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 Weißschuhthomas.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");while (start_addr + hop < end_addr) { if (write(fd, (void *)(start_addr + hop), 1) != 1) return 1;@@ -152,7 +151,6 @@ static int validate_complete_va_space(void)
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?