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

[Dave Hansen]

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@xxxxxxxxx/

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?