linux-kernel - Re: [PATCH] Inconsistent mmap()/mremap() flags

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Message-ID: <Pine.LNX.4.64.0710021304230.26719@blonde.wat.veritas.com>
Date:	Tue, 2 Oct 2007 13:19:15 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Thayne Harbaugh <thayne@...net>
cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, discuss@...-64.org
Subject: Re: [PATCH] Inconsistent mmap()/mremap() flags

On Tue, 2 Oct 2007, Thayne Harbaugh wrote:
> On Tue, 2007-10-02 at 07:15 +0200, Andi Kleen wrote:
> 
> > For mmap you can emulate it by passing a low hint != 0 (e.g. getpagesize()) 
> > in address but without MAP_FIXED and checking if the result is not beyond
> > your range.
> 
> Cool.  That's a much better solution for multiple reasons - like you
> mention, MAP_32BIT is only 2GB as well as it's only available on x86_64.
> 
> > > > Given for mremap() it is not that easy because there is no "hint" argument
> > > > without MREMAP_FIXED; but unless someone really needs it i would prefer
> > > > to not propagate the hack. If it's really needed it's probably better
> > > > to implement a start search hint for mremap()

I think you can do it already, without us complicating mremap further
with such a start search hint.

First call mmap with a low hint address, the new size you'll be wanting
from the mremap, PROT_NONE, MAP_ANONYMOUS, -1, 0.  Then call mremap with
old address, old size, new size, MREMAP_MAYMOVE|MREMAP_FIXED, and new
address as returned by the preparatory mmap.

Hugh
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