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@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.
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
