You're correct it doesn't prefault the mappings, but that's irrelevant: it accounts them as allocated, and a later allocation which goes over the limit will immediately fail.
Remember, the limit is artificial and defined by the user with overcommit=2, by overcommit_ratio and user_reserve_kbytes. Using overcommit=2 necessarily wastes RAM (renders a larger portion of it unusable).
> Using overcommit=2 necessarily wastes RAM (renders a larger portion of it unusable).
The RAM is not unusable, it will be used. Some portion of ram may be unallocatable, but that doesn't mean it's wasted.
There's a tradeoff. With overcommit disabled, you will get allocation failure rather than OOM killer. But you'll likely get allocation failures at memory pressure below that needed to trigger the OOM killer. And if you're running a wide variety of software, you'll run into problems because overcommit is the mainstream default for Linux, so many things are only widely tested with it enabled.
> The RAM is not unusable, it will be used. Some portion of ram may be unallocatable
I think that's a meaningless distinction: if userspace can't allocate it, it is functionally wasted.
I completely agree with your second paragraph, but again, some portion of RAM obtainable with overcommit=0 will be unobtainable with overcommit=2.
Maybe a better way to say it is that a system with overcommit=2 will fail at a lower memory pressure than one with overcommit=0. Additional RAM would have to be added to the former system to successfully run the same workload. That RAM is waste.
it's absolutely wasted if apps on server don't use disk (disk cache is pretty much only thing that can use that reserved memory).
You can have simple web server that took less than 100MB of RAM take gigabytes, just because it spawned few COW-ed threads
If the overcommit ratio is 1, there is no portion rendered unusable? This seems to contradict your "necessarily" wastes RAM claim?
Read the comment again, that wasn't the only one I mentioned.
Please point out what you're talking about, because the comment is short and I read it fully multiple times now.