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.

Link: https://lore.kernel.org/20260628215447.96166-4-sj@kernel.org
Link: https://lore.kernel.org/20260608112455.274231F00893@smtp.kernel.org [1]
Fixes: 82a08bde3cf7 ("samples/damon: implement a DAMON module for memory tiering")
Signed-off-by: SJ Park <sj@kernel.org>
Reviewed-by: Zenghui Yu <zenghui.yu@linux.dev>
Cc: <stable@vger.kernel.org> # 6.16.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 10 insertions, 1 deletions

diff --git a/samples/damon/mtier.c b/samples/damon/mtier.c
index 3785b0c7ffb1..564212e054de 100644
--- a/samples/damon/mtier.c
+++ b/samples/damon/mtier.c
@@ -177,6 +177,7 @@ free_out:
 static int damon_sample_mtier_start(void)
 {
 	struct damon_ctx *ctx;
+	int err;
 
 	ctx = damon_sample_mtier_build_ctx(true);
 	if (!ctx)
@@ -188,7 +189,15 @@ 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)