[RFC PATCH] mm/damon: reset thread status parameters upon kdamond termination

Liew Rui Yan
2026-03-30T16:43:52Z

Problem
=======
kdamond does not actively modify the module's
(DAMON_LRU_SORT/DAMON_RECLAIM) 'enabled' and 'kdamond_pid' after
exiting. After an unexpected termination, user will still see 'enabled'
as 'Y' and 'kdamond_pid' as a value other than -1. Furthermore, user
cannot restart the unexpectedly terminated kdamond by executing 'echo
Y/N > enabled' again.

Solution
========
Introduce a 'thread_status' structure to link the internal kdamond
state with module parameters ('enabled' and 'kdamond_pid').

Specifically:
1. Extend 'struct damon_ctx' to include pointers to the module's
parameters.
2. Initialize these pointers in damon_lru_sort_apply_parameters()
and damon_reclaim_apply_parameters() to point the respective
module variables.
3. Implement damon_update_thread_status() to reset 'enabled' to
false and 'kdamond_pid' to -1 when the kdamond thread finishes.

Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>
---
include/linux/damon.h | 7 +++++++
mm/damon/core.c | 19 ++++++++++++++++++-
mm/damon/lru_sort.c | 5 +++--
mm/damon/reclaim.c | 5 +++--
4 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index d9a3babbafc1..66cdac1eea93 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -763,6 +763,11 @@ struct damon_attrs {
unsigned long aggr_samples;
};

+struct damon_thread_status {
+ int *kdamond_pid;
+ bool *enabled;
+};
+
/**
* struct damon_ctx - Represents a context for each monitoring. This is the
* main interface that allows users to set the attributes and get the results
@@ -841,6 +846,8 @@ struct damon_ctx {

struct list_head adaptive_targets;
struct list_head schemes;
+
+ struct damon_thread_status thread_status;
};

static inline struct damon_region *damon_next_region(struct damon_region *r)
diff --git a/mm/damon/core.c b/mm/damon/core.c
index db6c67e52d2b..6c71203beec5 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1353,6 +1353,9 @@ int damon_commit_ctx(struct damon_ctx *dst, struct damon_ctx *src)
dst->addr_unit = src->addr_unit;
dst->min_region_sz = src->min_region_sz;

+ dst->thread_status.kdamond_pid = src->thread_status.kdamond_pid;
+ dst->thread_status.enabled = src->thread_status.enabled;
+
dst->maybe_corrupted = false;
return 0;
}
@@ -2941,6 +2944,14 @@ static void kdamond_init_ctx(struct damon_ctx *ctx)
}
}

+static void damon_update_thread_status(struct damon_ctx *ctx)
+{
+ if (ctx->thread_status.kdamond_pid)
+ *ctx->thread_status.kdamond_pid = -1;
+ if (ctx->thread_status.enabled)
+ *ctx->thread_status.enabled = false;
+}
+
/*
* The monitoring daemon that runs as a kernel thread
*/
@@ -3065,17 +3076,23 @@ static int kdamond_fn(void *data)
kdamond_call(ctx, true);
damos_walk_cancel(ctx);

- pr_debug("kdamond (%d) finishes\n", current->pid);
mutex_lock(&ctx->kdamond_lock);
ctx->kdamond = NULL;
mutex_unlock(&ctx->kdamond_lock);

+ if (ctx->thread_status.enabled && *ctx->thread_status.enabled)
+ pr_debug("kdamond (%d) crashed\n", current->pid);
+ else
+ pr_debug("kdamond (%d) finishes\n", current->pid);
+
mutex_lock(&damon_lock);
nr_running_ctxs--;
if (!nr_running_ctxs && running_exclusive_ctxs)
running_exclusive_ctxs = false;
mutex_unlock(&damon_lock);

+ damon_update_thread_status(ctx);
+
return 0;
}

diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
index 554559d72976..7196010ee9b4 100644
--- a/mm/damon/lru_sort.c
+++ b/mm/damon/lru_sort.c
@@ -294,6 +294,9 @@ static int damon_lru_sort_apply_parameters(void)
param_ctx->addr_unit = addr_unit;
param_ctx->min_region_sz = max(DAMON_MIN_REGION_SZ / addr_unit, 1);

+ param_ctx->thread_status.kdamond_pid = &kdamond_pid;
+ param_ctx->thread_status.enabled = &enabled;
+
if (!damon_lru_sort_mon_attrs.sample_interval) {
err = -EINVAL;
goto out;
@@ -388,8 +391,6 @@ static int damon_lru_sort_turn(bool on)

if (!on) {
err = damon_stop(&ctx, 1);
- if (!err)
- kdamond_pid = -1;
return err;
}

diff --git a/mm/damon/reclaim.c b/mm/damon/reclaim.c
index 86da14778658..61592be438d3 100644
--- a/mm/damon/reclaim.c
+++ b/mm/damon/reclaim.c
@@ -204,6 +204,9 @@ static int damon_reclaim_apply_parameters(void)
param_ctx->addr_unit = addr_unit;
param_ctx->min_region_sz = max(DAMON_MIN_REGION_SZ / addr_unit, 1);

+ param_ctx->thread_status.kdamond_pid = &kdamond_pid;
+ param_ctx->thread_status.enabled = &enabled;
+
if (!damon_reclaim_mon_attrs.aggr_interval) {
err = -EINVAL;
goto out;
@@ -290,8 +293,6 @@ static int damon_reclaim_turn(bool on)

if (!on) {
err = damon_stop(&ctx, 1);
- if (!err)
- kdamond_pid = -1;
return err;
}

--
2.53.0