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

[David Rientjes <rientjes@google.com>]

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.