linux-kernel - Re: [PATCH v3 1/5] mm: introduce MADV

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Message-ID: <343599f9-3d99-b74f-1732-368e584fa5ef@intel.com>
Date:   Thu, 27 Jun 2019 06:13:36 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org, Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Sonny Rao <sonnyrao@...gle.com>, oleksandr@...hat.com,
        hdanton@...a.com, lizeb@...gle.com,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: 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/

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?