
> structures.
> 
> Is it possible to bypass the corruption safeguard here? Should we check
> ctx->maybe_corrupted inside this loop to safely terminate the thread?

Good finding.  I will add that in the next spin.

> 
> > +			damos_walk_cancel(ctx);
> 
> If kdamond_call() processes a request that unpauses the context, this
> will immediately cancel any pending or concurrent damos_walk() requests
> before the loop condition is re-evaluated.
> 
> Could this cause spurious cancellations of walk requests when unpausing
> the context?

As I replied in the previous patch, that can happen and fine.  Users can do the
walk again in the case.  But, now I agree it may better to reduce such spurious
failures if it can be done in a simple way.  And seems it can be.  Just doing
this before the sleep.

> 
> > +		}
> >  		if (!list_empty(&ctx->schemes))
> >  			kdamond_apply_schemes(ctx);
> >  		else
> 
> 
> # end of sashiko.dev inline review
> # review url: https://sashiko.dev/#/patchset/20260321181343.93971-2-sj@kernel.org

So, I think below attaching fixup is needed.  Unless Andrew picks it up
together, I will post v2 tomorrow.


Thanks,
SJ
=== >8 ===
From 9b07cde80e61bb4cc7a559a62465dd0ee6e2b945 Mon Sep 17 00:00:00 2001
From: SeongJae Park <sj@kernel.org>
Date: Sat, 21 Mar 2026 13:22:52 -0700
Subject: [PATCH] mm/damon/core: fixup: care corruption and cancel damos_walk()
 immediately

As Sashiko suggested and I agreed.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index ce010009ca136..0ab2cfa848e69 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3005,12 +3005,12 @@ static int kdamond_fn(void *data)
 		if (ctx->maybe_corrupted)
 			break;
 		while (ctx->pause) {
-			if (kdamond_need_stop(ctx))
-				goto done;
+			damos_walk_cancel(ctx);
 			kdamond_usleep(ctx->attrs.sample_interval);
 			/* allow caller unset pause via damon_call() */
 			kdamond_call(ctx, false);
-			damos_walk_cancel(ctx);
+			if (kdamond_need_stop(ctx) || ctx->maybe_corrupted)
+				goto done;
 		}
 		if (!list_empty(&ctx->schemes))
 			kdamond_apply_schemes(ctx);
-- 
2.47.3

Re: (sashiko) [PATCH 10/10] selftets/damon/sysfs.py: pause DAMON before dumping status

[SeongJae Park]

TL; DR: need to add below attaching fixup.  Unless Andrew pick it together with
this, I will post v2 tomorrow.

On Sat, 21 Mar 2026 13:12:49 -0700 SeongJae Park <sj@kernel.org> wrote:

> Forwarding Sashiko review for doing discussions via mails.
> 
> # review url: https://sashiko.dev/#/patchset/20260321181343.93971-11-sj@kernel.org
> # start of sashiko.dev inline review
> commit 019bf04e4448bbce4f6d092792a1b959f718f39a
> Author: SeongJae Park <sj@kernel.org>
> 
> selftets/damon/sysfs.py: pause DAMON before dumping status
> 
> This isn't a bug, but there is a typo (selftets) in the subject line.

Good eye!  Unless Andrew fixes this up, I will post v2 with the fix tomorrow.

> 
> This commit updates the DAMON sysfs selftest to pause DAMON execution
> during state dumping. This prevents potential races caused by internal
> DAMON changes, such as the addition or removal of regions, while the
> dump is ongoing.
> 
> > diff --git a/tools/testing/selftests/damon/sysfs.py b/tools/testing/selftests/damon/sysfs.py
> > --- a/tools/testing/selftests/damon/sysfs.py
> > +++ b/tools/testing/selftests/damon/sysfs.py
> > @@ -193,18 +193,53 @@ def assert_ctx_committed(ctx, dump):
> >      assert_true(dump['pause'] == ctx.pause, 'pause', dump)
> >  
> >  def assert_ctxs_committed(kdamonds):
> > +    ctxs_paused_for_dump = []
> > +    # pause for safe state dumping
> > +    for kd in kdamonds.kdamonds:
> > +        for ctx in kd.contexts:
> > +            if ctx.pause is False:
> > +                ctx.pause = True
> > +                ctxs_paused_for_dump.append(ctx)
> > +        if len(ctxs_paused_for_dump) > 0:
> > +            err = kd.commit()
> 
> Since ctxs_paused_for_dump is initialized outside the outer loop, if the
> first kdamond appends a context to this list, won't len(ctxs_paused_for_dump)
> > 0 evaluate to true for all subsequent kdamonds?
> 
> This seems like it would trigger kd.commit() on every subsequent kdamond
> even if none of its contexts were actually modified.

Good eye.  There is no multiple kdamonds use case here, but why not make the
code more complete?  I will fix this, like below.

'''
@@ -201,13 +201,16 @@ def assert_ctx_committed(ctx, dump):

 def assert_ctxs_committed(kdamonds):
     ctxs_paused_for_dump = []
+    kdamonds_paused_for_dump = []
     # pause for safe state dumping
     for kd in kdamonds.kdamonds:
         for ctx in kd.contexts:
             if ctx.pause is False:
                 ctx.pause = True
                 ctxs_paused_for_dump.append(ctx)
-        if len(ctxs_paused_for_dump) > 0:
+                if not kd in kdamonds_paused_for_dump:
+                    kdamonds_paused_for_dump.append(kd)
+        if kd in kdamonds_paused_for_dump:
             err = kd.commit()
             if err is not None:
                 print('pause fail (%s)' % err)
'''

Note that this is not a complete fix.  Similar change will be needed to be made
to later part of this file, too.


> 
> [ ... ]
> 
> > +    # resume contexts paused for safe state dumping
> > +    for kd in kdamonds.kdamonds:
> > +        for ctx in ctxs_paused_for_dump:
> > +            ctx.pause = False
> > +        if len(ctxs_paused_for_dump) > 0:
> > +            err = kd.commit()
> 
> Does this inner loop repeatedly modify contexts across all kdamonds on
> every outer iteration?
> 
> Because ctxs_paused_for_dump is shared across all kdamonds, this appears
> to redundantly modify and commit contexts for every kdamond instance.
> Could ctxs_paused_for_dump be scoped per-kdamond instead to avoid this?

Good eye!  I will fix this like below.

'''
@@ -221,15 +224,14 @@ def assert_ctxs_committed(kdamonds):
         exit(1)

     # resume contexts paused for safe state dumping
-    for kd in kdamonds.kdamonds:
-        for ctx in ctxs_paused_for_dump:
-            ctx.pause = False
-        if len(ctxs_paused_for_dump) > 0:
-            err = kd.commit()
-            if err is not None:
-                print('resume fail (%s)' % err)
-                kdamonds.stop()
-                exit(1)
+    for ctx in ctxs_paused_for_dump:
+        ctx.pause = False
+    if kd in kdamonds_paused_for_dump:
+        err = kd.commit()
+        if err is not None:
+            print('resume fail (%s)' % err)
+            kdamonds.stop()
+            exit(1)

     # restore for comparison
     for ctx in ctxs_paused_for_dump:
'''

> 
> [ ... ]
> 
> > +    # restore for comparison
> > +    for ctx in ctxs_paused_for_dump:
> > +        ctx.pause = True
> 
> Since the context was forced to pause before the status dump, and the
> python object is forced to pause right here before the comparison,
> doesn't this mean the test only ever asserts that True == True?
> 
> It appears this entirely skips verifying whether the originally requested
> unpaused state was successfully committed to the kernel before the dump
> procedure began. Any kernel bug where the pause parameter is stuck as
> True might silently pass this test now.
> 
> Should the original state be verified by reading the specific sysfs file
> before overriding it?

I agree there is the room to improve.  But, there is no good way to test the
feature for making the test safe, at the moment.  We have kunit test for the
pause commit feature, though.  So, I'd not add change for this to this patch.

> 
> [ ... ]
> 
> 
> # end of sashiko.dev inline review
> # review url: https://sashiko.dev/#/patchset/20260321181343.93971-11-sj@kernel.org

So, below attaching fixup is needed for this patch.  Unless Andrew pick it
together with this patch, I will post v2 tomorrow.


Thanks,
SJ

=== >8 ===
From 90acf3dacea715790622c0dd6cf97c6ed97d1105 Mon Sep 17 00:00:00 2001
From: SeongJae Park <sj@kernel.org>
Date: Sat, 21 Mar 2026 13:43:50 -0700
Subject: [PATCH] selftests/damon/sysfs.py: fixup: avoid unnecessary commit()
 and resume setup

As Sashiko suggested and I agreed.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 tools/testing/selftests/damon/sysfs.py | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/damon/sysfs.py b/tools/testing/selftests/damon/sysfs.py
index 7a4cd915d5bc9..5f00e97f019f4 100755
--- a/tools/testing/selftests/damon/sysfs.py
+++ b/tools/testing/selftests/damon/sysfs.py
@@ -194,13 +194,16 @@ def assert_ctx_committed(ctx, dump):
 
 def assert_ctxs_committed(kdamonds):
     ctxs_paused_for_dump = []
+    kdamonds_paused_for_dump = []
     # pause for safe state dumping
     for kd in kdamonds.kdamonds:
         for ctx in kd.contexts:
             if ctx.pause is False:
                 ctx.pause = True
                 ctxs_paused_for_dump.append(ctx)
-        if len(ctxs_paused_for_dump) > 0:
+                if not kd in kdamonds_paused_for_dump:
+                    kdamonds_paused_for_dump.append(kd)
+        if kd in kdamonds_paused_for_dump:
             err = kd.commit()
             if err is not None:
                 print('pause fail (%s)' % err)
@@ -214,15 +217,14 @@ def assert_ctxs_committed(kdamonds):
         exit(1)
 
     # resume contexts paused for safe state dumping
-    for kd in kdamonds.kdamonds:
-        for ctx in ctxs_paused_for_dump:
-            ctx.pause = False
-        if len(ctxs_paused_for_dump) > 0:
-            err = kd.commit()
-            if err is not None:
-                print('resume fail (%s)' % err)
-                kdamonds.stop()
-                exit(1)
+    for ctx in ctxs_paused_for_dump:
+        ctx.pause = False
+    if kd in kdamonds_paused_for_dump:
+        err = kd.commit()
+        if err is not None:
+            print('resume fail (%s)' % err)
+            kdamonds.stop()
+            exit(1)
 
     # restore for comparison
     for ctx in ctxs_paused_for_dump:
-- 
2.47.3

Re: (sashiko) [PATCH 01/10] mm/damon/core: introduce damon_ctx->paused

[SeongJae Park]

On Sat, 21 Mar 2026 13:28:57 -0700 SeongJae Park <sj@kernel.org> wrote:

> TL; DR: I think this needs a few changes.  That can be a fixup (picked by
> Andrew) or respin for v2.

We already have 436 patches in mm-unstable and 40 patches in mm-new [1].
That's 476 patches in mm-new.  The total number was 392 five days ago, which
already made Andrew concerned [2] a little bit.  I also agree the concern, so I
will stop sedning non-RFC non-hotfix DAMON patches until v7.1-rc1 merge window.

That applies to this series, too.  That is, I will post the next spin of this
series with RFC tag again, tomorrow.

[1] https://github.com/sjp38/mm_git_dashboard/blob/master/summary/summary.md
[2] https://lore.kernel.org/20260316142636.0c29d431b1c8bf03e01c3630@linux-foundation.org/


Thanks,
SJ

[...]