linux-kernel - Re: [RFC 1/3] mm: mmap make MAP

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Date:	Tue, 28 Apr 2015 16:10:01 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, Cyril Hrubis <chrubis@...e.cz>,
	Hugh Dickins <hughd@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [RFC 1/3] mm: mmap make MAP_LOCKED really mlock semantic

On Tue, 28 Apr 2015 14:11:49 +0200 Michal Hocko <mhocko@...e.cz> wrote:

> The man page however says
> "
> MAP_LOCKED (since Linux 2.5.37)
>       Lock the pages of the mapped region into memory in the manner of
>       mlock(2).  This flag is ignored in older kernels.
> "

I'm trying to remember why we implemented MAP_LOCKED in the first
place.  Was it better than mmap+mlock in some fashion?

afaict we had a #define MAP_LOCKED in the header file but it wasn't
implemented, so we went and wired it up.  13 years ago:
https://lkml.org/lkml/2002/9/18/108

Anyway...  the third way of doing this is to use plain old mmap() while
mlockall(MCL_FUTURE) is in force.  Has anyone looked at that, checked
that the behaviour is sane and compared it with the mmap+mlock
behaviour, the MAP_LOCKED behaviour and the manpages?

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