We don't use userspace program to allocate memory. We use kernel module to
do so.
Here is the sample kernel program you can use to test on a machine that has
more than 2 TB memory.

Use "insmod memtest.ko mb=2100000" to allocate more than 2 TB memory.
Use "rmmod memtest" to free the memory.

memtest.c
=============================================================
#include <linux/version.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/mm.h>
#include <linux/vmalloc.h>
#ifdef MODVERSIONS
#include <linux/modversions.h>
#endif
#include <asm/io.h>

static char *mem_ptr;
MODULE_LICENSE("GPL");
int mb = 0;
MODULE_PARM_DESC(mb, "alloc MB memory");
module_param(mb, int, S_IWUSR | S_IRUGO);

static int __init memtest_init(void) {
    mem_ptr = NULL;
    if (mb > 0) {
        unsigned long alloc_len;
        alloc_len = mb * 1024 * 1024;

        mem_ptr = vmalloc(alloc_len);

        if (mem_ptr) {
            printk("memtest: vmalloc for size %u MB.\n", mb);
        } else {
            printk("memtest: failed to vmalloc for bytes %u MB.\n", mb);
        }
    } else {
        printk("memtest len_MB\n");
    }
    return(0);
}

static void __exit memtest_exit(void) {
    if (mem_ptr) {
        vfree(mem_ptr);
        printk("memtest: vfree for size %u MB.\n", mb);
        mem_ptr = NULL;
    }
}

module_init(memtest_init);
module_exit(memtest_exit);

On Wed, Nov 4, 2020 at 7:38 PM Andrew Morton <akpm@linux-foundation.org>
wrote:

> On Wed, 4 Nov 2020 15:53:36 -0500 Hsin-Hui Wu <hsinhuiwu@gmail.com> wrote:
>
> > Did this require custom kernel changes?  If not, precisely which system
> > calls were used to cause this allocation attempt?
> >
> > [Frank] No. vmalloc().
> >
>
> vmalloc() is not a system call.
>
> Please fully describe how vmalloc() came to be called with such a large
> request size.  Fully.  Are you able to provide example userspace code?
>
>