RE: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory pin

On Sun, 7 Feb 2021, Song Bao Hua (Barry Song) wrote:

NUMA balancer is just one of many reasons for page migration. Even one
simple alloc_pages() can cause memory migration in just single NUMA
node or UMA system.

The other reasons for page migration include but are not limited to:
* memory move due to CMA
* memory move due to huge pages creation

Hardly we can ask users to disable the COMPACTION, CMA and Huge Page
in the whole system.

What about only for mlocked memory, i.e. disable 
vm.compact_unevictable_allowed?

Adding syscalls is a big deal, we can make a reasonable inference that 
we'll have to support this forever if it's merged.  I haven't seen mention 
of what other unevictable memory *should* be migratable that would be 
adversely affected if we disable that sysctl.  Maybe that gets you part of 
the way there and there are some other deficiencies, but it seems like a 
good start would be to describe how CONFIG_NUMA_BALANCING=n + 
vm.compact_unevcitable_allowed + mlock() doesn't get you mostly there and 
then look into what's missing.

If it's a very compelling case where there simply are no alternatives, it 
would make sense.  Alternative is to find a more generic way, perhaps in 
combination with vm.compact_unevictable_allowed, to achieve what you're 
looking to do that can be useful even beyond your originally intended use 
case.