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

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Message-ID: <20190627140203.GB5303@dhcp22.suse.cz>
Date:   Thu, 27 Jun 2019 16:02:03 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org,
        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 Thu 27-06-19 06:13:36, Dave Hansen wrote:
> 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:

But we already do have interfaces for migrating the memory
(move_pages(2)). Why should this interface duplicate that interface?
I believe the only purpose of these two new madvise modes is to provide
a non-destructive MADV_{DONTNEED,FREE} alteternatives. In other words,
pageout vs. age interface.
-- 
Michal Hocko
SUSE Labs