mm/damon/dbgfs: fix 'struct pid' leaks in 'dbgfs_target_ids_write()'

DAMON debugfs interface increases the reference counts of 'struct pid's
for targets from the 'target_ids' file write callback
('dbgfs_target_ids_write()'), but decreases the counts only in DAMON
monitoring termination callback ('dbgfs_before_terminate()').

Therefore, when 'target_ids' file is repeatedly written without DAMON
monitoring start/termination, the reference count is not decreased and
therefore memory for the 'struct pid' cannot be freed.  This commit
fixes this issue by decreasing the reference counts when 'target_ids' is
written.

Link: https://lkml.kernel.org/r/20211229124029.23348-1-sj@kernel.org
Fixes: 4bc05954d0 ("mm/damon: implement a debugfs-based user space interface")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org>	[5.15+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/damon/dbgfs.c

@@ -353,6 +353,7 @@ static ssize_t dbgfs_target_ids_write(struct file *file,
 		const char __user *buf, size_t count, loff_t *ppos)
 {
 	struct damon_ctx *ctx = file->private_data;
+	struct damon_target *t, *next_t;
 	bool id_is_pid = true;
 	char *kbuf, *nrs;
 	unsigned long *targets;
@@ -397,8 +398,12 @@ static ssize_t dbgfs_target_ids_write(struct file *file,
 		goto unlock_out;
 	}
 
-	/* remove targets with previously-set primitive */
-	damon_set_targets(ctx, NULL, 0);
+	/* remove previously set targets */
+	damon_for_each_target_safe(t, next_t, ctx) {
+		if (targetid_is_pid(ctx))
+			put_pid((struct pid *)t->id);
+		damon_destroy_target(t);
+	}
 
 	/* Configure the context for the address space type */
 	if (id_is_pid)