mmap failed to allocate virtual memory - linux

Question

I got the following output in ftrace:
mmap(0x200000000000, 17179869184, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = -1 ENOMEM (Cannot allocate memory)
My code:
void alloc_page_full_reverse()
{
printf("Allocating default pagesize pages > 128TB \n");
mmap_chunks_higher(24575, 0);
printf("Allocating default pagesize pages < 128TB \n");
/* Note: Allocating a 16GB chunk less due to heap space required
for other mappings */
mmap_chunks_lower(8190, 0);
}
int mmap_chunks_higher(unsigned long no_of_chunks, unsigned long hugetlb_arg)
{
unsigned long i;
char *hptr;
char *hint;
int mmap_args = 0;
for (i = 0; i < no_of_chunks; i++){
hint = hind_addr();
hptr = mmap(hint, MAP_CHUNK_SIZE, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | hugetlb_arg, -1, 0); // MAP_CHUNK_SIZE = 16GB
if (hptr == MAP_FAILED){
printf("\n Map failed at address %p < 384TB in iteration = %d \n", hptr, i);
exit(-1);
}
if (validate_addr(hptr, 1)){
printf("\n Address failed, not in > 128Tb iterator = %d\n", i);
exit(-1);
}
}
printf("> 128Tb: \n chunks allocated= %d \n", i);
}
static char *hind_addr(void)
{
int bits = 48 + rand() % 15;
return (char *) (1UL << bits);
}
Need to understand before mmap how to validate **void mmap(void addr, size_t length, int prot, int flags, int fd, off_t offset); all its argument are validated,
EX: size_t length is validated.

Answer 1

I still want to make sure I have enough memory before doing a mmap
There isn't an interface that allows a process to check this, and for good reason. Suppose such a syscall existed, and the kernel told a process it could allocate 1 GB of memory. However, it is possible the kernel is not able to allocate that memory by the time process actually requests the allocation. So, this information would not be useful.
Instead, you should attempt to allocate memory, and handle ENOMEM.