[PATCH V4 2/2] mm/damon/dbgfs: Add adaptive_targets list check before enable monitor_on

[Xin Hao <xhao@xxxxxxxxxxxxxxxxx>]

When the ctx->adaptive_targets list is empty,
I did some test on monitor_on interface like this.

    # cat /sys/kernel/debug/damon/target_ids
    #
    # echo on > /sys/kernel/debug/damon/monitor_on
    # damon: kdamond (5390) starts

Though the ctx->adaptive_targets list is empty, but the
kthread_run still be called, and the kdamond.x thread still
be created, this is meaningless.

So there adds a judgment in 'dbgfs_monitor_on_write',
if the ctx->adaptive_targets list is empty, return -EINVAL.

Signed-off-by: Xin Hao <xhao@xxxxxxxxxxxxxxxxx>
---
 include/linux/damon.h |  1 +
 mm/damon/core.c       |  5 +++++
 mm/damon/dbgfs.c      | 15 ++++++++++++---
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 715dadd21f7c..4fce5f1f6dad 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -316,6 +316,7 @@ void damon_destroy_scheme(struct damos *s);

 struct damon_target *damon_new_target(unsigned long id);
 void damon_add_target(struct damon_ctx *ctx, struct damon_target *t);
+bool damon_targets_empty(struct damon_ctx *ctx);
 void damon_free_target(struct damon_target *t);
 void damon_destroy_target(struct damon_target *t);
 unsigned int damon_nr_regions(struct damon_target *t);
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 2f6785737902..c3a1374dbe0b 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -156,6 +156,11 @@ void damon_add_target(struct damon_ctx *ctx, struct damon_target *t)
 	list_add_tail(&t->list, &ctx->adaptive_targets);
 }

+bool damon_targets_empty(struct damon_ctx *ctx)
+{
+	return list_empty(&ctx->adaptive_targets);
+}
+
 static void damon_del_target(struct damon_target *t)
 {
 	list_del(&t->list);
diff --git a/mm/damon/dbgfs.c b/mm/damon/dbgfs.c
index c90988a20fa4..a02cf6bee8e8 100644
--- a/mm/damon/dbgfs.c
+++ b/mm/damon/dbgfs.c
@@ -861,12 +861,21 @@ static ssize_t dbgfs_monitor_on_write(struct file *file,
 		return -EINVAL;
 	}

-	if (!strncmp(kbuf, "on", count))
+	if (!strncmp(kbuf, "on", count)) {
+		int i;
+
+		for (i = 0; i < dbgfs_nr_ctxs; i++) {
+			if (damon_targets_empty(dbgfs_ctxs[i])) {
+				kfree(kbuf);
+				return -EINVAL;
+			}
+		}
 		ret = damon_start(dbgfs_ctxs, dbgfs_nr_ctxs);
-	else if (!strncmp(kbuf, "off", count))
+	} else if (!strncmp(kbuf, "off", count)) {
 		ret = damon_stop(dbgfs_ctxs, dbgfs_nr_ctxs);
-	else
+	} else {
 		ret = -EINVAL;
+	}

 	if (!ret)
 		ret = count;
--
2.31.0