They ensure that regions of memory are backed by actual memory. This sounds like how to tell the kernel that you're actually need and will use this memory.
mlock() forces pages which have already been allocated and are already being used within an address range to stay resident in memory, so that they are not swapped out to disk. These pages were already being accounted for in swap reservations.
When disabling overcommit, what you'd need to tell Linux is actually quite different: it's that you are about to start to use a new address range, so please make sure that there is enough free swap space for this address range (and reserve it!).
Only after the latter is completed would the kernel be allowed to allocate new pages for this range and the program would be allowed to use them. The kernel would also be free to swap them out to disk like normal, unless mlock() would be called (but only after those pages are already being used, not before!).
So as you can see, mlock() accomplishes something very different and is orthogonal to the functionality I'm discussing, which means it can be used (or not) independently of this new feature to reserve swap space.
This new functionality (to notify Linux that you are about to use a new address range) would also implicitly allow Linux not to reserve swap space for any address range for which it hasn't been notified, which would allow Linux to use swap space much more efficiently and would allow users to disable memory overcommit on Linux without causing a bunch of unnecessary program failures / crashes.
mmap(), on the other hand, normally does two things:
1. It allocates a range of address space.
2. It reserves swap space for this address range (when memory overcommit is disabled).
Notably (and people get confused by this a lot), mmap() doesn't actually allocate memory (i.e. memory pages), it only assigns a range of address space for the program to use, exactly as large as the program requested, and then reserves swap space for it, if required.
What I'm proposing is to separate those two things, i.e. allow programs to allocate address ranges separately from doing the swap space reservation.
If you don't separate these two things then programs allocating a huge range of address space will simply fail due to lack of available swap space (again, when memory overcommit is disabled, which is the goal here).
