samples/damon/mtier: handle damon_{start,stop}() failures

RFC PATCH Superseded SeongJae Park 2 participants 7 messages Jun 9

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Stable branch Memory management DAMON

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SeongJae Park

author maintainer

2026-06-09 14:21 UTC #0 lore raw

v2 cover samples/damon/mtier: handle damon_{start,stop}() failures

DAMON sample module mtier is not correctly handling failures from damon_start() and damon_stop(). As a result, it can leak memory, disrupt next DAMON operations, and dereference freed and random memory. Fix those.

Changes from RFC v1
- RFC v1: https://lore.kernel.org/[email protected]
- Add damon_stop() failure handling fix to the series.

SeongJae Park (2):
samples/damon/mtier: handle damon_start() failure samples/damon/mtier: handle damon_stop() failure

 samples/damon/mtier.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

base-commit: f822d98f51b6e2b3ab41cad2e8bc1013d06f93bb

Signature · 1 line

2.47.3

SeongJae Park

author maintainer

2026-06-09 14:21 UTC #1 lore raw

RFC PATCH v2 1/2 samples/damon/mtier: handle damon_start() failure

damon_sample_mtier_start() callers assume it will clean up resources when it fails. And the function does the cleanup for context buildup failures. However, it is not doing the cleanup for damon_start() failure.

As a result, when damon_start() fails, it could leak the memory for DAMON context. Also, if damon_start() fails for only the second context, the first context will indefinitely run, and avoid starting other DAMON contexts since it is running in the exclusive mode. Stop possibly started DAMON context and free the contexts in case of the failure to fix the issues.

Note that the issue can reliably be reproduced because the module calls damon_start() in the exclusive mode. For example,

    $ sudo damo start
    $ echo Y | sudo tee /sys/module/damon_sample_mtier/parameters/enabled
    $ sudo cat /proc/allocinfo | grep damon_new_ctx

Because the first command is running another DAMON instance, the second command fails the damon_start() call because the new DAMON instance cannot exclusively run. And without this fix, by repeating the second and the third commands above, we can show the memory consumption is only increasing due to the leaks. It requires the sudo permission though.

The issue was discovered [1] by Sashiko.

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

Fixes 82a08bde3cf7 ("samples/damon: implement a DAMON module for memory tiering")

Cc <[email protected]> # 6.16.x

Signed-off-by SeongJae Park <[email protected]>

---
 samples/damon/mtier.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

samples/damon/mtier.c +10 -1

index eb1143de8df17..66b591f2180fa 100644
@@ -174,6 +174,7 @@ static struct damon_ctx *damon_sample_mtier_build_ctx(bool promote)
 static int damon_sample_mtier_start(void)
 {
 	struct damon_ctx *ctx;
+	int err;
 
 	ctx = damon_sample_mtier_build_ctx(true);
 	if (!ctx)

@@ -185,7 +186,15 @@ static int damon_sample_mtier_start(void)
 		return -ENOMEM;
 	}
 	ctxs[1] = ctx;
-	return damon_start(ctxs, 2, true);
+	err = damon_start(ctxs, 2, true);
+	if (!err)
+		return 0;
+
+	if (damon_is_running(ctxs[0]))
+		damon_stop(ctxs, 1);
+	damon_destroy_ctx(ctxs[0]);
+	damon_destroy_ctx(ctxs[1]);
+	return err;
 }
 
 static void damon_sample_mtier_stop(void)

Signature · 1 line

2.47.3

[email protected]

AI review

2026-06-09 14:35 UTC #2 lore raw

Re: [RFC PATCH v2 1/2] samples/damon/mtier: handle damon_start() failure

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Resource leak (struct damon_ctx, struct damos, struct damon_target, and struct pid) on damon_start() failure. --

commit e95eb25cfbc21e60927df5e7e4eadcf5468c1c41
Author: SeongJae Park <[email protected]>
samples/damon/mtier: handle damon_start() failure

This commit fixes a resource leak in the memory tiering DAMON sample module where failing to start the DAMON contexts leads to memory leaks and indefinitely running contexts.

Link https://lore.kernel.org/[email protected] open here

Quoted 3 lines

samples/damon/mtier.c +0 -0

[ ... ]

Quoted 5 lines

samples/damon/mtier.c +0 -1

@@ -185,7 +186,15 @@ static int damon_sample_mtier_start(void)
 		return -ENOMEM;
 	}
 	ctxs[1] = ctx;
-	return damon_start(ctxs, 2, true);

samples/damon/mtier.c static int damon_sample_mtier_start(void) · line 198

Quoted 10 lines

samples/damon/mtier.c +9 -0

+	err = damon_start(ctxs, 2, true);
+	if (!err)
+		return 0;
+
+	if (damon_is_running(ctxs[0]))
+		damon_stop(ctxs, 1);
+	damon_destroy_ctx(ctxs[0]);
+	damon_destroy_ctx(ctxs[1]);
+	return err;
 }

[Severity: High]
This is a pre-existing issue, but it looks like the exact same resource leak pattern exists in other DAMON sample modules when damon_start() fails.

Looking at damon_sample_prcl_start() in samples/damon/prcl.c:

	err = damon_start(&ctx, 1, true);
	if (err)
		return err;

And similarly in damon_sample_wsse_start() in samples/damon/wsse.c:

	err = damon_start(&ctx, 1, true);
	if (err)
		return err;

Since they return directly without calling damon_destroy_ctx() or putting the target_pidp reference, do these modules also leak their newly allocated contexts and PID references when damon_start() fails?

Signature · 1 line

Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1

SeongJae Park

author maintainer

2026-06-09 14:58 UTC #3 lore raw

Re: [RFC PATCH v2 1/2] samples/damon/mtier: handle damon_start() failure

Quoted 50 lines · On Tue, 09 Jun 2026 14:35:38 +0000 [email protected] wrote:

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
Pre-existing issues:
- [High] Resource leak (struct damon_ctx, struct damos, struct damon_target, and struct pid) on damon_start() failure. --
commit e95eb25cfbc21e60927df5e7e4eadcf5468c1c41
Author: SeongJae Park <[email protected]>
samples/damon/mtier: handle damon_start() failure
This commit fixes a resource leak in the memory tiering DAMON sample module where failing to start the DAMON contexts leads to memory leaks and indefinitely running contexts.
Link https://lore.kernel.org/[email protected] open here
Quoted 3 lines
samples/damon/mtier.c +0 -0
[ ... ]
Quoted 5 lines
samples/damon/mtier.c +0 -1
@@ -185,7 +186,15 @@ static int damon_sample_mtier_start(void)
 		return -ENOMEM;
 	}
 	ctxs[1] = ctx;
-	return damon_start(ctxs, 2, true);
samples/damon/mtier.c static int damon_sample_mtier_start(void) · line 198
Quoted 10 lines
samples/damon/mtier.c +9 -0
+	err = damon_start(ctxs, 2, true);
+	if (!err)
+		return 0;
+
+	if (damon_is_running(ctxs[0]))
+		damon_stop(ctxs, 1);
+	damon_destroy_ctx(ctxs[0]);
+	damon_destroy_ctx(ctxs[1]);
+	return err;
 }
[Severity: High]
This is a pre-existing issue, but it looks like the exact same resource leak pattern exists in other DAMON sample modules when damon_start() fails.
Looking at damon_sample_prcl_start() in samples/damon/prcl.c:
	err = damon_start(&ctx, 1, true);
	if (err)
		return err;
And similarly in damon_sample_wsse_start() in samples/damon/wsse.c:

Quoted 7 lines

	err = damon_start(&ctx, 1, true);
	if (err)
		return err;
Since they return directly without calling damon_destroy_ctx() or putting the target_pidp reference, do these modules also leak their newly allocated contexts and PID references when damon_start() fails?

Nice finding. Not a blocker of this patch, though. I will separately work on this.

Thanks,
SJ

[...]

SeongJae Park

author maintainer

2026-06-09 15:04 UTC #4 lore raw

Re: [RFC PATCH v2 1/2] samples/damon/mtier: handle damon_start() failure

Quoted 56 lines · On Tue, 09 Jun 2026 07:58:22 -0700 SeongJae Park <[email protected]> wrote:

Quoted 54 lines · On Tue, 09 Jun 2026 14:35:38 +0000 [email protected] wrote:
Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
Pre-existing issues:
- [High] Resource leak (struct damon_ctx, struct damos, struct damon_target, and struct pid) on damon_start() failure. --
commit e95eb25cfbc21e60927df5e7e4eadcf5468c1c41
Author: SeongJae Park <[email protected]>
samples/damon/mtier: handle damon_start() failure
This commit fixes a resource leak in the memory tiering DAMON sample module where failing to start the DAMON contexts leads to memory leaks and indefinitely running contexts.
Link https://lore.kernel.org/[email protected] open here
Quoted 3 lines
samples/damon/mtier.c +0 -0
[ ... ]
Quoted 5 lines
samples/damon/mtier.c +0 -1
@@ -185,7 +186,15 @@ static int damon_sample_mtier_start(void)
 		return -ENOMEM;
 	}
 	ctxs[1] = ctx;
-	return damon_start(ctxs, 2, true);
samples/damon/mtier.c static int damon_sample_mtier_start(void) · line 198
Quoted 10 lines
samples/damon/mtier.c +9 -0
+	err = damon_start(ctxs, 2, true);
+	if (!err)
+		return 0;
+
+	if (damon_is_running(ctxs[0]))
+		damon_stop(ctxs, 1);
+	damon_destroy_ctx(ctxs[0]);
+	damon_destroy_ctx(ctxs[1]);
+	return err;
 }
[Severity: High]
This is a pre-existing issue, but it looks like the exact same resource leak pattern exists in other DAMON sample modules when damon_start() fails.
Looking at damon_sample_prcl_start() in samples/damon/prcl.c:
	err = damon_start(&ctx, 1, true);
	if (err)
		return err;
And similarly in damon_sample_wsse_start() in samples/damon/wsse.c:
	err = damon_start(&ctx, 1, true);
	if (err)
		return err;

Quoted 6 lines

Quoted 3 lines
Since they return directly without calling damon_destroy_ctx() or putting the target_pidp reference, do these modules also leak their newly allocated contexts and PID references when damon_start() fails?
Nice finding. Not a blocker of this patch, though. I will separately work on this.

Hmm, but... They do destroy the ctx in damon_sample_{wsse,prcl}_stop(). And users cannot trigger damon_sample_{wsse,prcl}_start() without calling the stop function first, due to the current 'enabled' value based skippinng in damon_sample_wsse_enable_store(). So, this seems not a true issue. Let me know if I'm missing something.

Thanks,
SJ

[...]

SeongJae Park

author maintainer

2026-06-09 15:06 UTC #5 lore raw

Re: [RFC PATCH v2 1/2] samples/damon/mtier: handle damon_start() failure

Quoted 62 lines · On Tue, 09 Jun 2026 08:04:13 -0700 SeongJae Park <[email protected]> wrote:

Quoted 60 lines · On Tue, 09 Jun 2026 07:58:22 -0700 SeongJae Park <[email protected]> wrote:
Quoted 57 lines · On Tue, 09 Jun 2026 14:35:38 +0000 [email protected] wrote:
Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
Pre-existing issues:
- [High] Resource leak (struct damon_ctx, struct damos, struct damon_target, and struct pid) on damon_start() failure. --
commit e95eb25cfbc21e60927df5e7e4eadcf5468c1c41
Author: SeongJae Park <[email protected]>
samples/damon/mtier: handle damon_start() failure
This commit fixes a resource leak in the memory tiering DAMON sample module where failing to start the DAMON contexts leads to memory leaks and indefinitely running contexts.
Link https://lore.kernel.org/[email protected] open here
Quoted 3 lines
samples/damon/mtier.c +0 -0
[ ... ]
Quoted 5 lines
samples/damon/mtier.c +0 -1
@@ -185,7 +186,15 @@ static int damon_sample_mtier_start(void)
 		return -ENOMEM;
 	}
 	ctxs[1] = ctx;
-	return damon_start(ctxs, 2, true);
samples/damon/mtier.c static int damon_sample_mtier_start(void) · line 198
Quoted 10 lines
samples/damon/mtier.c +9 -0
+	err = damon_start(ctxs, 2, true);
+	if (!err)
+		return 0;
+
+	if (damon_is_running(ctxs[0]))
+		damon_stop(ctxs, 1);
+	damon_destroy_ctx(ctxs[0]);
+	damon_destroy_ctx(ctxs[1]);
+	return err;
 }
[Severity: High]
This is a pre-existing issue, but it looks like the exact same resource leak pattern exists in other DAMON sample modules when damon_start() fails.
Looking at damon_sample_prcl_start() in samples/damon/prcl.c:
	err = damon_start(&ctx, 1, true);
	if (err)
		return err;
And similarly in damon_sample_wsse_start() in samples/damon/wsse.c:
	err = damon_start(&ctx, 1, true);
	if (err)
		return err;
Since they return directly without calling damon_destroy_ctx() or putting the target_pidp reference, do these modules also leak their newly allocated contexts and PID references when damon_start() fails?

Quoted 8 lines

Quoted 2 lines
Nice finding. Not a blocker of this patch, though. I will separately work on this.
Hmm, but... They do destroy the ctx in damon_sample_{wsse,prcl}_stop(). And users cannot trigger damon_sample_{wsse,prcl}_start() without calling the stop function first, due to the current 'enabled' value based skippinng in damon_sample_wsse_enable_store(). So, this seems not a true issue. Let me know if I'm missing something.

Ok, I was missing this. damon_sample_wsse_enable_store() reset 'enabled' when damon_sample_wsse_start() fails. Same pattern for prcl and mtier. So seems the issue is real. I will separately work on this.

Thanks,
SJ

[...]

SeongJae Park

author maintainer

2026-06-09 14:21 UTC #6 lore raw

RFC PATCH v2 2/2 samples/damon/mtier: handle damon_stop() failure

In reply to SeongJae Park

damon_sample_mtier_stop() assumes its damon_stop() call will always successfully stops the two DAMON contexts. Hence it deallocates the two DAMON contexts after the damon_stop() call. However, if a given context is already stopped, damon_stop() fails and returns an error while letting the DAMON contexts that have not yet stopped keep running. This kind of unexpected early DAMON context stops could happen due to memory allocation failures in kdamond_fn(). Because damon_sample_mtier_stop() just deallocates all DAMON contexts with damon_target and damon_region objects that are linked to the contexts, the execution of the unstopped DAMON context (kdamond) ends up using the memory that freed (use-after-free). Fix the issue by separating the damon_stop() to be invoked per context.

Note that DAMON_SYSFS also allows multiple DAMON contexts execution. But, it calls damon_stop() for each context one by one. Hence this issue is only in mtier.

For the long term, it would be better to refactor damon_stop() to always ensure stopping all contexts regardless of the failures in the middle. Make this fix in the current way, though, to keep it simple and easy to backport. I will do the refactoring later.

The issue was discovered [1] by Sashiko.

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

Fixes 82a08bde3cf7 ("samples/damon: implement a DAMON module for memory tiering")

Cc <[email protected]> # 6.16.x

Signed-off-by SeongJae Park <[email protected]>

---
 samples/damon/mtier.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

samples/damon/mtier.c +2 -1

index 66b591f2180fa..faaaaa12e6206 100644
@@ -199,7 +199,8 @@ static int damon_sample_mtier_start(void)
 
 static void damon_sample_mtier_stop(void)
 {
-	damon_stop(ctxs, 2);
+	damon_stop(ctxs, 1);
+	damon_stop(&ctxs[1], 1);
 	damon_destroy_ctx(ctxs[0]);
 	damon_destroy_ctx(ctxs[1]);
 }

Signature · 1 line

2.47.3