[patch 255/262] mm/damon/dbgfs: add adaptive_targets list check before enable monitor_on

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

From: Xin Hao <xhao@xxxxxxxxxxxxxxxxx>
Subject: mm/damon/dbgfs: add adaptive_targets list check before enable monitor_on

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.

Link: https://lkml.kernel.org/r/0a60a6e8ec9d71989e0848a4dc3311996ca3b5d4.1634720326.git.xhao@xxxxxxxxxxxxxxxxx
Signed-off-by: Xin Hao <xhao@xxxxxxxxxxxxxxxxx>
Reviewed-by: SeongJae Park <sj@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 include/linux/damon.h |    1 +
 mm/damon/core.c       |    5 +++++
 mm/damon/dbgfs.c      |   15 ++++++++++++---
 3 files changed, 18 insertions(+), 3 deletions(-)

--- a/include/linux/damon.h~mm-damon-dbgfs-add-adaptive_targets-list-check-before-enable-monitor_on
+++ a/include/linux/damon.h
@@ -440,6 +440,7 @@ void damon_destroy_scheme(struct damos *
 
 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);
--- a/mm/damon/core.c~mm-damon-dbgfs-add-adaptive_targets-list-check-before-enable-monitor_on
+++ a/mm/damon/core.c
@@ -180,6 +180,11 @@ void damon_add_target(struct damon_ctx *
 	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);
--- a/mm/damon/dbgfs.c~mm-damon-dbgfs-add-adaptive_targets-list-check-before-enable-monitor_on
+++ a/mm/damon/dbgfs.c
@@ -878,12 +878,21 @@ static ssize_t dbgfs_monitor_on_write(st
 		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;
_