selftests/damon: test debugfs file reads/writes with huge count

DAMON debugfs interface users were able to trigger warning by writing
some files with arbitrarily large 'count' parameter. The issue is fixed
with commit db7a347b26fe ("mm/damon/dbgfs: use '__GFP_NOWARN' for
user-specified size buffer allocation"). This commit adds a test case
for the issue in DAMON selftests to avoid future regressions.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: SeongJae Park <[e-mail]>
Cc: Brendan Higgins <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/damon/.gitignore

···
+# SPDX-License-Identifier: GPL-2.0-only
+huge_count_read_write

tools/testing/selftests/damon/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 # Makefile for damon selftests
 
+TEST_GEN_FILES += huge_count_read_write
+
 TEST_FILES = _chk_dependency.sh
 TEST_PROGS = debugfs_attrs.sh
 

tools/testing/selftests/damon/debugfs_attrs.sh

···
 test_write_fail "$DBGFS/monitor_on" "on" "orig_monitor_on" "empty target ids"
 echo "$orig_target_ids" > "$DBGFS/target_ids"
 
+# Test huge count read write
+# ==========================
+
+dmesg -C
+
+for file in "$DBGFS/"*
+do
+	./huge_count_read_write "$file"
+done
+
+if dmesg | grep -q WARNING
+then
+	dmesg
+	exit 1
+else
+	exit 0
+fi
+
 echo "PASS"

tools/testing/selftests/damon/huge_count_read_write.c

···
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Author: SeongJae Park <[e-mail]>
+ */
+
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <stdio.h>
+
+void write_read_with_huge_count(char *file)
+{
+	int filedesc = open(file, O_RDWR);
+	char buf[25];
+	int ret;
+
+	printf("%s %s\n", __func__, file);
+	if (filedesc < 0) {
+		fprintf(stderr, "failed opening %s\n", file);
+		exit(1);
+	}
+
+	write(filedesc, "", 0xfffffffful);
+	perror("after write: ");
+	ret = read(filedesc, buf, 0xfffffffful);
+	perror("after read: ");
+	close(filedesc);
+}
+
+int main(int argc, char *argv[])
+{
+	if (argc != 2) {
+		fprintf(stderr, "Usage: %s <file>\n", argv[0]);
+		exit(1);
+	}
+	write_read_with_huge_count(argv[1]);
+
+	return 0;
+}