selftests/mm: clarify alternate unmapping in compaction_test

[ Upstream commit 69ba1d7 ]

Add a comment explaining that every other entry in the list is unmapped to
intentionally create fragmentation with locked pages before invoking
check_compaction().

Link: https://lore.kernel.org/da5e0a8d5152e54152c0d2f456aac2fac35af291.1779296493.git.sayalip@linux.ibm.com
Fixes: bd67d5c ("Test compaction of mlocked memory")
Signed-off-by: Sayali Patil <sayalip@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Sayali Patil

tools/testing/selftests/mm/compaction_test.c

@@ -234,6 +234,9 @@ int main(int argc, char **argv)
 		mem_fragmentable_MB -= MAP_SIZE_MB;
 	}
 
+	/* Unmap every other entry in the list to create fragmentation with
+	 * locked pages before invoking check_compaction().
+	 */
 	for (entry = list; entry != NULL; entry = entry->next) {
 		munmap(entry->map, MAP_SIZE);
 		if (!entry->next)