linux-kernel - Re: [PATCH] mmap.2: MAP_FIXED is no longer discouraged

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Message-ID: <20171202150554.GA30203@bombadil.infradead.org>
Date:   Sat, 2 Dec 2017 07:05:55 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     john.hubbard@...il.com
Cc:     Michael Kerrisk <mtk.manpages@...il.com>,
        linux-man <linux-man@...r.kernel.org>, linux-api@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH] mmap.2: MAP_FIXED is no longer discouraged

On Fri, Dec 01, 2017 at 06:16:26PM -0800, john.hubbard@...il.com wrote:
> MAP_FIXED has been widely used for a very long time, yet the man
> page still claims that "the use of this option is discouraged".

I think we should continue to discourage the use of this option, but
I'm going to include some of your text in my replacement paragraph ...

-Because requiring a fixed address for a mapping is less portable,
-the use of this option is discouraged.
+The use of this option is discouraged because it forcibly unmaps any
+existing mapping at that address.  Programs which use this option need
+to be aware that their memory map may change significantly from one run to
+the next, depending on library versions, kernel versions and random numbers.
+In a threaded process, checking the existing mappings can race against
+a new dynamic library being loaded, for example in response to another
+thread making a library call which causes a PAM module to be loaded.

(I don't love this text, in particular "PAM module".  I'm going off to
use the ATM machine now.  Please edit.)