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

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

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 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.

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)