Re: [PATCH v3 1/5] mm: introduce MADV_COLD

On 6/27/19 4:54 AM, Minchan Kim wrote:

This patch introduces the new MADV_COLD hint to madvise(2) syscall.
MADV_COLD can be used by a process to mark a memory range as not expected
to be used in the near future. The hint can help kernel in deciding which
pages to evict early during memory pressure.

It works for every LRU pages like MADV_[DONTNEED|FREE]. IOW, It moves

	active file page -> inactive file LRU
	active anon page -> inacdtive anon LRU

Is the LRU behavior part of the interface or the implementation?

I ask because we've got something in between tossing something down the
LRU and swapping it: page migration.  Specifically, on a system with
slower memory media (like persistent memory) we just migrate a page
instead of discarding it at reclaim:

https://lore.kernel.org/linux-mm/20190321200157.29678-4-keith.busch@intel.com/ (local)

So let's say I have a page I want to evict from DRAM to the next slower
tier of memory.  Do I use MADV_COLD or MADV_PAGEOUT?  If the LRU
behavior is part of the interface itself, then MADV_COLD doesn't work.

Do you think we'll need a third MADV_ flag for our automatic migration
behavior?  MADV_REALLYCOLD?  MADV_MIGRATEOUT?