Pretty much only glibc malloc still uses brk. GNU being GNU I guess. Modern mallocs like jemalloc use mmap exclusively.

FreeBSD straight up did not implement sbrk at all for aarch64 and riscv – very very few things broke because of that (one of the things that did break was, IIRC, GNU Emacs).

Fair, but I think this all amounts to "A linear patch of memory is fine primitive to build an allocator or standard library on."

The "not faulting on address zero" thing is interesting but not crazy -- only broken programs will break.

While you can build it that way it is not at all a good way to do it. You lose all the benefits of paging when handling fragmentation - you basically will never be able to shrink the data segment on most practical programs - and therefore never be able to return memory to the operating system.

brk is an ancient holdover from when memory protection was done via segmentation. Paging eliminates the need for this historical misfeature. I'm a bit sad WASM propagated it further into modern systems.

Isn't paging orthogonal to using brk? Or is "paging" something more specific than "virtual memory is handled by the OS in big chunks"?

I guess your allocator can try its best to prevent fragmentation, but you're right that returning memory to the OS isn't going to happen without explicit compaction (which nobody in native code is going to do.) Dunno if anyone ever expects memory use to go down though.

(Is less true than it used to be? -- I guess we used to make big swap files and now that's less fashionable.)

I guess if it's not too much more of a burden on the OS to make it keep track of a Swiss cheese virtual memory allocation then it is useful.

The OS implements brk under the hood with "swiss cheese virtual memory" so you don't get any real optimizations here. On AMD64 paging is mandatory. It does have a cost but the MMU hardware accelerates this to make it mostly not noticeable.

The brk system call places an unnecessary restriction on how the memory is used that is not imposed by the hardware. Back in the 70s when segmentation was a thing and virtual memory hadn't taken hold this would be different - but virtual memory has won.