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

[Dev Jain <dev.jain@arm.com>]

[-- Attachment #1: Type: text/plain, Size: 3162 bytes --]


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 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;
>   		}
>

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.

To test this behaviour, run the following program:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
intmain()
{
intfd;
fd = open("va_dump", O_CREAT| O_WRONLY, 0600);
unlink("va_dump");
// fd = open("/dev/null", O_WRONLY);
intret = munmap((void*)(1UL<< 30), 100);
if(!ret)
printf("munmap succeeded\n");
intres = write(fd, (void*)(1UL<< 30), 1);
if(res == 1)
printf("write succeeded\n");
return0;
}
The write will fail as expected, but if you comment out the va_dump
lines and use /dev/null, the write will succeed.

[-- Attachment #2: Type: text/html, Size: 7148 bytes --]