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

[Liew Rui Yan <aethernet65535@gmail.com>]

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