[PATCH 3/3] selftests/mm: virtual_address_range: Dump to /dev/null

["Thomas Weißschuh" <thomas.weissschuh@linutronix.de>]

During the execution of validate_complete_va_space() a lot of memory is
on the VM subsystem. When running on a low memory subsystem an OOM may
be triggered, when writing to the dump file as the filesystem may also
require memory.

On my test system with 1100MiB physical memory:

	Tasks state (memory values in pages):
	[  pid  ]   uid  tgid total_vm      rss rss_anon rss_file rss_shmem pgtables_bytes swapents oom_score_adj name
	[     57]     0    57 34359215953      695      256        0       439 1064390656        0             0 virtual_address

	Out of memory: Killed process 57 (virtual_address) total-vm:137436863812kB, anon-rss:1024kB, file-rss:0kB, shmem-rss:1756kB, UID:0 pgtables:1039444kB oom_score_adj:0
	<snip>
	fault_in_iov_iter_readable+0x4a/0xd0
	generic_perform_write+0x9c/0x280
	shmem_file_write_iter+0x86/0x90
	vfs_write+0x29c/0x480
	ksys_write+0x6c/0xe0
	do_syscall_64+0x9e/0x1a0
	entry_SYSCALL_64_after_hwframe+0x77/0x7f

Write the dumped data into /dev/null instead which does not require
additional memory during write(), making the code simpler as a
side-effect.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 tools/testing/selftests/mm/virtual_address_range.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/mm/virtual_address_range.c b/tools/testing/selftests/mm/virtual_address_range.c
index 484f82c7b7c871f82a7d9ec6d6c649f2ab1eb0cd..4042fd878acd702d23da2c3293292de33bd48143 100644
--- a/tools/testing/selftests/mm/virtual_address_range.c
+++ b/tools/testing/selftests/mm/virtual_address_range.c
@@ -103,10 +103,9 @@ static int validate_complete_va_space(void)
 	FILE *file;
 	int fd;
 
-	fd = open("va_dump", O_CREAT | O_WRONLY, 0600);
-	unlink("va_dump");
+	fd = open("/dev/null", O_WRONLY);
 	if (fd < 0) {
-		ksft_test_result_skip("cannot create or open dump file\n");
+		ksft_test_result_skip("cannot create or open /dev/null\n");
 		ksft_finished();
 	}
 
@@ -152,7 +151,6 @@ static int validate_complete_va_space(void)
 		while (start_addr + hop < end_addr) {
 			if (write(fd, (void *)(start_addr + hop), 1) != 1)
 				return 1;
-			lseek(fd, 0, SEEK_SET);
 
 			hop += MAP_CHUNK_SIZE;
 		}

-- 
2.47.1