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madvise() MADV_DONTNEED allows this - http://man7.org/linux/man-pages/man2/madvise.2.html

It's also a flag in posix_madvise(), so the concept is reasonably portable.

malloc() and free() is what I meant by "standard memory allocation functions". I'd say program that is in the position to use madvise() effectively has implemented it's own heap allocator.

Also, MADV_DONTNEED is only usable in some specific situations, like caches. I don't see how it could be used to implement things like "on low memory, trigger garbage collection and trim the heap to the smallest possible with munmap()".

You said: "In other words, it's just wouldn't be practically possible to create an application that uses standard memory allocation functions and reliably can free some memory back to the kernel."

So I thought you'd be interested to know that you can do just this with the standard functions mmap() and madvise().

No, it's not a replacement for malloc/free, but it does have value to some applications for some use cases.

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