On Wed, Jan 08, 2025 at 11:39:40AM +0530, Dev Jain wrote:

On 07/01/25 8:44 pm, Thomas Weißschuh wrote:

...

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 Weißschuhthomas.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;
  

  }

The reason I had not used /dev/null was that write() was succeeding to /dev/null even from an address not in my VA space. I was puzzled about this behaviour of /dev/null and I chose to ignore it and just use a real file.

That makes sense and I can reproduce your example. Switching to another dummy file which reads the written data like /dev/random also leads to OOM, so wouldn't help either.

Thanks for the explanation.

@Andrew, could you drop this patch?

To test this behaviour, run the following program:

[..]

PS: Your mail contained HTML and did not make it to the list archives. (And the text variant of the example program was corrupted)