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

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Date:   Sat, 2 Dec 2017 19:49:20 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     john.hubbard@...il.com, Michael Kerrisk <mtk.manpages@...il.com>,
        linux-man <linux-man@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <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 Sat, Dec 2, 2017 at 4:05 PM, Matthew Wilcox <willy@...radead.org> wrote:
> 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.

How about adding something explicit about when it's okay to use MAP_FIXED?
"This option should only be used to displace an existing mapping that is
controlled by the caller, or part of such a mapping." or something like that?

> +In a threaded process, checking the existing mappings can race against
> +a new dynamic library being loaded

malloc() and its various callers can also cause mmap() calls, which is probably
more relevant than library loading.