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@xxxxxxxxxxxxx>
> > ---
> > 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)
