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

[David Hildenbrand <david@redhat.com>]

On 08.01.25 07:09, 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 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.

What exactly do we want to achieve with the write? Verify that the 
output of /proc/self/map is reasonable and we can actually resolve a 
fault / map a page?

Why not access the memory directly+signal handler or using 
/proc/self/mem, so you can avoid the temp file completely?

-- 
Cheers,

David / dhildenb