FAILED: patch "[PATCH] mm/damon/reclaim: detect and use fresh enabled and" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 64a140afa5ed1c6f5ba6d451512cbdbbab1ba339
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026051255-roundup-crushing-4344@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 64a140afa5ed1c6f5ba6d451512cbdbbab1ba339 Mon Sep 17 00:00:00 2001
From: SeongJae Park <[email protected]>
Date: Sun, 19 Apr 2026 09:10:00 -0700
Subject: [PATCH] mm/damon/reclaim: detect and use fresh enabled and
kdamond_pid values

Patch series "mm/damon/modules: detect and use fresh status", v3.

DAMON modules including DAMON_RECLAIM, DAMON_LRU_SORT and DAMON_STAT commonly expose the kdamond running status via their parameters. Under certain scenarios including wrong user inputs and memory allocation failures, those parameter values can be stale. It can confuse users. For DAMON_RECLAIM and DAMON_LRU_SORT, it even makes the kdamond unable to be restarted before the system reboot.

The problem comes from the fact that there are multiple events for the status changes and it is difficult to follow up all the scenarios. Fix the issue by detecting and using the status on demand, instead of using a cached status that is difficult to be updated.

Patches 1-3 fix the bugs in DAMON_RECLAIM, DAMON_LRU_SORT and DAMON_STAT in the order.

This patch (of 3):

DAMON_RECLAIM updates 'enabled' and 'kdamond_pid' parameter values, which represents the running status of its kdamond, when the user explicitly requests start/stop of the kdamond. The kdamond can, however, be stopped in events other than the explicit user request in the following three events.

1. ctx->regions_score_histogram allocation failure at beginning of the execution,
2. damon_commit_ctx() failure due to invalid user input, and
3. damon_commit_ctx() failure due to its internal allocation failures.

Hence, if the kdamond is stopped by the above three events, the values of the status parameters can be stale. Users could show the stale values and be confused. This is already bad, but the real consequence is worse. DAMON_RECLAIM avoids unnecessary damon_start() and damon_stop() calls based on the 'enabled' parameter value. And the update of 'enabled' parameter value depends on the damon_start() and damon_stop() call results. Hence, once the kdamond has stopped by the unintentional events, the user cannot restart the kdamond before the system reboot. For example, the issue can be reproduced via below steps.

    # cd /sys/module/damon_reclaim/parameters
    #
    # # start DAMON_RECLAIM
    # echo Y > enabled
    # ps -ef | grep kdamond
    root         806       2  0 17:53 ?        00:00:00 [kdamond.0]
    root         808     803  0 17:53 pts/4    00:00:00 grep kdamond
    #
    # # commit wrong input to stop kdamond withou explicit stop request
    # echo 3 > addr_unit
    # echo Y > commit_inputs
    bash: echo: write error: Invalid argument
    #
    # # confirm kdamond is stopped
    # ps -ef | grep kdamond
    root         811     803  0 17:53 pts/4    00:00:00 grep kdamond
    #
    # # users casn now show stable status
    # cat enabled
    Y
    # cat kdamond_pid
    806
    #
    # # even after fixing the wrong parameter,
    # # kdamond cannot be restarted.
    # echo 1 > addr_unit
    # echo Y > enabled
    # ps -ef | grep kdamond
    root         815     803  0 17:54 pts/4    00:00:00 grep kdamond

The problem will only rarely happen in real and common setups for the following reasons. The allocation failures are unlikely in such setups since those allocations are arguably too small to fail. Also sane users on real production environments may not commit wrong input parameters. But once it happens, the consequence is quite bad. And the bug is a bug.

The issue stems from the fact that there are multiple events that can change the status, and following all the events is challenging. Dynamically detect and use the fresh status for the parameters when those are requested.

index 86da14778658..fe7fce26cf6c 100644
@@ -144,15 +144,6 @@ static unsigned long addr_unit __read_mostly = 1;
 static bool skip_anon __read_mostly;
 module_param(skip_anon, bool, 0600);
 
-/*
- * PID of the DAMON thread
- *
- * If DAMON_RECLAIM is enabled, this becomes the PID of the worker thread.
- * Else, -1.
- */
-static int kdamond_pid __read_mostly = -1;
-module_param(kdamond_pid, int, 0400);
-
 static struct damos_stat damon_reclaim_stat;
 DEFINE_DAMON_MODULES_DAMOS_STATS_PARAMS(damon_reclaim_stat,
 		reclaim_tried_regions, reclaimed_regions, quota_exceeds);

@@ -288,12 +279,8 @@ static int damon_reclaim_turn(bool on)
 {
 	int err;
 
-	if (!on) {
-		err = damon_stop(&ctx, 1);
-		if (!err)
-			kdamond_pid = -1;
-		return err;
-	}
+	if (!on)
+		return damon_stop(&ctx, 1);
 
 	err = damon_reclaim_apply_parameters();
 	if (err)

@@ -302,9 +289,6 @@ static int damon_reclaim_turn(bool on)
 	err = damon_start(&ctx, 1, true);
 	if (err)
 		return err;
-	kdamond_pid = damon_kdamond_pid(ctx);
-	if (kdamond_pid < 0)
-		return kdamond_pid;
 	return damon_call(ctx, &call_control);
 }
 

@@ -332,42 +316,83 @@ module_param_cb(addr_unit, &addr_unit_param_ops, &addr_unit, 0600);
 MODULE_PARM_DESC(addr_unit,
 	"Scale factor for DAMON_RECLAIM to ops address conversion (default: 1)");
 
+static bool damon_reclaim_enabled(void)
+{
+	if (!ctx)
+		return false;
+	return damon_is_running(ctx);
+}
+
 static int damon_reclaim_enabled_store(const char *val,
 		const struct kernel_param *kp)
 {
-	bool is_enabled = enabled;
-	bool enable;
 	int err;
 
-	err = kstrtobool(val, &enable);
+	err = kstrtobool(val, &enabled);
 	if (err)
 		return err;
 
-	if (is_enabled == enable)
+	if (damon_reclaim_enabled() == enabled)
 		return 0;
 
 	/* Called before init function.  The function will handle this. */
 	if (!damon_initialized())
-		goto set_param_out;
+		return 0;
 
-	err = damon_reclaim_turn(enable);
-	if (err)
-		return err;
+	return damon_reclaim_turn(enabled);
+}
 
-set_param_out:
-	enabled = enable;
-	return err;
+static int damon_reclaim_enabled_load(char *buffer,
+		const struct kernel_param *kp)
+{
+	return sprintf(buffer, "%c\n", damon_reclaim_enabled() ? 'Y' : 'N');
 }
 
 static const struct kernel_param_ops enabled_param_ops = {
 	.set = damon_reclaim_enabled_store,
-	.get = param_get_bool,
+	.get = damon_reclaim_enabled_load,
 };
 
 module_param_cb(enabled, &enabled_param_ops, &enabled, 0600);
 MODULE_PARM_DESC(enabled,
 	"Enable or disable DAMON_RECLAIM (default: disabled)");
 
+static int damon_reclaim_kdamond_pid_store(const char *val,
+		const struct kernel_param *kp)
+{
+	/*
+	 * kdamond_pid is read-only, but kernel command line could write it.
+	 * Do nothing here.
+	 */
+	return 0;
+}
+
+static int damon_reclaim_kdamond_pid_load(char *buffer,
+		const struct kernel_param *kp)
+{
+	int kdamond_pid = -1;
+
+	if (ctx) {
+		kdamond_pid = damon_kdamond_pid(ctx);
+		if (kdamond_pid < 0)
+			kdamond_pid = -1;
+	}
+	return sprintf(buffer, "%d\n", kdamond_pid);
+}
+
+static const struct kernel_param_ops kdamond_pid_param_ops = {
+	.set = damon_reclaim_kdamond_pid_store,
+	.get = damon_reclaim_kdamond_pid_load,
+};
+
+/*
+ * PID of the DAMON thread
+ *
+ * If DAMON_RECLAIM is enabled, this becomes the PID of the worker thread.
+ * Else, -1.
+ */
+module_param_cb(kdamond_pid, &kdamond_pid_param_ops, NULL, 0400);
+
 static int __init damon_reclaim_init(void)
 {
 	int err;

DAMON_LRU_SORT updates 'enabled' and 'kdamond_pid' parameter values, which represents the running status of its kdamond, when the user explicitly requests start/stop of the kdamond. The kdamond can, however, be stopped in events other than the explicit user request in the following three events.

1. ctx->regions_score_histogram allocation failure at beginning of the execution,
2. damon_commit_ctx() failure due to invalid user input, and
3. damon_commit_ctx() failure due to its internal allocation failures.

Hence, if the kdamond is stopped by the above three events, the values of the status parameters can be stale. Users could show the stale values and be confused. This is already bad, but the real consequence is worse. DAMON_LRU_SORT avoids unnecessary damon_start() and damon_stop() calls based on the 'enabled' parameter value. And the update of 'enabled' parameter value depends on the damon_start() and damon_stop() call results. Hence, once the kdamond has stopped by the unintentional events, the user cannot restart the kdamond before the system reboot. For example, the issue can be reproduced via below steps.

    # cd /sys/module/damon_lru_sort/parameters
    #
    # # start DAMON_LRU_SORT
    # echo Y > enabled
    # ps -ef | grep kdamond
    root         806       2  0 17:53 ?        00:00:00 [kdamond.0]
    root         808     803  0 17:53 pts/4    00:00:00 grep kdamond
    #
    # # commit wrong input to stop kdamond withou explicit stop request
    # echo 3 > addr_unit
    # echo Y > commit_inputs
    bash: echo: write error: Invalid argument
    #
    # # confirm kdamond is stopped
    # ps -ef | grep kdamond
    root         811     803  0 17:53 pts/4    00:00:00 grep kdamond
    #
    # # users casn now show stable status
    # cat enabled
    Y
    # cat kdamond_pid
    806
    #
    # # even after fixing the wrong parameter,
    # # kdamond cannot be restarted.
    # echo 1 > addr_unit
    # echo Y > enabled
    # ps -ef | grep kdamond
    root         815     803  0 17:54 pts/4    00:00:00 grep kdamond

The problem will only rarely happen in real and common setups for the following reasons. The allocation failures are unlikely in such setups since those allocations are arguably too small to fail. Also sane users on real production environments may not commit wrong input parameters. But once it happens, the consequence is quite bad. And the bug is a bug.

The issue stems from the fact that there are multiple events that can change the status, and following all the events is challenging. Dynamically detect and use the fresh status for the parameters when those are requested.

(port parts of 42b7491af14c ("mm/damon/core: introduce damon_call()") and d2b5be741a50 ("mm/damon/sysfs: use DAMON core API damon_is_running()") for damon_is_running() dependency)
Cc: Liew Rui Yan <[email protected]>
Cc: Andrew Morton <[email protected]>
---
This depends on other two backported patches [1,2]. Please apply this after those.

[1] https://lore.kernel.org/[email protected]
[2] https://lore.kernel.org/[email protected]

 include/linux/damon.h |  1 +
 mm/damon/core.c       | 16 ++++++++
 mm/damon/lru_sort.c   | 88 +++++++++++++++++++++++++++----------------
 3 files changed, 73 insertions(+), 32 deletions(-)

index 77f8f05cf26fb..e92e9e8a81375 100644
@@ -778,6 +778,7 @@ static inline unsigned int damon_max_nr_accesses(const struct damon_attrs *attrs
 
 int damon_start(struct damon_ctx **ctxs, int nr_ctxs, bool exclusive);
 int damon_stop(struct damon_ctx **ctxs, int nr_ctxs);
+bool damon_is_running(struct damon_ctx *ctx);
 int damon_kdamond_pid(struct damon_ctx *ctx);
 
 int damon_set_region_biggest_system_ram_default(struct damon_target *t,

index 75d391b4ad72f..df721c13c2ca8 100644
@@ -1163,6 +1163,22 @@ int damon_stop(struct damon_ctx **ctxs, int nr_ctxs)
 	return err;
 }
 
+/**
+ * damon_is_running() - Returns if a given DAMON context is running.
+ * @ctx:	The DAMON context to see if running.
+ *
+ * Return: true if @ctx is running, false otherwise.
+ */
+bool damon_is_running(struct damon_ctx *ctx)
+{
+	bool running;
+
+	mutex_lock(&ctx->kdamond_lock);
+	running = ctx->kdamond != NULL;
+	mutex_unlock(&ctx->kdamond_lock);
+	return running;
+}
+
 /**
  * damon_kdamond_pid() - Return pid of a given DAMON context's worker thread.
  * @ctx:	The DAMON context of the question.

2.47.3

On Tue, 12 May 2026 21:46:58 -0700 SeongJae Park <[email protected]> wrote:

This is a wrong patch that mistakenly sent. Please ignore.

Thanks,
SJ

[...]

Patch series "mm/damon/modules: detect and use fresh status", v3.

DAMON modules including DAMON_RECLAIM, DAMON_LRU_SORT and DAMON_STAT commonly expose the kdamond running status via their parameters. Under certain scenarios including wrong user inputs and memory allocation failures, those parameter values can be stale. It can confuse users. For DAMON_RECLAIM and DAMON_LRU_SORT, it even makes the kdamond unable to be restarted before the system reboot.

The problem comes from the fact that there are multiple events for the status changes and it is difficult to follow up all the scenarios. Fix the issue by detecting and using the status on demand, instead of using a cached status that is difficult to be updated.

Patches 1-3 fix the bugs in DAMON_RECLAIM, DAMON_LRU_SORT and DAMON_STAT in the order.

This patch (of 3):

DAMON_RECLAIM updates 'enabled' and 'kdamond_pid' parameter values, which represents the running status of its kdamond, when the user explicitly requests start/stop of the kdamond. The kdamond can, however, be stopped in events other than the explicit user request in the following three events.

1. ctx->regions_score_histogram allocation failure at beginning of the execution,
2. damon_commit_ctx() failure due to invalid user input, and
3. damon_commit_ctx() failure due to its internal allocation failures.

Hence, if the kdamond is stopped by the above three events, the values of the status parameters can be stale. Users could show the stale values and be confused. This is already bad, but the real consequence is worse. DAMON_RECLAIM avoids unnecessary damon_start() and damon_stop() calls based on the 'enabled' parameter value. And the update of 'enabled' parameter value depends on the damon_start() and damon_stop() call results. Hence, once the kdamond has stopped by the unintentional events, the user cannot restart the kdamond before the system reboot. For example, the issue can be reproduced via below steps.

    # cd /sys/module/damon_reclaim/parameters
    #
    # # start DAMON_RECLAIM
    # echo Y > enabled
    # ps -ef | grep kdamond
    root         806       2  0 17:53 ?        00:00:00 [kdamond.0]
    root         808     803  0 17:53 pts/4    00:00:00 grep kdamond
    #
    # # commit wrong input to stop kdamond withou explicit stop request
    # echo 3 > addr_unit
    # echo Y > commit_inputs
    bash: echo: write error: Invalid argument
    #
    # # confirm kdamond is stopped
    # ps -ef | grep kdamond
    root         811     803  0 17:53 pts/4    00:00:00 grep kdamond
    #
    # # users casn now show stable status
    # cat enabled
    Y
    # cat kdamond_pid
    806
    #
    # # even after fixing the wrong parameter,
    # # kdamond cannot be restarted.
    # echo 1 > addr_unit
    # echo Y > enabled
    # ps -ef | grep kdamond
    root         815     803  0 17:54 pts/4    00:00:00 grep kdamond

The problem will only rarely happen in real and common setups for the following reasons. The allocation failures are unlikely in such setups since those allocations are arguably too small to fail. Also sane users on real production environments may not commit wrong input parameters. But once it happens, the consequence is quite bad. And the bug is a bug.

The issue stems from the fact that there are multiple events that can change the status, and following all the events is challenging. Dynamically detect and use the fresh status for the parameters when those are requested.

---
This depends on other two backported patches [1,2]. Please apply this after those.

[1] https://lore.kernel.org/[email protected]
[2] https://lore.kernel.org/[email protected]

 mm/damon/reclaim.c | 88 +++++++++++++++++++++++++++++-----------------
 1 file changed, 56 insertions(+), 32 deletions(-)

index 65842e6854fd1..9df096218beb7 100644
@@ -137,15 +137,6 @@ module_param(monitor_region_end, ulong, 0600);
 static bool skip_anon __read_mostly;
 module_param(skip_anon, bool, 0600);
 
-/*
- * PID of the DAMON thread
- *
- * If DAMON_RECLAIM is enabled, this becomes the PID of the worker thread.
- * Else, -1.
- */
-static int kdamond_pid __read_mostly = -1;
-module_param(kdamond_pid, int, 0400);
-
 static struct damos_stat damon_reclaim_stat;
 DEFINE_DAMON_MODULES_DAMOS_STATS_PARAMS(damon_reclaim_stat,
 		reclaim_tried_regions, reclaimed_regions, quota_exceeds);

@@ -247,60 +238,93 @@ static int damon_reclaim_turn(bool on)
 {
 	int err;
 
-	if (!on) {
-		err = damon_stop(&ctx, 1);
-		if (!err)
-			kdamond_pid = -1;
-		return err;
-	}
+	if (!on)
+		return damon_stop(&ctx, 1);
 
 	err = damon_reclaim_apply_parameters();
 	if (err)
 		return err;
 
-	err = damon_start(&ctx, 1, true);
-	if (err)
-		return err;
-	kdamond_pid = ctx->kdamond->pid;
-	return 0;
+	return damon_start(&ctx, 1, true);
+}
+
+static bool damon_reclaim_enabled(void)
+{
+	if (!ctx)
+		return false;
+	return damon_is_running(ctx);
 }
 
 static int damon_reclaim_enabled_store(const char *val,
 		const struct kernel_param *kp)
 {
-	bool is_enabled = enabled;
-	bool enable;
 	int err;
 
-	err = kstrtobool(val, &enable);
+	err = kstrtobool(val, &enabled);
 	if (err)
 		return err;
 
-	if (is_enabled == enable)
+	if (damon_reclaim_enabled() == enabled)
 		return 0;
 
 	/* Called before init function.  The function will handle this. */
 	if (!ctx)
-		goto set_param_out;
+		return 0;
 
-	err = damon_reclaim_turn(enable);
-	if (err)
-		return err;
+	return damon_reclaim_turn(enabled);
+}
 
-set_param_out:
-	enabled = enable;
-	return err;
+static int damon_reclaim_enabled_load(char *buffer,
+		const struct kernel_param *kp)
+{
+	return sprintf(buffer, "%c\n", damon_reclaim_enabled() ? 'Y' : 'N');
 }
 
 static const struct kernel_param_ops enabled_param_ops = {
 	.set = damon_reclaim_enabled_store,
-	.get = param_get_bool,
+	.get = damon_reclaim_enabled_load,
 };
 
 module_param_cb(enabled, &enabled_param_ops, &enabled, 0600);
 MODULE_PARM_DESC(enabled,
 	"Enable or disable DAMON_RECLAIM (default: disabled)");
 
+static int damon_reclaim_kdamond_pid_store(const char *val,
+		const struct kernel_param *kp)
+{
+	/*
+	 * kdamond_pid is read-only, but kernel command line could write it.
+	 * Do nothing here.
+	 */
+	return 0;
+}
+
+static int damon_reclaim_kdamond_pid_load(char *buffer,
+		const struct kernel_param *kp)
+{
+	int kdamond_pid = -1;
+
+	if (ctx) {
+		kdamond_pid = damon_kdamond_pid(ctx);
+		if (kdamond_pid < 0)
+			kdamond_pid = -1;
+	}
+	return sprintf(buffer, "%d\n", kdamond_pid);
+}
+
+static const struct kernel_param_ops kdamond_pid_param_ops = {
+	.set = damon_reclaim_kdamond_pid_store,
+	.get = damon_reclaim_kdamond_pid_load,
+};
+
+/*
+ * PID of the DAMON thread
+ *
+ * If DAMON_RECLAIM is enabled, this becomes the PID of the worker thread.
+ * Else, -1.
+ */
+module_param_cb(kdamond_pid, &kdamond_pid_param_ops, NULL, 0400);
+
 static int damon_reclaim_handle_commit_inputs(void)
 {
 	int err;

2.47.3

TL; DR: Sashiko found upstream commit fed48693bdfe ("mm/damon/reclaim: use parameter context correctly") is required for 6.12.y regardless of this patch, but not yet picked by stable kernel maintainers. Stable kernel maintainers, could you please add the patch to 6.12.y?

Other Sashiko-found issues are no issue in my opinion.

Please read below if you are interested in more details.

The bug is introduced by commit 11ddcfc257a3 ("mm/damon/reclaim: use damon_commit_ctx()"), which was merged into the mainline by 6.11. So the broken commit is in 6.12.y.

The fix was made by commit fed48693bdfe ("mm/damon/reclaim: use parameter context correctly") which was merged into the mainline by 6.17. And the fix commit has correct Fixes: tag, but somehow didn't picked by stable kernel maintainers.

Stable kernel maintainers, could you please backport the fix commit to 6.12.y? Let me know if any different action from my side is needed.

I informed this and where the dependent patches can be found, on the commentary area of the patch.

DAMON_RECLAIM is a static module. Userspace cannot write sysfs files before the static module initializations are finished to my understanding. So this kind of race cannot happen. Please let me know if I'm wrong.

I informed this and where the dependent patches can be found on the commentary area of this patch.

Thanks,
SJ