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

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Message-ID: <20190627145302.GC5303@dhcp22.suse.cz>
Date:   Thu, 27 Jun 2019 16:53:02 +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 07:36:50, Dave Hansen wrote:
[...]
> For MADV_COLD, if we defined it like this, I think we could use it for
> both purposes (demotion and LRU movement):
> 
> 	Pages in the specified regions will be treated as less-recently-
> 	accessed compared to pages in the system with similar access
> 	frequencies.  In contrast to MADV_DONTNEED, the contents of the

you meant s@...V_DONTNEED@...V_FREE@ I suppose

> 	region are preserved.
> 
> It would be nice not to talk about reclaim at all since we're not
> promising reclaim per se.

Well, I guess this is just an implementation detail. MADV_FREE is really
only about aging. It is up to the kernel what to do during the reclaim
and the advice doesn't and shouldn't make any difference here.

Now MADV_PAGEOUT would be more tricky in that direction because it
defines an immediate action to page out the range. I do understand your
argument about NUMA unaware applications which might want to get
something like MADV_DEMOTE which would move a page to a secondary memory
(whatever that is) but I think this is asking for its own madvise.
MADV_PAGEOUT has a quite simple semnatic - move to the backing storage -
and I would rather not make it more complex.
-- 
Michal Hocko
SUSE Labs