If/When does the does deallocated heap memory get reclaimed?

Question

I have been running overnight memory tests on an embedded Linux system. Using vmstat I have observed that the free memory steadily decreases over time. According to some

Answer 1

User calls to malloc and free (or new and delete), to the best of my knowledge never return no-longer used pages to the O/S. Instead, they just remember what memory has been freed so that if you do a malloc/new of a size that can be satisfied by previously freed memory, then it will use that, rather than going to the O/S and using sbrk to get more memory.

Thus this code:

for (;;)
{
    struct { char data[200 * 1024 * 1024] } HugeBuffer;
    HugeBuffer *buff = new HugeBuffer;
    delete buff;
}

Will allocate 200Mb once, and then just steadily use that memory forever. It will go to the O/S once on the original allocation, and then loop fiddling around in user space.